2023-03-08 15:43:29 +03:00
<!doctype html> < html lang = en class = no-js > < head > < meta charset = utf-8 > < meta name = viewport content = "width=device-width,initial-scale=1" > < meta name = author content = "Niels Lohmann" > < link href = https://json.nlohmann.me/api/basic_json/value/ rel = canonical > < link rel = icon href = ../../../assets/images/favicon.png > < meta name = generator content = "mkdocs-1.4.2, mkdocs-material-8.5.10" > < title > value - JSON for Modern C++< / title > < link rel = stylesheet href = ../../../assets/stylesheets/main.975780f9.min.css > < link rel = stylesheet href = ../../../assets/stylesheets/palette.2505c338.min.css > < link rel = preconnect href = https://fonts.gstatic.com crossorigin > < link rel = stylesheet href = "https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CJetBrains+Mono:400,400i,700,700i&display=fallback" > < style > : root { --md-text-font : "Roboto" ; --md-code-font : "JetBrains Mono" } < / style > < link rel = stylesheet href = ../../../css/custom.css > < script > _ _md _scope = new URL ( "../../.." , location ) , _ _md _hash = e => [ ... e ] . reduce ( ( e , _ ) => ( e << 5 ) - e + _ . charCodeAt ( 0 ) , 0 ) , _ _md _get = ( e , _ = localStorage , t = _ _md _scope ) => JSON . parse ( _ . getItem ( t . pathname + "." + e ) ) , _ _md _set = ( e , _ , t = localStorage , a = _ _md _scope ) => { try { t . setItem ( a . pathname + "." + e , JSON . stringify ( _ ) ) } catch ( e ) { } } < / script > < / head > < body dir = ltr data-md-color-scheme = default data-md-color-primary = indigo data-md-color-accent = indigo > < script > var palette = _ _md _get ( "__palette" ) ; if ( palette && "object" == typeof palette . color ) for ( var key of Object . keys ( palette . color ) ) document . body . setAttribute ( "data-md-color-" + key , palette . color [ key ] ) < / script > < input class = md-toggle data-md-toggle = drawer type = checkbox id = __drawer autocomplete = off > < input class = md-toggle data-md-toggle = search type = checkbox id = __search autocomplete = off > < label class = md-overlay for = __drawer > < / label > < div data-md-component = skip > < a href = #nlohmannbasic_jsonvalue class = md-skip > Skip to content < / a > < / div > < div data-md-component = announce > < / div > < header class = md-header data-md-component = header > < nav class = "md-header__inner md-grid" aria-label = Header > < a href = ../../.. title = "JSON for Modern C++" class = "md-header__button md-logo" aria-label = "JSON for Modern C++" data-md-component = logo > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54Z" / > < / svg > < / a > < label class = "md-header__button md-icon" for = __drawer > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M3 6h18v2H3V6m0 5h18v2H3v-2m0 5h18v2H3v-2Z" / > < / svg > < / label > < div class = md-header__title data-md-component = header-title > < div class = md-header__ellipsis > < div class = md-header__topic > < span class = md-ellipsis > JSON for Modern C++ < / span > < / div > < div class = md-header__topic data-md-component = header-topic > < span class = md-ellipsis > value < / span > < / div > < / div > < / div > < form class = md-header__option data-md-component = palette > < input class = md-option data-md-color-media = "(prefers-color-scheme: light)" data-md-color-scheme = default data-md-color-primary = indigo data-md-color-accent = indigo aria-label = "Switch to dark mode" type = radio name = __palette id = __palette_1 > < label class = "md-header__button md-icon" title = "Switch to dark mode" for = __palette_2 hidden > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12 20 8.69Z" / > < / svg > < / label > < input class = md-option data-md-color-media = "(prefers-color-scheme: dark)" data-md-color-scheme = slate data-md-color-primary = indigo data-md-color-accent = indigo aria-label = "Switch to light mode" type = radio name = __palette id = __palette_2 > < label class = "md-header__button md-icon" title = "Switch to light mode" for = __palette_1 hidden > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M12 18c- . 89 0-1 . 74- . 2-2 . 5- . 55C11 . 56 16 . 5 13 14 . 42 13 12c0-2 . 42-1 . 44-4 . 5-3 . 5-5 . 45C10 . 26 6 . 2 11 . 11 6 12 6a6 6 0 0 1 6 6 6 6 0 0
< span class = k > template< / span > < span class = o > < < / span > < span class = k > class< / span > < span class = w > < / span > < span class = nc > ValueType< / span > < span class = o > > < / span > < span class = w > < / span >
< span class = n > ValueType< / span > < span class = w > < / span > < span class = n > value< / span > < span class = p > (< / span > < span class = k > const< / span > < span class = w > < / span > < span class = k > typename< / span > < span class = w > < / span > < span class = nc > object_t< / span > < span class = o > ::< / span > < span class = n > key_type< / span > < span class = o > & < / span > < span class = w > < / span > < span class = n > key< / span > < span class = p > ,< / span > < span class = w > < / span >
< span class = w > < / span > < span class = n > ValueType< / span > < span class = o > & & < / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > )< / span > < span class = w > < / span > < span class = k > const< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = c1 > // (2)< / span >
