How does one create a shared_ptr to pass to a function that takes a void *

Question

That's pretty much it. I need to allocate memory and pass it to a function that takes a void *. I'd like to use a shared_ptr but I don't know how to do it.

Answer 1

Do you mean something like:

boost::shared_ptr<int> myInt(new int(5));

call_some_function(myInt.get());

This only let's the function use the int*. It shouldn't try to delete it or take ownership.

---

If you want just raw memory, use a vector:

std::vector<char> memory(blockSize);

call_some_function(&blockSize[0]);

Again, the memory belongs to the vector.

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If your function does want ownership, there's no need to wrap it into something, since you won't be managing it:

call_some_function(new int);
call_some_function(new char[blockSize]);

Make sure the function will be releasing it with a call to delete/delete[]. If not (for example, it intends to use free()), you'll need to use malloc instead:

template <typename T>
T* malloc_new(void)
{
    void* memory = std::malloc(sizeof(T));
    if (!memory)
        throw std::bad_alloc();

    try
    {
        return new (memory) T();
    }
    catch(...)
    {
        std::free(memory);
        throw;
    }
}

call_some_function(malloc_new<int>());
call_some_function(malloc(blockSize));

Answer 2

you can also use std::string as a reference counted memory blob container. It is as efficient as shared_ptr on char * vector (maybe better)