Template class method does not cause error -- is this part of the standard?

Question

When I compile the following with g++ --std=c++98 -Wall -Werror -Wpedantic Test.cc, there's no error.

template <class T>
struct TemplateClass {
  T *ptr;

  TemplateClass(T *p): ptr(p) {}

  int foo() {
    return ptr->bar();
  }
};

struct ExampleClass {
};

int main() {
  TemplateClass<ExampleClass> x(new ExampleClass());
}

I expected that the compiler would complain that ExampleClass doesn't implement method bar.

But it looks like it only complains if I actually use the method foo.

Can I rely on this behavior on any C++98 and C++11 compliant compilers?

My understanding of templates before was that whenever a template is instantiated, the entirety of the body is copied with T replaced with the template argument. Is this not how templates work?

Answer 1

This is the correct behaviour according to the standard. The definition of foo is not instantiated until used in a context that requires it to exist. Emphasis mine in the below:

C++03, [temp.inst]/1:

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions or default arguments, of the class member functions, member classes, static data members and member templates; and it causes the implicit instantiation of the definitions of member anonymous unions. Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; ...

C++11, [temp.inst]/1 and [temp.inst]/2:

Unless a class template specialization has been explicitly instantiated (14.7.2) or explicitly specialized (14.7.3), the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type or when the completeness of the class type affects the semantics of the program. The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions or default arguments, of the class member functions, member classes, scoped member enumerations, static data members and member templates; ... Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; ...