c++ - 编译器编译一个 'io_service_' 变量显示为 : cannot appear in a constant-expression

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) ) 对象插入模板来改进 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类的主要函数如下:

#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，编译器报错如下:

..\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

std::string message = make_daytime_string;

你忘记了 ()，应该是:

 std::string message = make_daytime_string();