How is a template instantiated?

c++, instantiation, templates

Solution

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)

Problem

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?

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