Can a C++ class determine whether it's on the stack or heap?

Question

I have

class Foo {
....
}

Is there a way for Foo to be able to separate out:

function blah() {
  Foo foo; // on the stack
         


        

Answer 1

It is possible if you compare the value of 'this' with the current value of the stack pointer. If this < sp then you have been allocated in the stack.

Try this out (using gcc in x86-64):

#include 

class A
{
public:
    A()
    {
        int x;

        asm("movq %1, %%rax;"
            "cmpq %%rsp, %%rax;"
            "jbe Heap;"
            "movl $1,%0;"
            "jmp Done;"
            "Heap:"
            "movl $0,%0;"
            "Done:"
            : "=r" (x)
            : "r" (this)
            );

        std::cout << ( x ? " Stack " : " Heap " )  << std::endl; 
    }
};

class B
{
private:
    A a;
};

int main()
{
    A a;
    A *b = new A;
    A c;
    B x;
    B *y = new B;
    return 0;
}

It should output:

Stack 
Heap 
Stack 
Stack 
Heap