Question

I want get the type of lambda as the template argument. How to do this?

template<class T>
class Foo
{
public:
    Foo(T t){ t(); }
};

int main()
{   
    Foo< type?? > foo([](){ cout<<"construct OK"; });

    system("pause");
    return 0;
}

Answer 1

It’s possible to deduce the type of a lambda-expression when it is the argument to a function template function parameter whose type is deduced.

Lambda expressions however are explicitly forbidden to appear inside an unevaluated operand — this includes decltype. It was found that this includes several yet unhandled special cases and it was found that there is no real utility to allow lambda expressions inside decltype and friends. Recall that every lambda expression creates a new unique type.

You can instead use std::function<void()>, which is able to store any function object that can be called without arguments and yields void as return type.

Foo< std::function<void()> > foo([](){ cout<<"construct OK"; });

Of course, as I said above, function templates can deduce their argument type, so you can also make the constructor a template

class Foo
{
public:
    template<typename T>
    Foo(T t){ t(); }
};

Foo foo([](){ cout<<"construct OK"; });

But I take it that this is not really useful in your case, since you presumably want to do something with T inside your class definition. Like, storing the object as a non-static data member. With std::function, that’s possible.