How is a template instantiated?

Question

It\'s an exercise from C++ Primer 5th Edition:

Exercise 16.27: For each labeled statement explain what, if any, instantiations happen. If a tem

Answer 1

In your specific case a declaration doesn't mean an instantiation

#include <iostream>
using namespace std;


template <typename T> class Stack {
  typedef typename T::ThisDoesntExist StaticAssert; // T::NotExisting doesn't exist at all!
};


void f1(Stack<char>); // No instantiation, compiles

class Exercise {
  Stack<double> &rsd; // No instantiation, compiles (references don't need instantiation, are similar to pointers in this)

  Stack<int>    si; // Instantiation! Doesn't compile!!
};


int main(){

  Stack<char> *sc; // No Instantiation, this compiles successfully since a pointer doesn't need instantiation

  f1(*sc); // Instantiation of Stack<char>! Doesn't compile!!

  int iObj = sizeof(Stack< std::string >); // Instantiation of Stack<std::string>, doesn't compile!!

}

notice the pointer/reference stuff: they don't require instantiation since no data is actually allocated (a pointer is just a few bytes to contain the address, has no need to have all the data stored.. take a look at the pimpl idiom ).

Only when stuff is allocated then the template has to be completely resolved (and that happens at compile-time, that's why they usually need both declaration and definition.. there's no linking phase yet)

Answer 2

Regarding e and d I will quote the standard 14.7.1

Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist. Unless a call is to a function template explicit specialization or to a member function of an explicitly specialized class template, a default argument for a function template or a member function of a class template is implicitly instantiated when the function is called in a context that requires the value of the default argument.

Example also from the standard

template<class T> struct Z {
    void f();
    void g();
};

void h() {
    Z<int> a;     // instantiation of class Z<int> required
    Z<char>* p;   // instantiation of class Z<char> not required
    Z<double>* q; // instantiation of class Z<double> not required
    a.f();        // instantiation of Z<int>::f() required
    p->g();       // instantiation of class Z<char> required, and instantiation of Z<char>::g() required
}

This means that no instantiation happens in d. While it will be instantiated in e if that function actually needed to call a function from that type ( could be a copy constructor or any other function called inside the function).