C++ invoke explicit template constructor

Question

hello.

Can you tell me how to invoke template constructor explicitly (in initializer list)? for example:

struct T { 
    template<class> T();
};

struct U {
    U() : t<void>() {} //does not work
    T t;
};

thanks

Answer 1

It's not possible. The Standard also has a note on this at 14.8.1/5

[Note: because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates. ]

If you can live with it, you can work it around

struct T { 
    template<class U> T(identity<U>);
};

struct U {
    U() : t(identity<void>()) {} //does not work
    T t;
};

Given identity like it's defined in boost

template<typename T> struct identity { typedef T type; };