Question

What’s wrong with this code?

class School {
public:
    template<typename T> size_t count() const;
private:
    vector<Boy*> boys;
    vector<Girl*> girls;
};
template<> size_t School::count<Boy>() const {
    return boys.size();
}

My compile says

error: specialization of ‘size_t School::count() [with T = Boy]’
after instantiation

Could you please help?

ps. This is how I’m going to use it later:

School s;
size_t c = s.count<Boy>();

Answer 1

Have you accidentally called count<Boy> in School before it is declared? One way to reproduce your error is

class Boy;
class Girl;

class School {
public:
    template<typename T> size_t count() const;
    size_t count_boys() const { return count<Boy>(); }
    // ^--- instantiation
private:
    std::vector<Boy*> boys;
    std::vector<Girl*> girls;
};

template<> size_t School::count<Boy>() const { return boys.size(); }
// ^--- specialization

int main () { School g; return 0; }

You need to move the definition of count_boys() after all template members are specialized.