How to: Work with JSON Data (C++ REST SDK)
You can parse JSON data faster by using the C++ REST SDK (codename "Casablanca") web::json namespace. This page shows two examples. The first shows how to extract JSON data from an HTTP GET response. The second example builds a JSON value in memory and iterates through its values.
Warning
This topic contains information for the C++ REST SDK 1.0 (codename "Casablanca"). If you are using a later version from the Codeplex Casablanca web page, then use the local documentation at http://casablanca.codeplex.com/documentation.
A more complete example that shows #include and using statements follows these examples.
To extract JSON data from an HTTP GET response
Here’s how to use the web::http::client::http_response::extract_json method to extract JSON data from an HTTP GET response. For a more basic version that retrieves a server response but doesn’t work with JSON, see How to: Connect to HTTP servers.
// Retrieves a JSON value from an HTTP request.
pplx::task<void> RequestJSONValueAsync()
{
// TODO: To successfully use this example, you must perform the request
// against a server that provides JSON data.
// This example fails because the returned Content-Type is text/html and not application/json.
http_client client(L"https://www.fourthcoffee.com");
return client.request(methods::GET).then([](http_response response) -> pplx::task<json::value>
{
if(response.status_code() == status_codes::OK)
{
return response.extract_json();
}
// Handle error cases, for now return empty json value...
return pplx::task_from_result(json::value());
})
.then([](pplx::task<json::value> previousTask)
{
try
{
const json::value& v = previousTask.get();
// Perform actions here to process the JSON value...
}
catch (const http_exception& e)
{
// Print error.
wostringstream ss;
ss << e.what() << endl;
wcout << ss.str();
}
});
/* Output:
Content-Type must be application/json to extract (is: text/html)
*/
}
To build a JSON value in memory and iterate through its values
Here’s how to use the web::json::value class to build a JSON value in memory and loop through its values. The web::json::value::cbegin and web::json::value::cend methods return read-only iterators for the value collection.
// Demonstrates how to iterate over a JSON object.
void IterateJSONValue()
{
// Create a JSON object.
json::value obj;
obj[L"key1"] = json::value::boolean(false);
obj[L"key2"] = json::value::number(44);
obj[L"key3"] = json::value::number(43.6);
obj[L"key4"] = json::value::string(U("str"));
// Loop over each element in the object.
for(auto iter = obj.cbegin(); iter != obj.cend(); ++iter)
{
// Make sure to get the value as const reference otherwise you will end up copying
// the whole JSON value recursively which can be expensive if it is a nested object.
const json::value &str = iter->first;
const json::value &v = iter->second;
// Perform actions here to process each string and value in the JSON object...
std::wcout << L"String: " << str.as_string() << L", Value: " << v.to_string() << endl;
}
/* Output:
String: key1, Value: false
String: key2, Value: 44
String: key3, Value: 43.6
String: key4, Value: str
*/
}
Complete example
Here’s the complete example.
#include <http_client.h>
#include <iostream>
#include <json.h>
using namespace web;
using namespace web::http;
using namespace web::http::client;
// Retrieves a JSON value from an HTTP request.
pplx::task<void> RequestJSONValueAsync()
{
// TODO: To successfully use this example, you must perform the request
// against a server that provides JSON data.
// This example fails because the returned Content-Type is text/html and not application/json.
http_client client(L"https://www.fourthcoffee.com");
return client.request(methods::GET).then([](http_response response) -> pplx::task<json::value>
{
if(response.status_code() == status_codes::OK)
{
return response.extract_json();
}
// Handle error cases, for now return empty json value...
return pplx::task_from_result(json::value());
})
.then([](pplx::task<json::value> previousTask)
{
try
{
const json::value& v = previousTask.get();
// Perform actions here to process the JSON value...
}
catch (const http_exception& e)
{
// Print error.
wostringstream ss;
ss << e.what() << endl;
wcout << ss.str();
}
});
/* Output:
Content-Type must be application/json to extract (is: text/html)
*/
}
// Demonstrates how to iterate over a JSON object.
void IterateJSONValue()
{
// Create a JSON object.
json::value obj;
obj[L"key1"] = json::value::boolean(false);
obj[L"key2"] = json::value::number(44);
obj[L"key3"] = json::value::number(43.6);
obj[L"key4"] = json::value::string(U("str"));
// Loop over each element in the object.
for(auto iter = obj.cbegin(); iter != obj.cend(); ++iter)
{
// Make sure to get the value as const reference otherwise you will end up copying
// the whole JSON value recursively which can be expensive if it is a nested object.
const json::value &str = iter->first;
const json::value &v = iter->second;
// Perform actions here to process each string and value in the JSON object...
std::wcout << L"String: " << str.as_string() << L", Value: " << v.to_string() << endl;
}
/* Output:
String: key1, Value: false
String: key2, Value: 44
String: key3, Value: 43.6
String: key4, Value: str
*/
}
int wmain()
{
// This example uses the task::wait method to ensure that async operations complete before the app exits.
// In most apps, you typically don�t wait for async operations to complete.
wcout << L"Calling RequestJSONValueAsync..." << endl;
RequestJSONValueAsync().wait();
wcout << L"Calling IterateJSONValue..." << endl;
IterateJSONValue();
}