Default args in template

Is it possible to have something like this:

template<class T = unsigned = 64>
class X
{
};

Which basically says that if nothing is specified then unsigned 开发者_如何学Goshould be the type and its value should be 64.

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Taken literally, it doesn't make sense. Explain what are you trying to do first.

Templates in C++ have three kind of parameters: 1) type parameters, 2) value (non-type) parameters, 3) template parameters.

When you declare a parameter as class T it is a type parameter (first kind). It's "value" is actually a type. unsigned, for example. I.e. the "value" of T is unsigned. unsigned will be substituted instead of T in this instantiation of class X template. Parameter T can't have any numerical value, like 64.

Yet, you seem to be trying to create some kind of hybrid between kind 1 and kind 2 parameters. Firstly it is impossible. Secondly, there's no way to figure out what semantic meaning it could possibly have. Please, clarify your intent.

If you want to pass a numerical value, you have to create a template parameter of the second kind: a value parameter, as in

template <unsigned N = 64>
class X {
};

So, you need to decide first, what kind of template parameter you really need: a type parameter or a value parameter. Or maybe both. You haven't provided any details about your class X, so there's no way to guess what you are trying to do.

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Not really. You could do something close though:

template < typename T = unsigned, T value = 64>
struct X{};

Edit: Dawned on me reading your other answer that there's something else you could do if you really wanted to:

template < unsigned N = 64 >
struct unsigned_
{
  typedef unsigned type;
  enum { value = N };
};

// alternative versions...int, long, etc...

template < typename T = unsigned_<64> >
struct X {};

This creates a bunch more work for the user though. You could address some of that by using boost::enable_if and is_fundamental so that the user could supply a fundamental type without you breaking down because you're looking for T::type and T::value.

So, while there is no way to do exactly what you're looking for...depending on how ugly you want to get you could get pretty darn close from the user's perspective.