why is_base_of fails when both are just plain char type

Question

Do you see why the static_assert fail:

template <typename T>
void 
foo(const T &c)
{

    static_assert(std::is_base_of<T, char>::value, "T must be char");  // Fails !

}

int main()
{
    char c = 'a';

    foo<char>(c);

    return 0;
}

I swapped T and "char", still fails.

maybe because `char` is not a class... – Brian Bi Jun 02 '14 at 03:07 — Brian Bi, Jun 02 '14 at 03:07

score 5 · Answer 1 · answered Jun 02 '14 at 03:13

You might want to consider adding additional checks:

template <typename T> void foo(const T &c) 
{
    static_assert(
            std::is_base_of<T, char>::value || std::is_same<T, char>::value, 
            "T must be char");

}

But if you are only concerned about chars, then you might as well do:

static_assert(std::is_same<T, char>::value, "T must be char");

Also consider is_fundamental, and is_convertible for more complex assertions.

score 4 · Accepted Answer · answered Jun 02 '14 at 03:08

4

The documentation for std::is_base_of states:

If Derived is derived from Base or if both are the same non-union class, provides the member constant value equal to true. Otherwise value is false.

But char is not a class : it is a primitive type, so this is the expected result.

answered Jun 02 '14 at 03:08

quantdev

23,517
5
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oh, ok, then how do I restrict that the type T must be char? – my_question Jun 02 '14 at 03:11
3

Why put a template in the first place ? Just declare a function that takes a char, no ? – quantdev Jun 02 '14 at 03:12
The real use case is complex, I need to make a template function which takes several types including char. – my_question Jun 02 '14 at 03:13
Do you need *several types*? Or *any* type that inherits from a particular base plus `char`? If it is *several* in general, you should control what types are specifically used. – David Rodríguez - dribeas Jun 02 '14 at 03:15
Yes, it is char with other types which are derived from same type. – my_question Jun 02 '14 at 03:29

why is_base_of fails when both are just plain char type

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