如果type是指针，则在编译时检查

Question

I"m trying to find a way to tell if a type is a pointer at compile time. That is something like this:

#include <type_traits>
#if std::is_pointer<char*>::value
#pragma message("blah")
#endif

However, this gives "warning C4067: unexpected tokens following preprocessor directive - expected a newline" twice (I think the :: is what confuses it) and it doesn't print blah. When I hoover over ::value the compiler tells me if it's true, which means it's known at compile time so this should work.

The reason for this is that I want to be able to do something like this:

T pHead;
#if std::is_pointer<T>::value
pHead= NULL;
#endif

where I NULL the variable if it's a pointer. It has to be a compile time check because if T is a struct I cannot assign NULL to its variable. Ie the following code won't compile when T is a struct:

T pHead;
if (std::is_pointer<T>::value)
    pHead= NULL;

Thanks
Matt

Answer 1

You can initialize the variable to its default value, which is NULL for a pointer type.

T pHead = T();

Works for most structs as well.

Answer 2

You can use templates for this:

template<typename A>
void foo(A a)
{

}

template<typename A>
void foo(A*& a)
{
   a = NULL;
}

Calling foo with a pointer type will enter the second function, otherwise the first one.

You can't do it with preprocessor directives, because, as the name suggests, that happens before compilation.

But template resolution happens during compilation, so you can use this solution.

I'm assuming you're already using templates since your code also provides a generic type:

T pHead;
#if std::is_pointer<T>::value
pHead= NULL;
#endif

you could use

T pHead;
foo(pHead);