Not fixing the type of a non-type template parameter

[Juha Nieminen]

Non-type template parameters can be used for example like this:

template<int N> class Test { ... };

However, this fixes that non-type parameter to be of type int. Is it in
any way possible to make the instantiating code decide what that type
actually is? In other words, rather than the template itself, it's
the insantiating code that decides whether it's an int, and unsigned,
or something else. In other words, these would be different:

Test<5> test1;
Test<5U> test2;
Test<5LL> test3;

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[Ondra Holub]

Dne pondělí 2. května 2016 9:09:05 UTC+2 Juha Nieminen napsal(a):

> Non-type template parameters can be used for example like this:
>
> template<int N> class Test { ... };
>
> However, this fixes that non-type parameter to be of type int. Is it in
> any way possible to make the instantiating code decide what that type
> actually is? In other words, rather than the template itself, it's
> the insantiating code that decides whether it's an int, and unsigned,
> or something else. In other words, these would be different:
>
> Test<5> test1;
> Test<5U> test2;
> Test<5LL> test3;
>

I do not know any nice way. You can do this:

template<typename T, T VALUE>
struct X
{
static const T Value = VALUE;
};

To make it easier, you can define wrapping macro:

#define MAKE_X(val) X<std::decay<decltype(val)>::type, (val)>

MAKE_X(20) x1;
MAKE_X('a') x2;
MAKE_X(L'a') x3;
MAKE_X(0ULL) x4;

std::cout << sizeof(x1.Value) << '\n';
std::cout << sizeof(x2.Value) << '\n';
std::cout << sizeof(x3.Value) << '\n';
std::cout << sizeof(x4.Value) << '\n';

For integral types it should work in C++11.

[Ondra Holub]

Dne pondělí 2. května 2016 9:09:05 UTC+2 Juha Nieminen napsal(a):

> Non-type template parameters can be used for example like this:
>
> template<int N> class Test { ... };
>
> However, this fixes that non-type parameter to be of type int. Is it in
> any way possible to make the instantiating code decide what that type
> actually is? In other words, rather than the template itself, it's
> the insantiating code that decides whether it's an int, and unsigned,
> or something else. In other words, these would be different:
>
> Test<5> test1;
> Test<5U> test2;
> Test<5LL> test3;
>

Of course, std::decay is not needed, just

#define MAKE_X(val) X<decltype(val), (val)>

should be enough :-)