< span class = k > template< / span > < span class = o > < < / span > < span class = k > class< / span > < span class = w > < / span > < span class = nc > ValueType< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = k > class< / span > < span class = w > < / span > < span class = nc > KeyType< / span > < span class = o > > < / span > < span class = w > < / span >
< span class = n > ValueType< / span > < span class = w > < / span > < span class = n > value< / span > < span class = p > (< / span > < span class = n > KeyType< / span > < span class = o > & & < / span > < span class = w > < / span > < span class = n > key< / span > < span class = p > ,< / span > < span class = w > < / span >
< span class = w > < / span > < span class = n > ValueType< / span > < span class = o > & & < / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > )< / span > < span class = w > < / span > < span class = k > const< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = c1 > // (3)< / span >
< span class = k > template< / span > < span class = o > < < / span > < span class = k > class< / span > < span class = w > < / span > < span class = nc > ValueType< / span > < span class = o > > < / span > < span class = w > < / span >
< span class = n > ValueType< / span > < span class = w > < / span > < span class = n > value< / span > < span class = p > (< / span > < span class = k > const< / span > < span class = w > < / span > < span class = n > json_pointer< / span > < span class = o > & < / span > < span class = w > < / span > < span class = n > ptr< / span > < span class = p > ,< / span > < span class = w > < / span >
< span class = w > < / span > < span class = k > const< / span > < span class = w > < / span > < span class = n > ValueType< / span > < span class = o > & < / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > )< / span > < span class = w > < / span > < span class = k > const< / span > < span class = p > ;< / span > < span class = w > < / span >
< / code > < / pre > < / div > < ol > < li > < p > Returns either a copy of an object's element at the specified key < code > key< / code > or a given default value if no element with key < code > key< / code > exists.< / p > < p > The function is basically equivalent to executing < div class = highlight > < pre > < span > < / span > < code > < span class = k > try< / span > < span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = k > return< / span > < span class = w > < / span > < span class = n > at< / span > < span class = p > (< / span > < span class = n > key< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span > < span class = k > catch< / span > < span class = p > (< / span > < span class = n > out_of_range< / span > < span class = p > )< / span > < span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = k > return< / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span >
< / code > < / pre > < / div > < / p > < / li > < li > < p > See 1. This overload is only available if < code > KeyType< / code > is comparable with < code class = highlight > < span class = k > typename< / span > < span class = w > < / span > < span class = nc > object_t< / span > < span class = o > ::< / span > < span class = n > key_type< / span > < span class = w > < / span > < / code > and < code class = highlight > < span class = k > typename< / span > < span class = w > < / span > < span class = nc > object_comparator_t< / span > < span class = o > ::< / span > < span class = n > is_transparent< / span > < span class = w > < / span > < / code > denotes a type.< / p > < / li > < li > < p > Returns either a copy of an object's element at the specified JSON pointer < code > ptr< / code > or a given default value if no value at < code > ptr< / code > exists.< / p > < p > The function is basically equivalent to executing < div class = highlight > < pre > < span > < / span > < code > < span class = k > try< / span > < span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = k > return< / span > < span class = w > < / span > < span class = n > at< / span > < span class = p > (< / span > < span class = n > ptr< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span > < span class = k > catch< / span > < span class = p > (< / span > < span class = n > out_of_range< / span > < span class = p > )< / span > < span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = k > return< / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span >
