Quiz – Where am I ?

· 3 min read

The question is, in C++, how do detect if an object is allocated on the stack or heap.

On Windows, the stack address is in the range of 0x80000000. If the address of the variable is in this range, then you could say that the object is allocated on the stack; else it is allocated on the heap. This technique of detecting is not preferable since it may not work on other operating systems (such as linux), and deals with the platform specific information making it a non-portable solution.

Let us try to use standard C++ stuff to solve the problem. Ok, we want to know where an object is allocated. In C++, the new operator is responsible for allocating an object.

It was very thoughtful of Stroustoup to keep the object allocation and initialization\construction separate. The new operator is responsible only for allocation. The compiler is responsible for weaving up the code for allocation and calling the constructor. It was also very thoughtful of being able to specify the location where the object needs to be allocated, which of course does not necessarily require overloading the new operator.

C++ allows taking control of object allocation by overloading the new operator. By taking control, you would be able to detect where an object is allocated, and also keep a count of the objects allocated on the stack \ heap. The following code snippet illustrates the same:-

// SomeClass.h
//

#include <iostream>
#include <stdlib.h>
#include <deque>
#include <algorithm>

using namespace std;

class SomeClass;
typedef std::deque<SomeClass*>      SomeClassDB;
typedef SomeClassDB::iterator    SomeClassDBIter;
typedef SomeClassDB::const_iterator SomeClassDBConstIter;

class SomeClass
{
private: static SomeClassDB heapObjects;
private: static SomeClassDB stackObjects;

private: double value;
private: bool isOnHeap;

public: SomeClass(double d) : value(d), isOnHeap(SomeClass::IsOnHeap(this))
        {
           if (!IsOnHeap())
           {
              SomeClass::stackObjects.push_back(this);
           }

           PrintLocationInfo();
        }

public: ~SomeClass()
        {
           SomeClassDBIter iter = std::find(SomeClass::heapObjects.begin(),
                    SomeClass::heapObjects.end(),
                    this);

           if (iter != SomeClass::heapObjects.end())
           {
              SomeClass::heapObjects.erase(iter);
           }
        }

public: double Value() const
        {
           return this->value;
        }

public: bool IsOnHeap() const
        {
           return this->isOnHeap;
        }

public: bool IsOnStack() const
        {
           return !IsOnHeap();
        }

public: std::string Location() const
        {
           return IsOnHeap() ? "Heap" : "Stack";
        }

public: void PrintLocationInfo() const
        {
           std::cout << Value() << " is on " << Location().c_str() << std::endl;
        }

private: static bool IsOnHeap(SomeClass* scPtr)
         {
            SomeClassDBConstIter iter = std::find(SomeClass::heapObjects.begin(), SomeClass::heapObjects.end(), scPtr);
            return iter != SomeClass::heapObjects.end();
         }

private: static bool IsOnStack(SomeClass* scPtr)
         {
            return !IsOnHeap(scPtr);
         }

public: static void* operator new(size_t size)
        {
           SomeClass* scPtr = (SomeClass*)malloc(size);
           SomeClass::heapObjects.push_back(scPtr);
           return scPtr;
        }

public: static void operator delete(void* ptr)
        {
           free(ptr);
        }

public: static size_t HeapCount()
        {
           return SomeClass::heapObjects.size();
        }

public: static size_t StackCount()
        {
           return SomeClass::stackObjects.size();
        }

public: static void PrintObjectCount()
        {
           std::cout << "OnHeap: " << HeapCount() << " .... OnStack: " << StackCount() << std::endl;
        }
};

//
// SomeClass.cpp
//

#include "SomeClass.h"

SomeClassDB SomeClass::heapObjects;
SomeClassDB SomeClass::stackObjects;

int main (int argc, char *argv[])
{
   SomeClass sc(0.123);
   SomeClass* scPtr = new SomeClass(1.234);

   SomeClass::PrintObjectCount();

   {
      SomeClass sc1(2.345);
      SomeClass::PrintObjectCount();
   }

   delete scPtr;

   SomeClass::PrintObjectCount();

   return 0;
}

You should be aware of implementing the custom delete if you have provided a custom new operator. It is logical because only a custom implementation that allocated memory for the object could possibly know how to deallocate. The above technique of overloading new \ delete is portable and safe since it is standard C++. As always, writing standard C++ is fun.

But why would one care to know where an object is allocated or keep a count of objects. I don’t think it would something you would require for production purposes. It could be for development\debugging purposes; maybe you want to detect memory leaks or a general distribution of objects. You could take control of the allocation by overloading new for a particular class; Or you can declare a global new operator to make it applicable for all classes.