Is this defined?

Question

Suppose I have the following class:

struct A{
  void method(A& otherA) const{
    /* Mutate otherA */
  }
};

And then later I have this:

A myA;
myA.method(myA);

I have told the compiler that method will not change the this instance, but does the compiler realize that I could pass in the this instance as a parameter?

Could I be breaking stuff by doing this? Is this defined behavior?

Answer 1

This is totally fine, and not a problem. What you're doing in this example is sometimes called "aliasing" - when two arguments actually refer to the same object.

Consider the even simpler case in plain C:

void foo(int* a, const int* b) { *a += *b; }

That function takes two pointers to ints, and adds the second one to the first one. And of course this code to use my foo function is perfectly valid:

int x = 10;
foo(&x, &x); // now x is 20

If you don't like that behavior in this case, the best thing to do would be probably to add a check within your method like

void A::method(A& otherA) const {
    if (this == &otherA) { /* aliasing detected */ }
    else { /* proceed as normal */ }
}