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Crow has built in support for JSON data.


The types of values that rvalue and wvalue can take are as follows:

  • False: from type bool.
  • True: from type bool.
  • Number
    • Floating_point: from type double.
    • Signed_integer: from type int.
    • Unsigned_integer: from type unsigned int.
  • String: from type std::string.
  • List: from type std::vector.
  • Object: from type crow::json::wvalue or crow::json::rvalue.
    This last type means that rvalue or wvalue can have keys.


JSON read value, used for taking a JSON string and parsing it into crow::json.

You can read individual items of the rvalue, but you cannot add items to it.
To do that, you need to convert it to a wvalue, which can be done by simply writing crow::json::wvalue wval (rval); (assuming rval is your rvalue).

For more info on read values go here.


JSON write value, used for creating, editing and converting JSON to a string.


setting a wvalue to object type can be done by simply assigning a value to whatever string key you like, something like wval["key1"] = val1;. Keep in mind that val1 can be any of the above types.

A wvalue can be treated as an object or even a list (setting a value by using json[3] = 32 for example). Please note that this will remove the data in the value if it isn't of List type.

An object type wvalue uses std::unordered_map by default, if you want to have your returned wvalue key value pairs be sorted (using std::map) you can add #define CROW_JSON_USE_MAP to the top of your program.

A JSON wvalue can be returned directly inside a route handler, this will cause the content-type header to automatically be set to Application/json and the JSON value will be converted to string and placed in the response body. For more information go to Routes.

For more info on write values go here.