Incorrect vtable indexing in C++ to C transformation #701
Description
The following C++ example is actually taken from Back to Basics: Object-Oriented Programming in C++ - Andreas Fertig - CppCon 2024
#include <stdio.h>
struct Fruit {
double mWeight{};
virtual ~Fruit() { puts("~Fruit"); }
virtual void Print() { puts("Fruit's Print"); }
};
struct Apple : Fruit {
int mRipeGrade{5};
void Print() override { printf("Apple's Print: %d\n", mRipeGrade); }
};
struct PinkLady : Apple {
int mColorGrade{8};
void Print() override { printf("Pink Ladies Print: %d\n", mColorGrade); }
};
int main() {
PinkLady delicious{};
delicious.Print();
Fruit *f{static_cast<Fruit *>(&delicious)};
f->Print();
}which outputs:
$ g++ -o example ./example.c && ./example
Pink Ladies Print: 8
Pink Ladies Print: 8
~FruitBut the generated code which can be found here, its main is like:
// ...
int __main(void)
{
PinkLady delicious;
Constructor_PinkLady((PinkLady *)&delicious);
(*((void (*)(PinkLady *))((&delicious)->__vptrFruit[0]).f))((((PinkLady *)(char *)(&delicious)) + ((&delicious)->__vptrFruit[0]).d));
Fruit * f = (Fruit *)&delicious;
(*((void (*)(Fruit *))((f)->__vptrFruit[0]).f))((((Fruit *)(char *)(f)) + ((f)->__vptrFruit[0]).d));
return 0;
(*((void (*)(PinkLady *))((&delicious)->__vptrFruit[0]).f))((((PinkLady *)(char *)(&delicious)) + ((&delicious)->__vptrFruit[0]).d));
}
// ...
__mptr __vtbl_Fruit[2] = {{0, 0, (__vptp)Destructor_Fruit}, {0, 0, (__vptp)PrintFruit}};
__mptr __vtbl_Apple[2] = {{0, 0, (__vptp)Destructor_Apple}, {0, 0, (__vptp)PrintApple}};
__mptr __vtbl_PinkLady[2] = {{0, 0, (__vptp)Destructor_PinkLady}, {0, 0, (__vptp)PrintPinkLady}};
__mptr * __vtbl_array[3] = {__vtbl_Fruit, __vtbl_Apple, __vtbl_PinkLady};
// ...which outputs:
$ gcc -o example ./example.c && ./example
~Fruit
~Fruitwhich doesn't match the C++ example output. I enabled only the following two options:
- Show all implicit casts
- Show C++ to C transformation
I think the problem is that the code doesn't use correct indecies __vptrFruit[i] to refer to the correct functions to be used since they're pointing to the Destructors, not the Print functions.