检查一类具有指针数据成员 [英] Check if a class has a pointer data member
问题描述
有没有一种方法来测试,如果一个类有一个指针数据成员?
Is there a way to test if a class has a pointer data member?
class Test
{
int* p;
}
template< typename T >
foo( T bla )
{
}
这不应该编译。因为测试有一个指针数据成员。
This should not compile. because Test has a pointer data member.
Test test;
foo( test )
也许我可以用一个特点来禁用模板?或者是我唯一的选择宏?也许有人知道,如果提升能做到吗?
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
以下可以作为保护工作,但成员变量必须是可访问的(公共
),否则它不会工作:
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 );
}
当然,你需要知道的成员变量来检查的名称,因为没有内置到C ++没有一般的反射机制。
Of course you need to know the name of the member variable to check, as there is no general reflection mechanism built into C++.
这避免了歧义的另一种方法是创建一个 has_pointer
助手:
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" );
// ...
}
请注意,我只是增加了一个 static_assert
作为第一线的功能得到一个不错的,可读的错误信息。你可以,当然,也禁用该功能本身是这样的:
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 )
{
// ...
}
这篇关于检查一类具有指针数据成员的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!