为什么结构指针在Visual C ++中会发生错误，但GCC不会？

Question

We use a structure below.

struct  S
{
    int i;
    int *p;
};

Main process:

int main()
{
    S s;
    int *p = &s.i;
    p[0] = 4;
    p[1] = 3;
    printf("p[0]=%d\n", p[0]);
    printf("p[1]=%d\n", p[1]);
    s.p = p;
    s.p[0] = 1;
    s.p[1] = 2;

    printf("p[0]=%d\n", p[0]);
    printf("p[1]=%d\n", p[1]);

    return 0;
}

Then our process run it occurred memory error in sp[1] = 1 when we compiled it with Visual C++.

But it can run when we compiled it with GCC.

Why does it occurred error in VC++ but GCC doesn't?

Answer 1

The behaviour of p[1] = 3; is undefined .

p is not pointing to an array of at least two elements.

Answer 2

With this statement:

int* p = &s.i;

you are initializing your pointer p with the address of one object, not an array. But with:

p[1] = 3;

and:

s.p[1] = 2;

you are trying to set the value of some second array element but there is no array to begin with. This causes undefined behavior .

That being said your p inside the main has nothing to do with the int* p data member inside your class. It is also highly unlikely this works with GCC as _tmain is a Microsoft Visual C++ extension.