写一个使用WSASend()和WSARecv()函数进行异步操作的列子

好的，下面是一个使用WSASend()和WSARecv()函数进行异步操作的示例：

#include <winsock2.h>
#include <ws2tcpip.h>
#include <iostream>

#pragma comment(lib, "ws2_32.lib")

#define DEFAULT_PORT "27015"
#define DEFAULT_BUFLEN 512

int main() {
    WSADATA wsaData;
    int result = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (result != 0) {
        std::cout << "WSAStartup failed: " << result << std::endl;
        return 1;
    }

    struct addrinfo* addrResult = nullptr, hints;
    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    result = getaddrinfo("localhost", DEFAULT_PORT, &hints, &addrResult);
    if (result != 0) {
        std::cout << "getaddrinfo failed: " << result << std::endl;
        WSACleanup();
        return 1;
    }

    SOCKET connectSocket = INVALID_SOCKET;
    for (struct addrinfo* ptr = addrResult; ptr != nullptr; ptr = ptr->ai_next) {
        connectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (connectSocket == INVALID_SOCKET) {
            std::cout << "socket failed: " << WSAGetLastError() << std::endl;
            WSACleanup();
            return 1;
        }

        result = connect(connectSocket, ptr->ai_addr, static_cast<int>(ptr->ai_addrlen));
        if (result == SOCKET_ERROR) {
            closesocket(connectSocket);
            connectSocket = INVALID_SOCKET;
            continue;
        }

        break;
    }

    freeaddrinfo(addrResult);

    if (connectSocket == INVALID_SOCKET) {
        std::cout << "Unable to connect to server!" << std::endl;
        WSACleanup();
        return 1;
    }

    char sendBuffer[DEFAULT_BUFLEN] = "Hello, server!";
    char recvBuffer[DEFAULT_BUFLEN];
    DWORD sendBytes = 0, recvBytes = 0, flags = 0;
    WSABUF sendWsaBuf, recvWsaBuf;
    sendWsaBuf.buf = sendBuffer;
    sendWsaBuf.len = static_cast<ULONG>(strlen(sendBuffer));
    recvWsaBuf.buf = recvBuffer;
    recvWsaBuf.len = DEFAULT_BUFLEN;

    result = WSASend(connectSocket, &sendWsaBuf, 1, &sendBytes, flags, nullptr, nullptr);
    if (result == SOCKET_ERROR) {
        std::cout << "WSASend failed: " << WSAGetLastError() << std::endl;
        closesocket(connectSocket);
        WSACleanup();
        return 1;
    }

    result = WSARecv(connectSocket, &recvWsaBuf, 1, &recvBytes, &flags, nullptr, nullptr);
    if (result == SOCKET_ERROR) {
        std::cout << "WSARecv failed: " << WSAGetLastError() << std::endl;
        closesocket(connectSocket);
        WSACleanup();
        return 1;
    }

    std::cout << "Bytes sent: " << sendBytes << std::endl;
    std::cout << "Bytes received: " << recvBytes << std::endl;
    std::cout << "Server says: " << recvBuffer << std::endl;

    closesocket(connectSocket);
    WSACleanup();

    return 0;
}

该示例连接到本地主机的默认端口（27015），并向服务器发送“Hello, server!”的消息。发送和接收操作都是异步的，因此在发送和接收完成之前，程序将不会阻塞。当发送和接收完成时，将输出发送和接收的字节数，并输出服务器发送的响应消息。