用到的库:
假设现在 Client 想要获取 Person 喜欢的语言,需要发送 Person 的个人信息到 Server,Server 根据用户姓名返回他喜欢的语言。
struct Book {
std::string name;
};
struct Person {
std::string name;
int age;
std::vector<Book> favoriteBooks;
};
struct FavoriteLanguage {
std::string name;
std::vector<std::string> languages;
};
这个过程需要三个处理步骤:
- Client 将 Person 信息转为 json,并通过 curl 发送出去
- Server 收到数据后解 json 处理并返回该 Person 喜欢的语言
- Client 收到 Response 后解 json 得到喜欢的语言
Book 转 json:
void to_json(json &j, const Book &book) {
j = {
{"name", book.name}
};
}
Person 转 json:
void to_json(json &j, const Person &p) {
j = {
{"name", p.name},
{"age", p.age}
};
if (!p.favoriteBooks.empty())
j.push_back({"books", p.favoriteBooks});
}
Person p{"Aland", 18, books};
json j = p; // Person 转 json
std::string dump = j.dump();
通过 curl 发送 http POST 请求,数据为 json 格式。
std::string url = "127.0.0.1:9999/favorbooks";
CURL *curl;
curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
curl_easy_setopt(curl, CURLOPT_POST, 1L); // POST
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, dump.c_str()); // 要发送的数据
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback); // 处理 response 的回调函数
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &favor); // 存储返回的结果
curl_easy_perform(curl);
curl_easy_cleanup(curl);
Client 解析返回的 response:
static size_t write_callback(void *ptr, size_t size, size_t nmemb, void *userp) {
size_t realsize = size * nmemb;
auto j = json::parse(std::string((char *) ptr));
FavoriteLanguage *favor = (FavoriteLanguage *) userp;
*favor = j; // json 转 FavoriteLanguage
return realsize;
}
json 转结构体:
void from_json(const json &j, FavoriteLanguage &favor) {
favor.name = j["name"];
favor.languages = j["languages"].get<std::vector<std::string>>();
}
解析后的打印结果:
{"age":18,"books":[{"name":"TCP/IP Illustrated, Volume 1"},{"name":"Advanced Programming in the UNIX Environment"}],"name":"Aland"}
{"languages":["C++","Golang","Python"],"name":"Aland"}
Name:Aland
Languages:
C++
Golang
Python
注: 详细代码见 https://github.com/alandtsang/cppdemo/tree/master/src/curljson
遇到的错误:
could not find to_json() method in T's namespace
这个问题通常是两点引起的(下列解法对to_json
和 from_json
都适用):
- 没有对要转换的结构体做 to_json 转换。
- 外层结构体做了 to_json 转换,但是结构体包含的结构体没有做 to_json 转换。
参考
C libcurl get output into a string
creating nested json object in c++ using nlohmann json
Unable to locate ‘to_json()’ and ‘from_json()’ methods in the same namespace
Why not also look inside the type for (static) to_json and from_json funtions?