Explicit instantiation of class template not instantiating constructor

Question

I'm working on a project in C++ and am having trouble understanding what members of a template class get explicitly instantiated when I explicitly instantiate the template class. I've written the following file, which I then compile using Visual C++ 2008 Express Edition's Release configuration and then pop into a disassembler.

template<typename T> class test {
public:
    template<typename T> test(T param) {
        parameter = param;
    };
    ~test() {};
    int pop();
    int push();
    T parameter;
};

template<typename T> int test<T>::push() {return 1;}
template<typename T> int test<T>::pop() {return 2;}

template class test<int>;

int main() {
    return 0;
}

Ignoring that this file doesn't really need templates for the moment, this compiles fine. I throw the exe into the disassembler and it tells me that test<int>::pop(void), test<int>::push(void), and test<int>::~test<int>(void) are functions in the exe, but I don't see the constructor. I know I can explicitly instantiate the constructor with

template test<int>::test(int);

which causes test<int>::test<int>(int) to appear in the disassembly as a function with the other ones. My understanding of explicit instantiation is that it is supposed to tell the compiler to instantiate all members of a template class for a given set of arguments, so why is it that the constructor is not explicitly instantiated along with all the other member functions?

Your class template is invalid because the constructor's template parameter shadows the class' template parameter. But it's not surprising that the VC++ compiler accepts it silently. — Praetorian
How can it instantiate the constructor if its template parameters aren't known? — 0x499602D2

R Sahu R Sahu · Accepted Answer · 2015-06-22T18:23:36

When the constructor is a template member function, they are not instantiated unless explicitly used.

You would see the code for the constructor if you make it a non-template member function.

template<typename T> class test {
public:

    /***
    template<typename T> test(T param) {
        parameter = param;
    };
    ***/

    test(T param) : parameter(param) {}
    ~test() {}
    int pop();
    int push();
    T parameter;
};

Explicit instantiation of class template not instantiating constructor

1 Answers