std :: function的typename关键字

Question

I have a class

class Consumer
{
public:
    typedef std::function<void()> EventHandler;
    ...
};

which I would like to use in this as template

template<class Consumer>
class ConsumerGroup
{
public:
    typename Consumer::EventHandler EventHandler;
    ConsumerGroup(EventHandler handler);
};

But above one results compilation error saying EventHandler is not a type. How should I use typename keyword in this case?

Answer 1

You need a type alias for dependent Consumer::EventHandler .

This should work:

using EventHandler =  typename Consumer::EventHandler;

for lower compiler versions(before C++11)

typedef  typename Consumer::EventHandler EventHandler;

Answer 2

The typename keyword isn't used to import or define new type-aliases. What you're doing is really defining a member variable named EventHandler .

You need to use typdef again to define a type-alias:

typedef typename Consumer::EventHandler EventHandler;

Or with modern C++ using using :

using EventHandler = typename Consumer::EventHandler;