Understanding C++ Template Syntax

c++templates

How do I fix this syntax error?

struct A {
  template < typename T >
  void f () {}
};

template < typename C, typename U >
struct B {
  void g () {
    U::f < C > ();   // expected primary-expression before »>« token
  }
};

int main () {
  B<int,A> b;
  b.g ();
}

Best Answer

U is a dependent type so you need to specify that f is a template member:

U::template f<C>();

This is still invalid when U is A, though, as f is not a static member of A.

Related Solutions

C++ – Syntax of Template Template Parameters

I am not sure what is your question exactly, but here is the explanation for the two examples you gave.

template <typename T, template <typename> class Cont>
class Stack;

Stack is a class template with two template parameters. The first parameter, T can be any type (including built-in types, user-defined types, template instantiations and so on). The second parameter, Cont, must be a class template taking one parameter. The parameter is unnamed because it would not make much sense (the parameter is never bound to anything).

template <template <typename,typename> class Cont>
class Wrapper3;

Wrapper3 is a class template with a single parameter, Cont. Cont must be a class template with two parameters.

The syntax to define a template template parameter is the same as the one to define a class template (template <typename [param1], typename [param2], ...> class Name), so I don't really understand what is your problem.

However, I agree that the syntax can become a bit awkward when you start "nesting" template template parameters:

// class template whose parameter must be a class template whose parameter
// must be a class template
template <template <template <typename> class > class C >
struct Wow {};

Doesn't happen that often, though...

Best Answer

Related Solutions

C++ – Syntax of Template Template Parameters

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