如何：使用 JSON 数据 (C++ REST SDK)

通过使用 C++ REST SDK (codename "Casablanca") web::json 命名空间，你可以更快速地分析
JSON 数据。 此页显示两个示例。 第一个示例显示如何从 HTTP GET 响应提取 JSON 数据。 第二个示例在内存中生成 JSON 值，并循环访问它的值。

警告
本主题包含有关 C++ REST SDK 1.0 (codename "Casablanca") 的信息。 如果你正在从 Codeplex Casablanca 网页 使用更高版本，则使用 http://casablanca.codeplex.com/documentation 上的本地文档。

在这些示例之后，有一个更完整的示例显示 #include 和 using 语句。

从
HTTP GET 响应提取 JSON 数据

以下是使用 web::http::client::http_response::extract_json 方法从 HTTP GET 响应提取 JSON 数据的方法。 有关检索服务器响应但不使用
JSON 的更基础的版本,请参阅如何：连接到 HTTP 服务器。

C++

// 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"http://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)
*/
}

在内存中生成
JSON 值，并循环访问它的值

以下是使用 web::json::value 类在内存中生成 JSON 值，并循环访问它的值的方法。 web::json::value::cbegin 和web::json::value::cend 方法返回值集合的只读迭代器。

C++

// 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
*/
}

完整示例

以下是完整示例。

C++

#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"http://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();
}