parts/6.43.Redispatching-PPH.md

6.43 Redispatching [PPH]

6.43.1 Applicability to language

The vulnerability as described in ISO/IEC TR 24772-1:2019 clause 6.43 exists in C++ for virtual functions, except for constructors and destructors which are not dispatching. An example of the infinite recursion is:

#include <iostream>

class A {
public:
    virtual void f() { std::cout << "A::f()\n"; }
    virtual void g() { std::cout << "A::g()\n"; A::f(); }  //call to f() will not dispatch.
    virtual void h() { std::cout << "A::h()\n"; }
    virtual void i() { std::cout << "A::i()\n"; h(); } //call to h() will dispatch
                                                      //showing the vulnerability
};

class B : public A {
public:
    void f() override { std::cout << "B::f()\n"; g(); }
    void h() override { std::cout << "B::h()\n"; i(); }
};

int main() {
    B b;
    A * pA = &b;
    pA->f(); // no problem
    std::cout << "---\n";
    pA->h(); // infinite recursion
}

In C++, the call to a member function can be qualified, as shown in the above example, and avoids the vulnerability.

6.43.2 Avoidance mechanisms for language users

To avoid the vulnerability or mitigate its ill effects, C++ software developers can:

• At a call site, consider whether virtual dispatch is desired: if not, construct the call using the qualified name.

• Be suspicious of any call from a virtual member function of the derived class to any member function of any of its base classes.