Class 模板参数默认值

Question

There is a class template definition for time_point template class from standard C++ library:

template<
    class Clock,
    class Duration = typename Clock::duration
> class time_point;

Can someone please explain me the line:

class Duration = typename Clock::duration

? I know that we are defining a default value for the second template parameter Duration but the thing i don't understand is the ::duration part after Clock type name. Should't it be just:

class Duration = typename Clock

?

Answer 1

This would use the type of the Clock :

class Duration = typename Clock

However, the Duration is a type different from Clock . There is a named requirement Clock . Among other things it specifies that a Clock should have a duration member alias.

A member alias is something like this:

 struct my_clock {
      using duration = int;
 };

So when you instantiate a time_point<my_clock> , the second template parameter becomes my_clock::duration . The keyword typename is needed because it is a dependent name. You need to assure the compiler that Clock::duration really is a type.

Answer 2

Marek has already explain almost everything in his answer, this one is to improve it.

typename is need to tell compiler "look, duration is nested in Clock and depend on it, you have to resolve Clock first and then deduce Clock::duration "

here there's a stupid example of a dependent type, is not meant to be used in production code.

template<typename T>
class MyBase
{
public:
   using UnderlyingType = T;
   // ...
   double foo() { return 5.5; }
};

class MyIntChild : public MyBase<int>{};

class MyDoubleChild : public MyBase<double>{};

template<typename T, typename D = typename T::UnderlyingType>
D makeFoo(T& t)
{
   return t.foo();
}

MyIntChild a;
makeFoo<MyIntChild>(); // return 5 because return type is int;

Answer 3

This is called dependent type.

Note class can contain typedef ( using = ) inside. Clock::duration is just referring to this type definition which is expected inside type passed as first argument.

Now typename is needed to explain to compiler that depending symbol must be a type (it could be constant).

Take a look on some clock documentation it has typedef for duration:

std::chrono::system_clock - cppreference.com

Member types

Member type	Definition
rep	signed arithmetic type representing the number of ticks in the clock's duration
period	a std::ratio type representing the tick period of the clock, in seconds
duration	std::chrono::duration <rep, period>, capable of representing negative durations
time_point	std::chrono::time_point <std::chrono::system_clock>