C++ polymorphism different virtual functions can be called by same function name?

Question

#include<iostream>
using namespace std;
class Class1 {
public:
    virtual void f() {
        cout << "Function f() in Class1\n";
    }
};
class Class2 {
public:
    virtual void h() {
        cout << "Function h() in Class2\n";
    }
};
int main() {
    Class1 object1, *p;
    Class2 object2;
    p = &object1;
    p = (Class1*)&object2;
    p->f();//possibly abnormal program termination. but in fact, it will call function h(),Why?
    return 0;
}

The code must be wrong in theory because p->f() there is no function f() in the Class2.

But in fact, the code can be run, and it will call Function h() in the Class2, It's so strange,why??

There is no polymorphism here. `Class1` and `Class2` are unrelated and the behavior of your code is undefined. — Miles Budnek, Apr 14 '18 at 10:53
`object2` is not an instance of `Class1`, but (using an explicit type conversion that stops the compiler complaining) you force `p` to point at it. Calling `p->f()` then gives undefined behaviour. When behaviour is undefined, any result is possible, whether it makes sense to you or not. — Peter, Apr 14 '18 at 10:57
p = (Class1*)&object2; Do not use this kind of cast anyway. You should use static_cast(&object2); That would be a compile error. It is a good idea to NEVER use C-style casts in C++ code: https://stackoverflow.com/questions/1609163/what-is-the-difference-between-static-cast-and-c-style-casting — Germán Diago, Apr 14 '18 at 11:01

Janne Husberg · Answer 1 · 2018-04-14T11:06:48.513

3

C++ doesn't have many run-time checks for performance reasons, so it will just do what you tell it to do even if it is illogical.

You are telling it to call the virtual method Class1::f on a pointer organised in the memory layout of Class2 and apparently Class2::h aligns with Class1::f so it gets called.

This is not standardised behaviour and may differ between compilers. See Virtual method table.

edited Apr 14 '18 at 11:06

answered Apr 14 '18 at 10:55

Janne Husberg

294
1
9

gflegar · Accepted Answer · 2018-04-14T11:19:57.793

There is a problem with the code, and as @tkausi commented, the behavior is undefined.

The problem is that you are using C-style cast which is dangerous and should be avoided in C++. From cppreference:

When the C-style cast expression is encountered, the compiler attempts to interpret it as the following cast expressions, in this order:

const_cast<new_type>(expression);

static_cast<new_type>(expression), with extensions: pointer or reference to a derived class is additionally allowed to be cast to pointer or reference to unambiguous base class (and vice versa) even if the base class is inaccessible (that is, this cast ignores the private inheritance specifier). Same applies to casting pointer to member to pointer to member of unambiguous non-virtual base;

static_cast (with extensions) followed by const_cast;

reinterpret_cast<new_type>(expression);

reinterpret_cast followed by const_cast.

The first choice that satisfies the requirements of the respective cast operator is selected, even if it cannot be compiled (see example). If the cast can be interpreted in more than one way as static_cast followed by a const_cast, it cannot be compiled. In addition, C-style cast notation is allowed to cast from, to, and between pointers to incomplete class type. If both expression and new_type are pointers to incomplete class types, it's unspecified whether static_cast or reinterpret_cast gets selected.

Thus, in your case the compiler will select reinterpret_cast which will just reinterpret the raw bytes of your object as something else (and it just happens that the first thing in the virtual table of p points to method h).

To make your program safe, you should instead use static_cast or dynamic_cast:

#include<iostream>
using namespace std;
class Class1 {
public:
    virtual void f() {
        cout << "Function f() in Class1\n";
    }
};
class Class2 {
public:
    virtual void h() {
        cout << "Function h() in Class2\n";
    }
};
int main() {
    Class1 object1, *p;
    Class2 object2;
    p = &object1;
    p = static_cast<Class1*>(&object2); // fails to compile
    p->f();
    return 0;
}

In this case, compilation fails as expected. Live example: https://godbolt.org/g/noErNr

Note that if we replace static_cast line with p = dynamic_cast<Class1*>(&object2); the code will compile, but p will be set to nullptr at runtime, and trying to call p->f() will result in an error.

score 1 · Answer 3 · answered Apr 14 '18 at 11:34

The code must be wrong in theory because p->f() there is no function f() in the Class2.

The code is not only wrong in theory, but it is wrong in pratice too. The thing is that C++ does not hold your hand and does not wrap you in cotton wool, if you do wrong stuff, bad things can happen. Typesafety is your friend, but if you betray it, you are on your own.

In this line:

p = (Class1*)&object2;

you pretend that you could cast a Class2 to a Class1. Anything beyond that is undefined behaviour, which is sloopy speaking a way of saying: c++ does not care what is the output of programs that dont obey the rules.

The way to stay friends with the typesystem is to use static_cast instead of a C-style cast, which would tell you that the cast is not allowed.

C++ polymorphism different virtual functions can be called by same function name?

3 Answers3