关于c ++：编译器编译’io_service_’变量显示为：不能出现在常量表达式中

Compiler compile a 'io_service_' variable show as : cannot appear in a constant-expression

我为包含类名server_datetime创建了一个serverservice命名空间。Server_DateTime类是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Endpoint(TCP:：Endpoint(TCP:：v4()，Size_Data))对象插入模板来改进了Server_DateTime类。我以下面的例子为例：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

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_;
};
}

主函数按如下方式调用了server_datetime类：

1
2
3
4
5
6
7
8
9
10

#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;
}

在编译器编译主函数之后，编译器将错误显示为：

1
2
3
4
5
6
7

..\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

1	std::string message = make_daytime_string;

您忘记了()，应该是：

1	std::string message = make_daytime_string();

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，在本例中，如果您向服务器提供服务和端点，那么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；下面是一些更正(尽管可能仍然存在问题，我可能已经介绍了一些自己的错误)：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55

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.
}