检查类是否有指针数据成员 [英] Check if a class has a pointer data member

问题描述

有没有方法来测试类是否有指针数据成员？

Is there a way to test if a class has a pointer data member?

class Test
{
  int* p;
}

template< typename T >
foo( T bla )
{
}

不编译。因为Test有一个指针数据成员。

This should not compile. because Test has a pointer data member.

Test test;
foo( test )

也许我可以使用trait来禁用模板？或者是我唯一的选项宏？也许有人知道boost是否可以做到？

Maybe I can use a trait to disable the template? Or is my only option macros? Maybe someone knows if boost can do it?

推荐答案

以下可以作为保护，但成员变量必须可访问（ public ），否则将无法工作：

The following can work as a protection, but the member variable has to be accessible (public), otherwise it won't work:

#include <type_traits>

class Test
{
public:
  int* p;
};

template< typename T >
typename std::enable_if< std::is_pointer< decltype( T::p ) >::value >::type
foo( T bla ) { static_assert( sizeof( T ) == 0, "T::p is a pointer" ); }

template< typename T >
void foo( T bla )
{
}

int main()
{
    Test test;
    foo( test );
}

活动示例

当然，您需要知道会员的姓名

Of course you need to know the name of the member variable to check, as there is no general reflection mechanism built into C++.

另一种避免歧义的方法是创建a has_pointer 帮助器：

Another way which avoid the ambiguity is to create a has_pointer helper:

template< typename, typename = void >
struct has_pointer : std::false_type {};

template< typename T >
struct has_pointer< T, typename std::enable_if<
                         std::is_pointer< decltype( T::p ) >::value
                       >::type > : std::true_type {};

template< typename T >
void foo( T bla )
{
    static_assert( !has_pointer< T >::value, "T::p is a pointer" );
    // ...
}

生活示例

请注意我只是添加了一个 static_assert 作为第一行函数，以获得一个漂亮，可读的错误消息。当然，你也可以用这样的函数禁用函数本身：

Note that I simply added a static_assert as the first line to the function to get a nice, readable error message. You could, of course, also disable the function itself with something like this:

template< typename T >
typename std::enable_if< !has_pointer< T >::value >::type
foo( T bla )
{
    // ...
}

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