Problem with writing full specialization for identifying void type

Question

I am trying to specialize is_void struct. Here is my code that I am trying to compile:

#include <iostream>
using namespace std;

template<typename T>
struct is_void
{
    static constexpr bool value = false;
};

template<>
struct is_void<void>
{
    static constexpr bool value = true;
};

int main()
{
    printf("%d\n", is_void<int>::value);
    printf("%d\n", is_void<void>::value);
}

But it fails with the compile error:

error: explicit specialization of non-template struct 'is_void'
error: redefinition of 'is_void' as different kind of symbol

Right now I do not see any problems with this code. Appreciate any help.

cannot reproduce this:[godbolt](https://godbolt.org/z/0wjmJy), which compiler are you using? — Mike van Dyke, Aug 02 '19 at 08:38
What version of clang? Because I tried some and they worked as well, maybe its because of Mac — Mike van Dyke, Aug 02 '19 at 08:43
Yes, the problem was in using namespace std; but why is that? — Oleg, Aug 02 '19 at 08:51
Because there is [`std::is_void`](https://en.cppreference.com/w/cpp/types/is_void) which is defined differently. — rustyx, Aug 02 '19 at 08:55
Please avoid `using namespace std;`. It is considered bad practice and will ruin your life (as shown in this example). See [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/q/1452721) — L. F., Aug 02 '19 at 09:30

Vlad from Moscow · Accepted Answer · 2019-08-02T09:27:19.617

Just remove the directive

using namespace std;

Otherwise there can be a conflict with the standard class std::is_void declared in the header <type_traits>.

The using directive in general is a source of problems with name resolutions.

Another approach is to use a qualified name as for example

::is_void<int>::value

But even a better approach is to place your declarations in a user namespace as for example

namespace usr
{
template<typename T>
struct is_void
{
    static constexpr bool value = false;
};
}

Here is a demonstrative program

#include <iostream>
#include <cstdio>

namespace usr
{

template<typename T>
struct is_void
{
    static constexpr bool value = false;
};

}

template<>
struct usr::is_void<void>
{
    static constexpr bool value = true;
};

int main()
{
    printf("%d\n", usr::is_void<int>::value);
    printf("%d\n", usr::is_void<void>::value);
}

Also if you are using the C function printf then you should include header <cstdio>.

worth to mention [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) — francesco, Aug 02 '19 at 09:21

Problem with writing full specialization for identifying void type

1 Answers1