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Question #160 Difficulty: 2

According to the C++23 standard, what is the output of this program? 

    #include <iostream>

struct A {
    virtual void foo (int a = 1) {
        std::cout << "A" << a;
    }
};

struct B : A {
    virtual void foo (int a = 2) {
        std::cout << "B" << a;
    }
};

int main () {
    A *b = new B;
    b->foo();
}

Correct!

In the first line of main, we create a new B object, with an A pointer b pointing to it.

On the next line, we call b->foo(), where b has the static type A, and the dynamic type B. Since foo() is virtual, the dynamic type of b is used to ensure B::foo() gets called rather than A::foo().

However, which default argument is used for the int a parameter to foo()? §[dcl.fct.default]¶10 in the standard:

A virtual function call (§[class.virtual]) uses the default arguments in the declaration of the virtual function determined by the static type of the pointer or reference denoting the object. An overriding function in a derived class does not acquire default arguments from the function it overrides.

So B::foo() is called, but with the default argument from A::foo(), and the output is B1.

You can explore this question further on C++ Insights or Compiler Explorer!

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