method chaining child with parent classes using c++

Question

Is there a way to chain calls to superclass from subclass without casting, overriding the method or using interfaces. E.g. when doing

class A {
public: 
    A& foo() { return *this; }
};

class B : public A {
public:
    B& bar() { return *this; }
};

int main(void) {
    B b;
    b.foo().bar();
    return 0;
}

When compiling with clang I'm getting the error

main.cpp:13:10: error: no member named 'bar' in 'A'
        b.foo().bar();
        ~~~~~~~ ^
1 error generated.

Which I can see why (since A returns reference to self), but I would like it to return it's subclass type B since it is called in that context. Is this possible? Or do I need to define B as

class B : public A {
public:
    B& bar() { return *this; }
    B& foo() { A::foo(); return *this; }
};

and make foo() virtual?

llllllllll · Accepted Answer · 2018-03-26 16:21:36Z

6

You can use CRTP pattern:

template<class Derived>
class A {
public:
    Derived& foo() { return *static_cast<Derived*>(this); }
};

class B : public A<B> {
public:
    B& bar() { return *this; }
};

int main(void) {
    B b;
    b.foo().bar();
    return 0;
}

answered Mar 26, 2018 at 16:21

llllllllll

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2 Comments

Johan Over a year ago

but with that pattern I won't be able to instantiate an object of class A?

llllllllll Over a year ago

@Johan You can. Just inherit an empty class. class just_a : public A<just_a> {};

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