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<!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 href = ../update/ rel = prev > < link href = ../value_t/ rel = next > < link rel = icon href = ../../../assets/images/favicon.png > < meta name = generator content = "mkdocs-1.5.3, mkdocs-material-9.4.7" > < title > value - JSON for Modern C++< / title > < link rel = stylesheet href = ../../../assets/stylesheets/main.4b4a2bd9.min.css > < link rel = stylesheet href = ../../../assets/stylesheets/palette.356b1318.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-
< 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 = 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 = 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 >
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< span class = c1 > // (2)< / span >
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< 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 = 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 = 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 >
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< span class = c1 > // (3)< / span >
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< 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 = 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 = 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 >
< / 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 = 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 = 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 = k > return< / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > ;< / span >
< span class = p > }< / 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 > < / 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 > < / 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 = 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 = 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 = k > return< / span > < span class = w > < / span > < span class = n > default_value< / span > < span class = p > ;< / span >
< span class = p > }< / 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 > < / code > for JSON integer numbers, < code class = highlight > < span class = kt > bool< / span > < / code > for JSON booleans, or < code class = highlight > < span class = n > std< / span > < span class = o > ::< / span > < span class = n > vector< / 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 href = #examples title = "Permanent link" > ¶ < / a > < / h2 > < details
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / 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 = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span >
< span class = p > {< / span >
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< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
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< 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 = p > {< / 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 = 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 = 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 = 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 = 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 = 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 = p > };< / span >
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< span class = w > < / span > < span class = c1 > // access existing values< / span >
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< 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 = 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 >
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< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
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< 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 = 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 >
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< span class = w > < / span > < span class = c1 > // output values< / span >
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< 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 = 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 = p > }< / 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 >
< / 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 > #include< / span > < span class = w > < / span > < span class = cpf > < string_view> < / span >
< span class = cp > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / 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 = 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 = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span >
< span class = p > {< / span >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
2023-11-26 16:01:20 +03:00
< 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 = p > {< / 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 = 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 = 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 = 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 = 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 = 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 = p > };< / span >
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< span class = w > < / span > < span class = c1 > // access existing values< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // output values< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 = p > }< / 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 >
< / 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 > #include< / span > < span class = w > < / span > < span class = cpf > < nlohmann/json.hpp> < / 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 = 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 = kt > int< / span > < span class = w > < / span > < span class = nf > main< / span > < span class = p > ()< / span >
< span class = p > {< / span >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // create a JSON object with different entry types< / span >
2023-11-26 16:01:20 +03:00
< 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 = p > {< / 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 = 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 = 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 = 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 = 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 = 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 = p > };< / span >
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< span class = w > < / span > < span class = c1 > // access existing values< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // access nonexisting values and rely on default value< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 >
2023-03-08 15:43:29 +03:00
< span class = w > < / span > < span class = c1 > // output values< / span >
2023-11-26 16:01:20 +03:00
< 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 = 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 = p > }< / 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 >
< / 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 > < button type = button 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 < / button > < / main > < footer class = md-footer > < 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 - 2023 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 6.4.2 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 2023 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 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z" / > < / svg > < / a > < a href = https://twitter.com/nlohmann target = _blank rel = noopener title = twitter.com class = md-social__link > < svg xmlns = http://www.w3.org/2000/svg viewbox = "0 0 512 512" > <!-- Font Awesome Free 6.4.2 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 2023 Fonticons, Inc. --> < path d = "M459.37 151 . 716c . 325 4 . 548 . 325 9 . 097 . 325 13 . 645 0 138 . 72-105 . 583 298 . 558-298 . 558 298 . 558-59 . 452 0-114 . 68-17 . 219-161 . 137-47 . 106 8 . 447 . 974 16 . 568 1 . 299 25 . 34 1 . 299 49 .
