There is no standard way to find this information. However, some implementations provide functions like msize to do this. For example:
- _msize on Windows
- malloc_size on MacOS
- malloc_usable_size on systems with glibc
Keep in mind though, that malloc will allocate a minimum of the size requested, so you should check if msize variant for your implementation actually returns the size of the object or the memory actually allocated on the heap.