Question

I saw the following example of enable_if for C++11:

struct is_64_bit
{
    static const bool value = sizeof(void*) == 8;
};

enable_if<is_64_bit::value, void>::type
my_memcpy(void* target, const void* source, size_t n)
{
    cout << "64 bit memcpy" << endl;
}

enable_if<!is_64_bit::value, void>::type
my_memcpy(void* target, const void* source, size_t n)
{
    cout << "32 bit memcpy" << endl;
}

As I understand, depending on the system architecture, the “my_memcpy” function will be available either for 32 or 64 bit versions. But I’m getting the following error at compilation :

error: ‘type’ in ‘struct std::enable_if<false, void>’ does not name a type

I’m a bit confused because I thought only the 32 version should be available (I’m using Linux Fedora 32 bits).

Maybe is something wrong with this example? or Am I missing something?

Thanks.

Answer 1

The template template< bool B, class T = void > struct enable_if is specialized so that it only has a typedef type when the condition B is true.

Your compiler is correct. There is no typedef for type in struct std::enable_if<false, void>. There is only a typedef in struct std::enable_if<true, void>.

For more info look here.

So to fix your problem you need to make sure that the enable_if which has a B that evaluates to false never gets compiled. You can achieve this with the help of SFINAE by making my_memcpy a function template. The compiler will then not report an error when failing to compile the function template where B evaluates to false and will successfully compile and use the function where B evaluates to true.

#include <iostream>
#include <type_traits>

using namespace std;


struct is_64_bit
{
   static const bool value = sizeof(void*) == 8;
};

template<typename T>
typename enable_if<is_64_bit::value, T>::type
my_memcpy(T* target, const T* source, size_t n)
{
  cout << "64 bit memcpy" << endl;
}

template<typename T>
typename enable_if<!is_64_bit::value, T>::type
my_memcpy(T* target, const T* source, size_t n)
{
  cout << "32 bit memcpy" << endl;
}