Search code examples
c++pointersclasspointer-arithmetic

Doing pointer math in a c++ class: Is it "legit"?

Ah-hoi, hoi,

I'm wondering if it's ok to do something like the following:

class SomeClass
{
   int bar;
};

SomeClass* foo = new SomeClass();
int offset = &(foo->bar) - foo;

SomeClass* another = new SomeClass();
*(another+offset) = 3; // try to set bar to 3

Just Curious, Dan O

Solution

  • I suppose tecnically it might work out.

    However, there are several problems. bar is private. You are mixing pointers of different types (pointer arithmetic relies on the pointer type: int* + 1 and char* + 1 have different results because int and char have a different size).

    Have you also considered pointers to members:

    #include <cassert>

struct SomeClass
{
   int bar;
};

int main() {
    int SomeClass::*mem_ptr = &SomeClass::bar;
    SomeClass foo;
    foo.*mem_ptr = 3;
    assert(foo.bar == 3);
}