c++ – Why do g ++ and msvc interpret code differently?

Question:

Day code:

int main() {
    int a = 8;
    int b = 100;
    float d;
    int c = ++a * a++;
    d = b / (--a);
    return 0;
}

but for some reason in g ++ the variable с is 90 , and in msvc it is 81 .

Answer:

The undefined behavior is classic:

 int c = ++a * a++;

For C ++ 98:

Writing twice to the variable a within one sequence point.

For C ++ 11:

Violation of the rules of the order of calculation.

The program is wrong. The compiler can react randomly.