Add string concatenation function

Add variadic concat() function for concatenating char *, char, and
string types.

include/nlohmann/detail/string_concat.hpp

@@ -0,0 +1,139 @@
+#pragma once
+
+#include <cstring> // strlen
+#include <string> // string
+#include <utility> // forward
+
+#include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/detected.hpp>
+
+namespace nlohmann
+{
+namespace detail
+{
+
+inline std::size_t concat_length()
+{
+    return 0;
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest);
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest);
+
+template<typename... Args>
+inline std::size_t concat_length(const char /*c*/, Args&& ... rest)
+{
+    return 1 + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest)
+{
+    // cppcheck-suppress ignoredReturnValue
+    return ::strlen(cstr) + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest)
+{
+    return str.size() + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType>
+inline void concat_into(OutStringType& /*out*/)
+{}
+
+template<typename StringType, typename Arg>
+using string_can_append = decltype(std::declval<StringType&>().append(std::declval < Arg && > ()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append = is_detected<string_can_append, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_op = decltype(std::declval<StringType&>() += std::declval < Arg && > ());
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_op = is_detected<string_can_append_op, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_iter = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().begin(), std::declval<const Arg&>().end()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_iter = is_detected<string_can_append_iter, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_data = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().data(), std::declval<const Arg&>().size()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_data = is_detected<string_can_append_data, StringType, Arg>;
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template<typename OutStringType, typename Arg, typename... Args,
+         enable_if_t<detect_string_can_append<OutStringType, Arg>::value, int> = 0>
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest)
+{
+    out.append(std::forward<Arg>(arg));
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, Arg&& arg, Args&& ... rest)
+{
+    out += std::forward<Arg>(arg);
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.begin(), arg.end());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.data(), arg.size());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType = std::string, typename... Args>
+inline OutStringType concat(Args && ... args)
+{
+    OutStringType str;
+    str.reserve(concat_length(std::forward<Args>(args)...));
+    concat_into(str, std::forward<Args>(args)...);
+    return str;
+}
+
+} // namespace detail
+} // namespace nlohmann

include/nlohmann/json.hpp

@@ -83,6 +83,7 @@ SOFTWARE.
 #include <nlohmann/detail/json_pointer.hpp>
 #include <nlohmann/detail/json_ref.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
+#include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/string_escape.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>

single_include/nlohmann/json.hpp

@@ -13187,6 +13187,149 @@ class json_ref
 
 // #include <nlohmann/detail/macro_scope.hpp>
 
+// #include <nlohmann/detail/string_concat.hpp>
+
+
+#include <cstring> // strlen
+#include <string> // string
+#include <utility> // forward
+
+// #include <nlohmann/detail/meta/cpp_future.hpp>
+
+// #include <nlohmann/detail/meta/detected.hpp>
+
+
+namespace nlohmann
+{
+namespace detail
+{
+
+inline std::size_t concat_length()
+{
+    return 0;
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest);
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest);
+
+template<typename... Args>
+inline std::size_t concat_length(const char /*c*/, Args&& ... rest)
+{
+    return 1 + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename... Args>
+inline std::size_t concat_length(const char* cstr, Args&& ... rest)
+{
+    // cppcheck-suppress ignoredReturnValue
+    return ::strlen(cstr) + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename StringType, typename... Args>
+inline std::size_t concat_length(const StringType& str, Args&& ... rest)
+{
+    return str.size() + concat_length(std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType>
+inline void concat_into(OutStringType& /*out*/)
+{}
+
+template<typename StringType, typename Arg>
+using string_can_append = decltype(std::declval<StringType&>().append(std::declval < Arg && > ()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append = is_detected<string_can_append, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_op = decltype(std::declval<StringType&>() += std::declval < Arg && > ());
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_op = is_detected<string_can_append_op, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_iter = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().begin(), std::declval<const Arg&>().end()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_iter = is_detected<string_can_append_iter, StringType, Arg>;
+
+template<typename StringType, typename Arg>
+using string_can_append_data = decltype(std::declval<StringType&>().append(std::declval<const Arg&>().data(), std::declval<const Arg&>().size()));
+
+template<typename StringType, typename Arg>
+using detect_string_can_append_data = is_detected<string_can_append_data, StringType, Arg>;
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > = 0 >
+inline void concat_into(OutStringType& out, const Arg& arg, Args && ... rest);
+
+template<typename OutStringType, typename Arg, typename... Args,
+         enable_if_t<detect_string_can_append<OutStringType, Arg>::value, int> = 0>
+inline void concat_into(OutStringType& out, Arg && arg, Args && ... rest)
+{
+    out.append(std::forward<Arg>(arg));
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && detect_string_can_append_op<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, Arg&& arg, Args&& ... rest)
+{
+    out += std::forward<Arg>(arg);
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && detect_string_can_append_iter<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.begin(), arg.end());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template < typename OutStringType, typename Arg, typename... Args,
+           enable_if_t < !detect_string_can_append<OutStringType, Arg>::value
+                         && !detect_string_can_append_op<OutStringType, Arg>::value
+                         && !detect_string_can_append_iter<OutStringType, Arg>::value
+                         && detect_string_can_append_data<OutStringType, Arg>::value, int > >
+inline void concat_into(OutStringType& out, const Arg& arg, Args&& ... rest)
+{
+    out.append(arg.data(), arg.size());
+    concat_into(out, std::forward<Args>(rest)...);
+}
+
+template<typename OutStringType = std::string, typename... Args>
+inline OutStringType concat(Args && ... args)
+{
+    OutStringType str;
+    str.reserve(concat_length(std::forward<Args>(args)...));
+    concat_into(str, std::forward<Args>(args)...);
+    return str;
+}
+
+} // namespace detail
+} // namespace nlohmann
+
 // #include <nlohmann/detail/string_escape.hpp>
 
