Determine if Type is a pointer in a template function

Question

If I have a template function, for example like this:

template<typename T>
void func(const std::vector<T>& v)

Is there any way I can determine within the function whether T is a pointer, or would I have to use another template function for this, ie:

template<typename T>
void func(const std::vector<T*>& v)

Thanks

If, like me, you want to use a function that may be different depending on whether `T` is a pointer or not, you may find this answer very useful: http://stackoverflow.com/questions/14466620/c-template-specialization-calling-methods-on-types-that-could-be-pointers-or/14466705 — davidhood2, Feb 17 '17 at 17:02

score 99 · Answer 1 · answered Nov 19 '08 at 08:54

99

Indeed, templates can do that, with partial template specialization:

template<typename T>
struct is_pointer { static const bool value = false; };

template<typename T>
struct is_pointer<T*> { static const bool value = true; };

template<typename T>
void func(const std::vector<T>& v) {
    std::cout << "is it a pointer? " << is_pointer<T>::value << std::endl;
}

If in the function you do things only valid to pointers, you better use the method of a separate function though, since the compiler type-checks the function as a whole.

You should, however, use boost for this, it includes that too: http://www.boost.org/doc/libs/1_37_0/libs/type_traits/doc/html/boost_typetraits/reference/is_pointer.html

answered Nov 19 '08 at 08:54

Johannes Schaub - litb

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What a lonely little answer you've got here. :) – GManNickG Jan 29 '11 at 23:42
@GMan haha :) How did you hit this old answer of me xD – Johannes Schaub - litb Jan 30 '11 at 00:17
@Johannes: Meh, I remembered you were abnormally high on the `templates` tag top rep. list, so I wanted to see why. :) – GManNickG Jan 30 '11 at 05:01
+1 Some answers never get old, Johannes. This is one such answer. – WhozCraig Aug 15 '13 at 06:54
2

@WhozCraig Actually this answer did get old, because for C++ 11 this other answer is better http://stackoverflow.com/a/15974269/55721 – dss539 Mar 19 '15 at 15:55

score 68 · Answer 2 · edited May 18 '18 at 03:59

C++ 11 has a nice little pointer check function built in: std::is_pointer<T>::value

This returns a boolean bool value.

From http://en.cppreference.com/w/cpp/types/is_pointer

#include <iostream>
#include <type_traits>

class A {};

int main() 
{
    std::cout << std::boolalpha;
    std::cout << std::is_pointer<A>::value << '\n';
    std::cout << std::is_pointer<A*>::value << '\n';
    std::cout << std::is_pointer<float>::value << '\n';
    std::cout << std::is_pointer<int>::value << '\n';
    std::cout << std::is_pointer<int*>::value << '\n';
    std::cout << std::is_pointer<int**>::value << '\n';
}

score 2 · Answer 3 · answered Jan 26 '23 at 13:33

From C++17, there is an even shorter built-in construct, std::is_pointer_v<T>:

#include <type_traits>
 
int main()
{
    class A {};
 
    static_assert(
           not std::is_pointer<A>::value
        && not std::is_pointer_v<A>); // equivalent to above, but shorter

    return 0;
}

Determine if Type is a pointer in a template function

3 Answers3

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