编译器编译“io_service_”变量显示为：不能出现在常量表达式中

Question

我创建了一个包含类名 server_datetime 的 ServerService 命名空间。 Server_datetime 类作为 Boost 示例中的教程，但我通过使用模板参数将 io_service ( boost::asio::io_service ) 和端点 ( tcp::endpoint(tcp::v4(),SIZE_DATA) ) 对象插入模板改进了 server_datetime 类.我按照下面的例子：

using boost::asio::ip::tcp;
namespace ServerService{
template<typename Service, typename Endpoint>
class server_datetime {
public:
    server_datetime(){
        acceptor_(service_, endpoint_);
        for(;;)
        {
            tcp::socket socket(Service);
            acceptor_.accept(socket);
            std::string message = make_daytime_string;

            boost::system::error_code ignored_error;
            boost::asio::write(socket, boost::asio::buffer(message),boost::asio::transfer_all(), ignored_error);
        }
    }
    std::string make_daytime_string(){
        std::time_t now = std::time(0);
        return std::ctime(&now);
    }
    virtual ~server_datetime();
private:
    tcp::acceptor acceptor_;
    Service service_;
    Endpoint endpoint_;
};
}

调用server_datetime类的主函数如下：

#include "server_datetime.hpp"
using namespace std;
using boost::asio::ip::tcp;
int main() {
    const boost::asio::io_service io_service_;
    const int SIZE_DATA = 13;
    ServerService::server_datetime<io_service_, tcp::endpoint(tcp::v4(),SIZE_DATA)  >  server;
    cout << "" << endl; // prints 
    return 0;
}

编译器编译main函数后，编译器显示错误为：

..\src\connectk.cpp: In function 'int main()':
..\src\connectk.cpp:10: error: 'io_service_' cannot appear in a constant-expression
..\src\connectk.cpp:10: error: 'boost::asio::ip::tcp::v4()' cannot appear in a constant-expression
..\src\connectk.cpp:10: error: a function call cannot appear in a constant-expression
..\src\connectk.cpp:10: error: template argument 1 is invalid
..\src\connectk.cpp:10: error: template argument 2 is invalid
..\src\connectk.cpp:10: error: invalid type in declaration before ';' token

Answer 1

模板参数用于在编译时指定类型和常量值，而不是在运行时注入对象；这就是普通函数/构造函数参数的用途。因此，在这种情况下，如果您向服务器提供服务和端点，请将它们作为参数传递给构造函数。

代码中还有一些其他错误；这里有一些更正（虽然可能还有问题，我自己可能已经介绍了一些）：

namespace ServerService{

// Put 'using' declarations inside the namespace,
// to avoid polluting the global namespace
using boost::asio::ip::tcp;
using boost::asio::io_service;

// Not a template - pass runtime objects as constructor arguments
class server_datetime {
public:
    server_datetime(io_service & service, tcp::endpoint const & endpoint) :
        // Initialise members in an initialiser list
        acceptor_(service, endpoint),
        service_(service)
    {}

    // Put the main loop in a separate function; it's rather odd
    // to have a constructor that doesn't return.
    void run(){
        for(;;)
        {
            // Argument must be the object service_, not the type Service
            tcp::socket socket(service_); 
            acceptor_.accept(socket);
            std::string message = make_daytime_string(); // missing parentheses

            boost::system::error_code ignored_error;
            boost::asio::write(socket, boost::asio::buffer(message),boost::asio::transfer_all(), ignored_error);
        }
    }
    std::string make_daytime_string(){
        std::time_t now = std::time(0);
        return std::ctime(&now);
    }
    // No need for a virtual destructor - this class is not polymorphic
private:
    boost::asio::io_service & service_; // must be a reference - io_service is not copyable
    tcp::acceptor acceptor_;
    // No need to store the endpoint - it's only used to initialise acceptor_.
};
}

int main() {
    using boost::asio::ip::tcp;

    // can't be const if you want to use it
    boost::asio::io_service io_service_;

    // renamed SIZE_DATA and given it the same type as the constructor argument
    const unsigned short port = 13; 

    ServerService::server_datetime server(io_service_, tcp::endpoint(tcp::v4(),port));
    server.run();
    // no need to explicitly return zero, unless you want to.
}

Answer 2

std::string message = make_daytime_string;

你忘记了()，应该是：

 std::string message = make_daytime_string();

Answer 3

server_datetime 模板需要类型名称（它在第一个源代码的顶部这么说），但您提供了值。

也许你不应该在 main 中创建 io_service_，而是让服务器来做？