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?
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)