Strings

JSON natively encodes strings. There are two options for mapping them to C++ data structures, fully processed and raw strings. Raw strings only find the end of the string and do not change the underlying values.

Escaped

{
  "uris": [
    "example.com",
    "B\u00FCcher.ch",
    "happy.cn",
    "happy\u5FEB\u4E50.cn",
    "\u5FEB\u4E50.cn",
    "happy.\u4E2D\u56FD",
    "\u5FEB\u4E50.\u4E2D\u56FD",
    "www.\u30CF\u30F3\u30C9\u30DC\u30FC\u30EB\u30B5\u30E0\u30BA.com",
    "\uD83E\uDD84.com"
  ]
}

The above JSON document consists of an array member named "uris" with an element type of string.

The C++ std::vector<std::string> of the above json should be equivalent to:

 ++
std::vector<string> uris = {
  "example.com", 
  "Bücher.ch",
  "happy.cn", 
  "happy快乐.cn",
  "快乐.cn", 
  "happy.中国", 
  "快乐.中国", 
  "www.ハンドボールサムズ.com", 
  "🦄.com"};

To see a working example using this code, refer to cookbook_escaped_strings1_test.cpp.

Below the mapping of the JSON object with C++ data structures.

 ++
struct WebData {
  std::vector<std::string> uris;
};
 
namespace daw::json {
  template<>
  struct json_data_contract<WebData> {
    using type = json_member_list<
      json_array<"uris", std::string>
    >;
 
    static inline auto
    to_json_data( WebData const &value ) {
      return std::forward_as_tuple( value.uris );
    }
  };
}

Trusted, already-escaped strings

By default, json_string validates UTF-8 and escapes the value during serialization. When an application can guarantee that a value is valid UTF-8 and is already escaped as JSON string content, the EscapeValidUTF8::AssumeValid mapping option writes those bytes directly:

 ++
struct WebData {
  std::string escaped_uri;
};
 
namespace daw::json {
  template<>
  struct json_data_contract<WebData> {
    using type = json_member_list<
      json_string<
        "escaped_uri",
        std::string,
        options::string_opt( options::EscapeValidUTF8::AssumeValid )>
    >;
 
    static inline auto to_json_data( WebData const &value ) {
      return std::forward_as_tuple( value.escaped_uri );
    }
  };
}

For example, a value containing the bytes already escaped: \"quoted\\"</tt> is safe for this mapping because its quotation marks already have JSON escape characters. A value containing unescaped quotation marks is not safe and can produce invalid JSON. This option affects serialization only. <tt>from_json</tt> continues to decode and unescape a <tt>json_string</tt> normally. Consequently, a value produced by parsing must not automatically be treated as suitable for <tt>AssumeValid</tt>; the application must establish that it is already escaped before serializing it. The default <tt>EscapeValidUTF8::Validate</tt> behavior should be used whenever that guarantee is unavailable. @section autotoc_md202 Raw strings Raw strings are useful where we don't want to process the strings, we know they will never be escaped, or we do not require processing. A raw string, is also simpler in that it only requires a constructor that requires a pointer and size, like <tt>std::string_view</tt> or <tt>std::string</tt>. <br> @icode{json} { "uris": [ "example.com", "B\u00FCcher.ch", "happy.cn", "happy\u5FEB\u4E50.cn", "\u5FEB\u4E50.cn", "happy.\u4E2D\u56FD", "\u5FEB\u4E50.\u4E2D\u56FD", "www.\u30CF\u30F3\u30C9\u30DC\u30FC\u30EB\u30B5\u30E0\u30BA.com", "\uD83E\uDD84.com" ] } @endicode The above JSON document consists of an array member named <tt>"uris"</tt> with an element type of string. The C++ <tt>std::vector\<std::string\></tt> of the above json should be equivalent to when used as <tt>json_string_raw</tt>: @icode{c} ++ std::vector<std::string> uris = { "example.com", "B\u00FCcher.ch", "happy.cn", "happy\u5FEB\u4E50.cn", "\u5FEB\u4E50.cn", "happy.\u4E2D\u56FD", "\u5FEB\u4E50.\u4E2D\u56FD", "www.\u30CF\u30F3\u30C9\u30DC\u30FC\u30EB\u30B5\u30E0\u30BA.com", "\uD83E\uDD84.com" }; @endicode Contrasting with the [Escaped]("#Raw Strings") example, the difference will be that the array element type is <tt>json_string_raw</tt> @icode{c} ++ struct WebData { std::vector<std::string> uris; }; namespace daw::json { template<> json_data_contract<WebData> { using type = json_member_list< json_array<"uris", json_string_raw_no_name<>> >;

static inline auto to_json_data( WebData const & value ) { return std::forward_as_tuple( value.uris ); } }; }