Question

I came up with this as a quick solution to a debugging problem – I have the pointer variable and its type, I know it points to an array of objects allocated on the heap, but I don’t know how many. So I wrote this function to look at the cookie that stores the number of bytes when memory is allocated on the heap.

template< typename T >
int num_allocated_items( T *p )
{
return *((int*)p-4)/sizeof(T);
}

//test
#include <iostream>
int main( int argc, char *argv[] )
{
    using std::cout; using std::endl;
    typedef long double testtype;
    testtype *p = new testtype[ 45 ];

    //prints 45
    std::cout<<"num allocated = "<<num_allocated_items<testtype>(p)<<std::endl;
    delete[] p;
    return 0;
}

I’d like to know just how portable this code is.

Answer 1

It is not even remotely portable.

An implementation can perform heap bookkeeping however it wants and there is absolutely no way to portably get the size of a heap allocation unless you keep track of it yourself (which is what you should be doing).