 // #include <nlohmann/detail/meta/cpp_future.hpp>

test/src/unit-convenience.cpp

@@ -37,6 +37,84 @@ using nlohmann::json;
 
 namespace
 {
+struct alt_string_iter
+{
+    alt_string_iter() = default;
+    alt_string_iter(const char* cstr)
+        : impl(cstr)
+    {}
+
+    void reserve(std::size_t s)
+    {
+        impl.reserve(s);
+    }
+
+    template<typename Iter>
+    void append(Iter first, Iter last)
+    {
+        impl.append(first, last);
+    }
+
+    std::string::const_iterator begin() const
+    {
+        return impl.begin();
+    }
+
+    std::string::const_iterator end() const
+    {
+        return impl.end();
+    }
+
+    std::size_t size() const
+    {
+        return impl.size();
+    }
+
+    alt_string_iter& operator+=(const char c)
+    {
+        impl += c;
+        return *this;
+    }
+
+    std::string impl{};
+};
+
+struct alt_string_data
+{
+    alt_string_data() = default;
+    alt_string_data(const char* cstr)
+        : impl(cstr)
+    {}
+
+    void reserve(std::size_t s)
+    {
+        impl.reserve(s);
+    }
+
+    void append(const char* p, std::size_t s)
+    {
+        impl.append(p, s);
+    }
+
+    const char* data() const
+    {
+        return impl.data();
+    }
+
+    std::size_t size() const
+    {
+        return impl.size();
+    }
+
+    alt_string_data& operator+=(const char c)
+    {
+        impl += c;
+        return *this;
+    }
+
+    std::string impl{};
+};
+
 void check_escaped(const char* original, const char* escaped = "", bool ensure_ascii = false);
 void check_escaped(const char* original, const char* escaped, const bool ensure_ascii)
 {
@@ -110,4 +188,39 @@ TEST_CASE("convenience functions")
 
         CHECK_THROWS_WITH_AS(check_escaped("\xC2"), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
     }
+
+    SECTION("string concat")
+    {
+        using nlohmann::detail::concat;
+
+        const char* expected = "Hello, world!";
+        alt_string_iter hello_iter{"Hello, "};
+        alt_string_data hello_data{"Hello, "};
+        std::string world = "world";
+
+        SECTION("std::string")
+        {
+            std::string str1 = concat(hello_iter, world, '!');
+            std::string str2 = concat(hello_data, world, '!');
+            std::string str3 = concat("Hello, ", world, '!');
+
+            CHECK(str1 == expected);
+            CHECK(str2 == expected);
+            CHECK(str3 == expected);
+        }
+
+        SECTION("alt_string_iter")
+        {
+            alt_string_iter str = concat<alt_string_iter>(hello_iter, world, '!');
+
+            CHECK(str.impl == expected);
+        }
+
+        SECTION("alt_string_data")
+        {
+            alt_string_data str = concat<alt_string_data>(hello_data, world, '!');
+
+            CHECK(str.impl == expected);
+        }
+    }
 }