I am working on legacy code where memory allocation/deallocation done in traditional C style, but want to wrap it in a unique_ptr with a custom deleter. Consider a case where a 2 dimensional array is allocated by legacy code by calling calloc/malloc. I need to call the corresponding legacy deallocator function taking two parameters - pointer to array and size of the array. How to define a custom deleter that takes 2 parameters in this case, both will be passed only at the time of initializing the unique_ptr to take ownership of already allocated array? Thanks in advance.

A simplified(yet contrived) example below:

    // function allocates memory and returns size of the buffer
    void legacyFunction(char **array, int *num)
    {
        char *p = (char *)calloc(5, sizeof(char));
        for (auto i=0; i<5;   i)
        {
            p[i] = 'a';
        }
        *num = 5;
        *array = p;
        return;
    }

int main()
{
    char *array = nullptr;
    int numofElts = 0;
    legacyFunction(&array, &numofElts);
    // unique_ptr here to take ownership of array
    // custom deleter should take both pointer and size
    // the size could be any number based on business use case

    return 0;
}

CodePudding user response：

The deleter is an object. It can hold onto the size.

struct LegacyDeleter
{
  int size;
  void operator()(char** ptr) const noexcept
  { legacyDeallocate(ptr, size); }
};
using legacy_ptr = std::unique_ptr<char*, LegacyDeleter>;

int main()
{
   legacy_ptr ptr(legacyAllocatorFunction(5), LegacyDeleter{5});
}