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 template is instantiated, explain why; if not, explain why not. P.677

template <typename T> class Stack { };

void f1(Stack<char>);                   // (a)

class Exercise {
    Stack<double> &rsd;                 // (b)
    Stack<int>    si;                   // (c)
};

int main() {
    Stack<char> *sc;                    // (d)
    f1(*sc);                            // (e)
    int iObj = sizeof(Stack< string >); // (f)
}

Below is what I tried:

(a) Stack<char> is instantiated , but no member of it is instantiated.

(b) Stack<double> is instantiated , but no member of it is instantiated.

(c) Stack<int> and its default constructor are instantiated.

(d) (e) totally no idea...

(f) Stack< string > is instantiated , but no member of it is instantiated.

Am I right? Can anyone tell me how this code is instantiated?

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::ThisDoesntExist 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)