c++ - "invalid use of incomplete type" error with partial template specialization

Question

The following code:

template <typename S, typename T>
struct foo {
   void bar();
};

template <typename T>
void foo <int, T>::bar() {
}

gives me the error

invalid use of incomplete type 'struct foo<int, T>'
declaration of 'struct foo<int, T>'

(I'm using gcc.) Is my syntax for partial specialization wrong? Note that if I remove the second argument:

template <typename S>
struct foo {
   void bar();
};

template <>
void foo <int>::bar() {
}

then it compiles correctly.

Answer 1

You can't partially specialize a function. If you wish to do so on a member function, you must partially specialize the entire template (yes, it's irritating). On a large templated class, to partially specialize a function, you would need a workaround. Perhaps a templated member struct (e.g. template <typename U = T> struct Nested) would work. Or else you can try deriving from another template that partially specializes (works if you use the this->member notation, otherwise you will encounter compiler errors).