Question

Does C++0x have (or was C++0x at some point in time going to have) template argument deduction for constructors? In An Overview of the Coming C++ (C++0x) Standard, I saw the following lines:

std::lock_guard l(m);   // at 7:00

std::thread t(f);       // at 9:00

Does this mean that delegating make_foo function templates are finally redundant?

Answer 1

Template argument deduction works for any function, including the constructor. But you can’t deduce the class template parameters from arguments passed to the constructor. And no, you can’t do it in
C++0x either.

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

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

int main()
{
   X x(3); //T is deduced to be int. OK in C++03 and C++0x; 
   Y y(3); //compiler error: missing template argument list. Error in 03 and 0x
}

lock_guard and thread aren’t class templates. They have constructor templates though.