Is it possible that a compiled program does not contain an instantiated template class?

Question

Consider this code:

template <typename T>
class A {
    T x;
    // A bunch of functions
};

std::size_t s = sizeof(A<double>);

Assume the sizeof operator is the only place where an instantiation of A<double> is required. Is it possible that the compiled program does not contain relevant code for A<double> (e.g. A<double>::~A())?

Answer 1

The class will be instantiated, but the compiler must not instantiate any member function definition, [temp.inst]/1:

[...] the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type[...]

[temp.inst]/2:

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions, default arguments, or noexcept-specifiers of the class member functions, [...]

Answer 2

Is it possible that the compiled program does not contain relevant code for A<double> (e.g. A<double>::~A())?

Sure that's possible.

std::size_t s = sizeof(A<double>);

is just a compile time operation, and doesn't need any runtime instance of A<double>, so there's no need for constructors, destructors, or other relevant code.

---

Even if there would be explicit instantiations of template function code like follows

 if(sizeof(A<double>) <= 4) {
      A<double> a; // Instantiation of constructor and destructor
      a.x = 3.5;
 }

the compiler is allowed to optimize that code away.