用C++写以上步骤

以下是用C++实现以上步骤的示例代码：

#include <iostream>
#include <string>
#include <curl/curl.h>
#include <jsoncpp/json/json.h>

using namespace std;

// callback function for curl
size_t callback(char* ptr, size_t size, size_t nmemb, string* data) {
    data->append(ptr, size * nmemb);
    return size * nmemb;
}

int main() {
    // set up curl
    CURL* curl = curl_easy_init();
    if (!curl) {
        cerr << "Error: failed to initialize curl" << endl;
        return 1;
    }

    // set up parameters
    string url = "https://api.openai.com/v1/engines/davinci-codex/completions";
    string prompt = "Hello,";
    string api_key = "<YOUR_API_KEY_HERE>";
    int max_tokens = 50;
    double temperature = 0.5;

    // encode parameters into json
    Json::Value json_data;
    json_data["prompt"] = prompt;
    json_data["max_tokens"] = max_tokens;
    json_data["temperature"] = temperature;

    Json::Value json_headers;
    json_headers["Content-Type"] = "application/json";
    json_headers["Authorization"] = "Bearer " + api_key;

    Json::Value json_request;
    json_request["data"] = json_data;
    json_request["headers"] = json_headers;

    string json_string = json_request.toStyledString();

    // set up curl options
    curl_easy_setopt(curl, CURLOPT_URL, url.c_str());
    curl_easy_setopt(curl, CURLOPT_POST, 1L);
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, json_string.c_str());
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, json_string.length());
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, callback);

    string response_body;
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response_body);

    // execute curl request
    CURLcode res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        cerr << "Error: curl_easy_perform() failed: " << curl_easy_strerror(res) << endl;
        curl_easy_cleanup(curl);
        return 1;
    }

    // parse json response
    Json::Value json_response;
    Json::CharReaderBuilder builder;
    Json::CharReader* reader = builder.newCharReader();
    string errors;

    if (!reader->parse(response_body.c_str(), response_body.c_str() + response_body.size(), &json_response, &errors)) {
        cerr << "Error: failed to parse json response: " << errors << endl;
        curl_easy_cleanup(curl);
        return 1;
    }

    string completions = json_response["choices"][0]["text"].asString();

    cout << completions << endl;

    // clean up
    delete reader;
    curl_easy_cleanup(curl);

    return 0;
}

这个程序使用了 `curl` 库来发送 HTTP 请求，使用了 `jsoncpp` 库来处理 JSON 数据。在运行这个程序之前需要先安装这两个库，并且将 `<YOUR_API_KEY_HERE>` 替换成你的 OpenAI API Key。