c++ - 使用 future 时获取 C2280(尝试引用已删除的函数)

Question

我正在使用 poco 库，我正在尝试将它们包装到一个更好的 HTTPClient 中，我可以在几行中随处使用，并使它们异步。为此，我使用 std::future 和自定义响应结构。但是，出于某种原因，它告诉我它正在尝试引用已删除的函数。我没有删除任何东西，所以我真的不知道为什么会这样说。

http客户端.h

#include <Poco/Net/HTTPSClientSession.h>
#include <Poco/Net/HTTPRequest.h>
#include <Poco/Net/HTTPResponse.h>
#include <Poco/Exception.h>
#include <Poco/URI.h>
#include <future>
#include <map>

typedef struct response {
    std::istream& is;
    Poco::Net::HTTPResponse& response;
} RES;

class HttpClient {
public:
    static std::future<RES> get(Poco::Net::HTTPSClientSession& session, Poco::URI url, std::map<std::string, std::string> headers = {});
};

http客户端.cpp

#include "httpclient.h"

std::future<RES> HttpClient::get(Poco::Net::HTTPSClientSession& session, Poco::URI url, std::map<std::string, std::string> headers) {
    return std::async(std::launch::async, [&session, url, headers](){
        try {
            std::string path(url.getPathAndQuery());
            if (path.empty()) path = "/";

            Poco::Net::HTTPRequest request(Poco::Net::HTTPRequest::HTTP_GET, path, Poco::Net::HTTPMessage::HTTP_1_1);
            request.add("Content-Length", "0");

            if (headers.size() > 0) {
                for (std::map<std::string, std::string>::const_iterator itr = headers.begin(); itr != headers.end(); ++itr) {
                    request.add(itr->first, itr->second);
                }
            }

            Poco::Net::HTTPResponse _res;
            session.sendRequest(request);
            std::istream& is = session.receiveResponse(_res);
            return RES { is, _res };
        }
        catch (Poco::Exception & exc) {
            OutputDebugStringA(exc.displayText().c_str());
        }
    });
}

主要.cpp

Poco::Net::initializeSSL();

    Poco::URI uri("https://www.google.com");
    const Poco::Net::Context::Ptr context = new Poco::Net::Context(Poco::Net::Context::CLIENT_USE, "", "", "", Poco::Net::Context::VERIFY_NONE, 9, false, "ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH");
    Poco::Net::HTTPSClientSession session(uri.getHost(), uri.getPort(), context);

    std::future<RES> response = HttpClient::get(session, uri, {});
    response.get();

这是我得到的精确错误:C2280: response &response::operator =(const response &)': attempted to reference a deleted function future:line 332。

谢谢!

Answer 1

该错误告诉您无法复制 response 对象，而您正试图这样做。

struct response { 
    std::istream& is; // can't be copied: istream( const istream&) = delete;
    Poco::Net::HTTPResponse& response; // also not copyable or movable
};

但是，您显示的代码中没有任何内容尝试这样做。

receiveResponse() 返回对 std::istream 的引用，如果抛出该引用就会出现问题。当您捕捉到异常时，您没有任何可返回的东西，因此您不会 - 并进入未定义行为的领域。

您不妨读取 async lambda 中的数据并将其直接存储在您的 RES 中。

#include <Poco/Exception.h>
#include <Poco/Net/HTTPRequest.h>
#include <Poco/Net/HTTPResponse.h>
#include <Poco/Net/HTTPSClientSession.h>
#include <Poco/Net/SecureStreamSocket.h>
#include <Poco/URI.h>

#include <future>
#include <iostream>
#include <map>
#include <vector>
#include <memory>

// a slightly modified version of your RES struct
struct RES {
    std::vector<std::string> data{}; // the document data

    // using a unique_ptr to make HTTPResponse easier to move
    std::unique_ptr<Poco::Net::HTTPResponse> 
        response = std::make_unique<Poco::Net::HTTPResponse>();

    bool ok = false;                 // if reading was done without an exception
};

class HttpClient {
public:
    static std::future<RES> get(Poco::Net::HTTPSClientSession& session,
                                Poco::URI url,
                                std::map<std::string, std::string> headers = {});
};

std::future<RES> HttpClient::get(Poco::Net::HTTPSClientSession& session,
                                 Poco::URI url,
                                 std::map<std::string, std::string> headers) {
    return std::async(std::launch::async, [&session, url, headers]() {

        RES res;

        try {
            Poco::Net::HTTPRequest request(Poco::Net::HTTPRequest::HTTP_GET,
                                           url.getPathAndQuery(),
                                           Poco::Net::HTTPMessage::HTTP_1_1);

            // add request headers
            for(const auto&[field, value]:  headers)
                request.add(field, value);

            session.sendRequest(request);
            std::istream& is = session.receiveResponse(*res.response);

            // read document data
            std::string line;
            while(std::getline(is, line))
                res.data.push_back(line);

            res.ok = true; // reading was done without an exception
        } catch(Poco::Exception& exc) {
            std::cout << exc.displayText().c_str() << "\n";
        }

        // always return according to what you declared
        return res;
    });
}

示例用法:

class SSLInitializer {
public:
    SSLInitializer() { Poco::Net::initializeSSL(); }
    ~SSLInitializer() { Poco::Net::uninitializeSSL(); }
};

int main() {
    SSLInitializer sslInitializer;

    Poco::URI uri{"https://www.google.com/"};

    const Poco::Net::Context::Ptr context = new Poco::Net::Context(
        Poco::Net::Context::CLIENT_USE, "", "", "", Poco::Net::Context::VERIFY_NONE, 9,
        false, "ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH");

    Poco::Net::HTTPSClientSession sess(uri.getHost(), uri.getPort(), context);

    std::future<RES> fut_res = HttpClient::get(sess, uri);

    fut_res.wait();

    RES res = fut_res.get();
    std::cout << std::boolalpha << "Response OK: " << res.ok << "\n---\n";
    if(res.ok) {
        Poco::Net::HTTPResponse& header = *res.response

        std::cout << "HTTPResponse header:\n";
        for(const auto& [field, value] : header) {
            std::cout << field << " = " << value << "\n";
        }

        std::cout << "--- DOCUMENT DATA ---\n";
        for(const auto& s : res.data) {
            std::cout << s << "\n";
        }
    }
}