typedefing non-type template parameter

Question

I would like to access a template parameter outside of a class. I usually do this as follows:

template <class T>
class A
{
  typedef typename T T;
}

A<int>::T;

I would like to be able to do the same for non-type template parameters. This doesn't work:

template <int T>
class A
{
  typedef typename T T;
}

A<3>::T;

I will clarify why I need this. I want to define a second class as follows:

template <class C>
class B
{
  static int func() {return C::T;}
}

B<A<3> >::func();

What is the correct way to do this? Thank you very much.

`class` is interchangeable with `typename`, `int` is not. Also, what are you trying to achieve? — antonijn, Feb 10 '13 at 15:33
Actually, you cannot use `typename` in either code since `T` isn’t a dependent name, and you cannot redefine `T` either. — Konrad Rudolph, Feb 10 '13 at 15:36
`typename` before unqualified names, such as the dependent unqualified name `T`, is not possible. — Johannes Schaub - litb, Feb 10 '13 at 16:38

score 5 · Accepted Answer · answered Feb 10 '13 at 15:34

That's because T is not a type name and you cannot typedef it. It is an int value and, if you want to access it as a static member of the class, you need a static member int. Seems like what you really want is this:

template <int T>
class A
{
  public:
    static const int x = T;
};

doSomething(A<5>::x);

score 2 · Answer 2 · answered Feb 10 '13 at 15:37

2

It's a value, and not a type, so perhaps:

template <int T>
class A
{
  static const int param = T;
};

And then you can access it as A<42>::param. Not that it helps much, unless A is itself used as a template parameter somewhere else.

answered Feb 10 '13 at 15:37

Bo Persson

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score 1 · Answer 3 · edited May 23 '17 at 12:15

in the second case, T is not a type, it's an int value. Therefore you should define it as a ~~const int or~~ static const int value.

template <int T>
class A {
    static const int T = T;
};

Note that it is customary to use T for types (in particular when the template is monadic, since there is no ambiguity on the type), and other names for constants, usually a more meaningful name, for instance SIZE or preferably Size (all caps symbols are best used for macros).

template <int Param>
class A {
    static const int param = Param;
};

See other SO questions (like this one) for the use of static const values in the context of a template definition.

typedefing non-type template parameter

3 Answers3