< / code > < / pre > < / div > < / p > < / li > < / ol > < div class = "admonition note" > < p class = admonition-title > Differences to < code > at< / code > and < code > operator[]< / code > < / p > < ul > < li > Unlike < a href = ../at/ > < code > at< / code > < / a > , this function does not throw if the given < code > key< / code > /< code > ptr< / code > was not found.< / li > < li > Unlike < a href = ../operator%5B%5D/ > < code > operator[]< / code > < / a > , this function does not implicitly add an element to the position defined by < code > key< / code > /< code > ptr< / code > key. This function is furthermore also applicable to const objects.< / li > < / ul > < / div > < h2 id = template-parameters > Template parameters< a class = headerlink href = #template-parameters title = "Permanent link" > ¶ < / a > < / h2 > < dl > < dt > < code > KeyType< / code > < / dt > < dd > A type for an object key other than < a href = ../../json_pointer/ > < code > json_pointer< / code > < / a > that is comparable with < a href = ../string_t/ > < code > string_t< / code > < / a > using < a href = ../object_comparator_t/ > < code > object_comparator_t< / code > < / a > . This can also be a string view (C++17).< / dd > < dt > < code > ValueType< / code > < / dt > < dd > type compatible to JSON values, for instance < code class = highlight > < span class = kt > int< / span > < span class = w > < / span > < / code > for JSON integer numbers, < code class = highlight > < span class = kt > bool< / span > < span class = w > < / span > < / code > for JSON booleans, or < code class = highlight > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > vector< / span > < span class = w > < / span > < / code > types for JSON arrays. Note the type of the expected value at < code > key< / code > /< code > ptr< / code > and the default value < code > default_value< / code > must be compatible.< / dd > < / dl > < h2 id = parameters > Parameters< a class = headerlink href = #parameters title = "Permanent link" > ¶ < / a > < / h2 > < dl > < dt > < code > key< / code > (in)< / dt > < dd > key of the element to access< / dd > < dt > < code > default_value< / code > (in)< / dt > < dd > the value to return if < code > key< / code > /< code > ptr< / code > found no value< / dd > < dt > < code > ptr< / code > (in)< / dt > < dd > a JSON pointer to the element to access< / dd > < / dl > < h2 id = return-value > Return value< a class = headerlink href = #return-value title = "Permanent link" > ¶ < / a > < / h2 > < ol > < li > copy of the element at key < code > key< / code > or < code > default_value< / code > if < code > key< / code > is not found< / li > < li > copy of the element at key < code > key< / code > or < code > default_value< / code > if < code > key< / code > is not found< / li > < li > copy of the element at JSON Pointer < code > ptr< / code > or < code > default_value< / code > if no value for < code > ptr< / code > is found< / li > < / ol > < h2 id = exception-safety > Exception safety< a class = headerlink href = #exception-safety title = "Permanent link" > ¶ < / a > < / h2 > < p > Strong guarantee: if an exception is thrown, there are no changes to any JSON value.< / p > < h2 id = exceptions > Exceptions< a class = headerlink href = #exceptions title = "Permanent link" > ¶ < / a > < / h2 > < ol > < li > The function can throw the following exceptions:< ul > < li > Throws < a href = ../../../home/exceptions/#jsonexceptiontype_error302 > < code > type_error.302< / code > < / a > if < code > default_value< / code > does not match the type of the value at < code > key< / code > < / li > < li > Throws < a href = ../../../home/exceptions/#jsonexceptiontype_error306 > < code > type_error.306< / code > < / a > if the JSON value is not an object; in that case, using < code > value()< / code > with a key makes no sense.< / li > < / ul > < / li > < li > See 1.< / li > < li > The function can throw the following exceptions:< ul > < li > Throws < a href = ../../../home/exceptions/#jsonexceptiontype_error302 > < code > type_error.302< / code > < / a > if < code > default_value< / code > does not match the type of the value at < code > ptr< / code > < / li > < li > Throws < a href = ../../../home/exceptions/#jsonexceptiontype_error306 > < code > type_error.306< / code > < / a > if the JSON value is not an object; in that case, using < code > value()< / code > with a key makes no sense.< / li > < / ul > < / li > < / ol > < h2 id = complexity > Complexity< a class = headerlink href = #complexity title = "Permanent link" > ¶ < / a > < / h2 > < ol > < li > Logarithmic in the size of the container.< / li > < li > Logarithmic in the size of the container.< / li > < li > Logarithmic in the size of the container.< / li > < / ol > < h2 id = examples > Examples< a class = headerlink
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / span > < span class = cp > < / span >
< span class = k > using< / span > < span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > nlohmann< / span > < span class = o > ::< / span > < span class = n > json< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span > < span class = w > < / span >
< span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
< span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = n > j< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " integer" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " floating" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " string" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " hello world" < / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " boolean" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > true< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " object" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {{< / span > < span class = s > " key1" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span > < span class = p > {< / span > < span class = s > " key2" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > }}},< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " array" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {< / span > < span class = mi > 1< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 3< / span > < span class = p > }}< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > };< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access existing values< / span >
