Question

I am confused about the memory allocation in C++. Can anyone guide me as to where each of the variable in the below snippet is getting allocated. How can I determine what is getting allocated on stack and what gets allocated on heap. Is there any good web reference for learning this.

   class Sample {
    private:
        int *p;
    public: 
        Sample() {
            p = new int;
        }
    };

    int* function(void) {
        int *p;
        p = new int;
        *p = 1;

        Sample s;

        return p;
    }

Answer 1

If it’s created via new, it’s in the heap. If it’s inside of a function, and it’s not static, then it’s on the stack. Otherwise, it’s in global (non-stack) memory.

class Sample {
    private:
        int *p;
    public: 
        Sample() {
            p = new int;  // p points to memory that's in the heap
        }

       // Any memory allocated in the constructor should be deleted in the destructor; 
       // so I've added a destructor for you:
        ~Sample() { delete p;}

        // And in fact, your constructor would be safer if written like this:
        // Sample() : p(new int) {}
        // because then there'd be no risk of p being uninitialized when
        // the destructor runs.
        //
        // (You should also learn about the use of shared_ptr,
        //  which is just a little beyond what you've asked about here.)
    };

    int* function(void) {
        static int stat; // stat is in global memory (though not in global scope)
        int *p;          // The pointer itself, p, is on the stack...
        p = new int;     // ... but it now points to memory that's in the heap
        *p = 1;

        Sample s;        // s is allocated on the stack

        return p;
    }

}

int foo; // In global memory, and available to other compilation units via extern

int main(int argc, char *argv[]) { 
// Your program here...