cpp core · advanced

Two-Phase Lookup: Why a Dependent Base's Member Won't Compile

Asked at Trading Technologies. A class template that inherits from a dependent base (Derived<T> : Base<T>) cannot see the base's members through an unqualified call. C++ uses two-phase name lookup: non-dependent names - like a no-argument, unqualified data() - are bound in phase 1 at definition time, when the compiler refuses to look inside Base<T> because a later specialization could change or remove its members. Make the name dependent so lookup is deferred to phase 2 (instantiation): this->data() (implicit this is dependent), Base<T>::data() (explicit qualification), or add using Base<T>::data; in the class body.

The code

#include <iostream>

template <typename T>
struct Base {
    void data() { std::cout << "Base::data()\n"; }
};

template <typename T>
struct Derived : Base<T> {        // Base<T> is a DEPENDENT base (it depends on T)
    void execute() {
        // data();                // (1) ERROR: 'data' was not declared in this scope
        this->data();             // (2) Fix 1 - 'this->' makes the name dependent
        // Base<T>::data();       // (3) Fix 2 - qualify with the base class
        // using Base<T>::data;   // (4) Fix 3 - a using-declaration in the class body
    }
};

int main() {
    Derived<int> obj;
    obj.execute();                // prints: Base::data()
}

What this lesson walks through

1. 01Looks obvious - and it won't compile
2. 02Templates compile in TWO phases
3. 03Phase-1 lookup skips the dependent base
4. 04Why the standard forbids the shortcut
5. 05Fix 1 - this-> defers the lookup
6. 06Three one-line fixes

Derived<int> inherits Base<int>, Base defines data(), and execute() just calls data(). Intuition says the call is found. The compiler says: 'data' was not declared in this scope. The reason is how templates are name-looked-up.