< span class = w > < / span > < span class = kt > int< / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " integer" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 0< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > double< / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " floating" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 47.11< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > string< / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " nonexisting" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " oops" < / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > bool< / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " nonexisting" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > false< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // output values< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > cout< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > boolalpha< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span >
< span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " < / span > < span class = se > \n< / span > < span class = s > " < / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span >
< / code > < / pre > < / div > < p > Output:< / p > < div class = highlight > < pre > < span > < / span > < code > < span class = mi > 1< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = w > < / span > < span class = err > oops< / span > < span class = w > < / span > < span class = kc > false< / span > < span class = w > < / span >
< / code > < / pre > < / div > < / details > < details class = example > < summary > Example: (2) access specified object element using string_view with default value< / summary > < p > The example below shows how object elements can be queried with a default value.< / p > < div class = highlight > < pre > < span > < / span > < code > < span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < iostream> < / span > < span class = cp > < / span >
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < string_view> < / span > < span class = cp > < / span >
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / span > < span class = cp > < / span >
< span class = k > using< / span > < span class = w > < / span > < span class = k > namespace< / span > < span class = w > < / span > < span class = nn > std< / span > < span class = o > ::< / span > < span class = nn > string_view_literals< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = k > using< / span > < span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > nlohmann< / span > < span class = o > ::< / span > < span class = n > json< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span > < span class = w > < / span >
< span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
< span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = n > j< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " integer" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " floating" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " string" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " hello world" < / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " boolean" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > true< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " object" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {{< / span > < span class = s > " key1" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span > < span class = p > {< / span > < span class = s > " key2" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > }}},< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " array" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {< / span > < span class = mi > 1< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 3< / span > < span class = p > }}< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > };< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access existing values< / span >
< span class = w > < / span > < span class = kt > int< / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " integer" < / span > < span class = n > sv< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 0< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > double< / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " floating" < / span > < span class = n > sv< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 47.11< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > string< / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " nonexisting" < / span > < span class = n > sv< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " oops" < / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > bool< / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " nonexisting" < / span > < span class = n > sv< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > false< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // output values< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > cout< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > boolalpha< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span >
< span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " < / span > < span class = se > \n< / span > < span class = s > " < / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span >
< / code > < / pre > < / div > < p > Output:< / p > < div class = highlight > < pre > < span > < / span > < code > < span class = mi > 1< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = w > < / span > < span class = err > oops< / span > < span class = w > < / span > < span class = kc > false< / span > < span class = w > < / span >
< / code > < / pre > < / div > < / details > < details class = example > < summary > Example: (3) access specified object element via JSON Pointer with default value< / summary > < p > The example below shows how object elements can be queried with a default value.< / p > < div class = highlight > < pre > < span > < / span > < code > < span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < iostream> < / span > < span class = cp > < / span >
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / span > < span class = cp > < / span >
< span class = k > using< / span > < span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > nlohmann< / span > < span class = o > ::< / span > < span class = n > json< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = k > using< / span > < span class = w > < / span > < span class = k > namespace< / span > < span class = w > < / span > < span class = nn > nlohmann< / span > < span class = o > ::< / span > < span class = nn > literals< / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span > < span class = w > < / span >
< span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
< span class = w > < / span > < span class = n > json< / span > < span class = w > < / span > < span class = n > j< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " integer" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " floating" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " string" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " hello world" < / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " boolean" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > true< / span > < span class = p > },< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " object" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {{< / span > < span class = s > " key1" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 1< / span > < span class = p > },< / span > < span class = w > < / span > < span class = p > {< / span > < span class = s > " key2" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > }}},< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > {< / span > < span class = s > " array" < / span > < span class = p > ,< / span > < span class = w > < / span > < span class = p > {< / span > < span class = mi > 1< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 2< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 3< / span > < span class = p > }}< / span > < span class = w > < / span >
< span class = w > < / span > < span class = p > };< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access existing values< / span >
< span class = w > < / span > < span class = kt > int< / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " /integer" < / span > < span class = n > _json_pointer< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mi > 0< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > double< / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " /floating" < / span > < span class = n > _json_pointer< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = mf > 47.11< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > string< / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " /nonexisting" < / span > < span class = n > _json_pointer< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = s > " oops" < / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = kt > bool< / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > =< / span > < span class = w > < / span > < span class = n > j< / span > < span class = p > .< / span > < span class = n > value< / span > < span class = p > (< / span > < span class = s > " /nonexisting" < / span > < span class = n > _json_pointer< / span > < span class = p > ,< / span > < span class = w > < / span > < span class = nb > false< / span > < span class = p > );< / span > < span class = w > < / span >
< span class = w > < / span > < span class = c1 > // output values< / span >
< span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > cout< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > boolalpha< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_integer< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_floating< / span > < span class = w > < / span >
< span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_string< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " " < / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = n > v_boolean< / span > < span class = w > < / span > < span class = o > < < < / span > < span class = w > < / span > < span class = s > " < / span > < span class = se > \n< / span > < span class = s > " < / span > < span class = p > ;< / span > < span class = w > < / span >
< span class = p > }< / span > < span class = w > < / span >
< / code > < / pre > < / div > < p > Output:< / p > < div class = highlight > < pre > < span > < / span > < code > < span class = mi > 1< / span > < span class = w > < / span > < span class = mf > 42.23< / span > < span class = w > < / span > < span class = err > oops< / span > < span class = w > < / span > < span class = kc > false< / span > < span class = w > < / span >
2023-09-23 18:28:29 +03:00
< / code > < / pre > < / div > < / details > < h2 id = see-also > See also< a class = headerlink href = #see-also title = "Permanent link" > ¶ < / a > < / h2 > < ul > < li > see < a href = ../at/ > < code > at< / code > < / a > for access by reference with range checking< / li > < li > see < a href = ../operator%5B%5D/ > < code > operator[]< / code > < / a > for unchecked access by reference< / li > < / ul > < h2 id = version-history > Version history< a class = headerlink href = #version-history title = "Permanent link" > ¶ < / a > < / h2 > < ol > < li > Added in version 1.0.0. Changed parameter < code > default_value< / code > type from < code > const ValueType& < / code > to < code > ValueType& & < / code > in version 3.11.0.< / li > < li > Added in version 3.11.0. Made < code > ValueType< / code > the first template parameter in version 3.11.2.< / li > < li > Added in version 2.0.2.< / li > < / ol > < hr > < div class = md-source-file > < small > Last update: < span class = "git-revision-date-localized-plugin git-revision-date-localized-plugin-date" > August 7, 2022< / span > < / small > < / div > < / article > < / div > < script > var tabs = _ _md _get ( "__tabs" ) ; if ( Array . isArray ( tabs ) ) e : for ( var set of document . querySelectorAll ( ".tabbed-set" ) ) { var tab , labels = set . querySelector ( ".tabbed-labels" ) ; for ( tab of tabs ) for ( var label of labels . getElementsByTagName ( "label" ) ) if ( label . innerText . trim ( ) === tab ) { var input = document . getElementById ( label . htmlFor ) ; input . checked = ! 0 ; continue e } } < / script > < / div > < a href = # class = "md-top md-icon" data-md-component = top hidden > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12Z" / > < / svg > Back to top < / a > < / main > < footer class = md-footer > < nav class = "md-footer__inner md-grid" aria-label = Footer > < a href = ../update/ class = "md-footer__link md-footer__link--prev" aria-label = "Previous: update" rel = prev > < div class = "md-footer__button md-icon" > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M20 11v2H8l5.5 5.5-1.42 1.42L4.16 12l7.92-7.92L13.5 5.5 8 11h12Z" / > < / svg > < / div > < div class = md-footer__title > < div class = md-ellipsis > < span class = md-footer__direction > Previous < / span > update < / div > < / div > < / a > < a href = ../value_t/ class = "md-footer__link md-footer__link--next" aria-label = "Next: value_t" rel = next > < div class = md-footer__title > < div class = md-ellipsis > < span class = md-footer__direction > Next < / span > value_t < / div > < / div > < div class = "md-footer__button md-icon" > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 24 24" > < path d = "M4 11v2h12l-5.5 5.5 1.42 1.42L19.84 12l-7.92-7.92L10.5 5.5 16 11H4Z" / > < / svg > < / div > < / a > < / nav > < div class = "md-footer-meta md-typeset" > < div class = "md-footer-meta__inner md-grid" > < div class = md-copyright > < div class = md-copyright__highlight > Copyright © 2013 - 2022 Niels Lohmann < / div > < / div > < div class = md-social > < a href = https://github.com/nlohmann target = _blank rel = noopener title = github.com class = md-social__link > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 496 512" > <!-- Font Awesome Free 6.2.1 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M165.9 397 . 4c0 2-2 . 3 3 . 6-5 . 2 3 . 6-3 . 3 . 3-5 . 6-1 . 3-5 . 6-3 . 6 0-2 2 . 3-3 . 6 5 . 2-3 . 6 3- . 3 5 . 6 1 . 3 5 . 6 3 . 6zm-31 . 1-4 . 5c- . 7 2 1 . 3 4 . 3 4 . 3 4 . 9 2 . 6 1 5 . 6 0 6 . 2-2s-1 . 3-4 . 3-4 . 3-5 . 2c-2 . 6- . 7-5 . 5 . 3-6 . 2 2 . 3zm44 . 2-1 . 7c-2 . 9 . 7-4 . 9 2 . 6-4 . 6 4 . 9 . 3 2 2 . 9 3 . 3 5 . 9 2 . 6 2 . 9- . 7 4 . 9-2 . 6 4 . 6-4 . 6- . 3-1 . 9-3-3 . 2-5 . 9-2 . 9zM244 . 8 8C106 . 1 8 0 113 . 3 0 252c0 110 . 9 69 . 8 205 . 8 169 . 5 239 . 2 12 . 8 2 . 3 17 . 3-5 . 6 17 . 3-12 . 1 0-6 . 2- . 3-40 . 4- . 3-61 . 4 0 0-70 15-84 . 7-29 . 8 0 0-11 . 4-29 . 1-27 . 8-36 . 6 0 0-22 . 9-15 . 7 1 . 6-15 . 4 0 0 24 . 9 2 38 . 6 25 . 8 21 . 9 38 . 6 58 . 6 27 . 5 72 . 9 20 . 9 2 . 3-16 8 . 8-27 . 1 16-33 . 7-55 . 9-6 . 2-112 . 3-14 . 3-112 . 3-110 . 5 0-27 . 5 7 . 6-41 . 3 23 . 6-58 . 9-2 . 6-6 . 5-11 . 1-33 . 3 2 . 6-67 . 9 20 . 9-6 . 5 69 27 69 27 20-5 . 6 41 . 5-8 . 5 62 . 8-8 . 5s42 . 8 2 . 9 62 . 8 8 . 5c0 0 48 . 1-33 . 6 69-27 13 . 7 34 . 7 5 . 2 61 . 4 2 . 6 67 . 9 16 17 . 7 25 . 8 31 . 5 25 . 8 58 . 9 0 96 . 5-58 . 9 104 . 2-114 . 8 110 . 5 9 . 2 7 . 9 17 22 . 9 17 46 . 4 0 33 . 7- . 3 75 . 4- . 3 83 . 6 0 6 . 5 4 . 6 14 . 4 17 . 3 12 . 1C428 . 2 457 . 8 496 362 . 9 496 252 496 113 . 3 383 . 5 8 244 . 8 8zM97 . 2 352 . 9c-1 . 3 1-1 3 . 3 . 7 5 . 2 1 . 6 1 . 6 3 . 9 2 . 3 5 . 2 1 1
