Parameters to operator new

Question

I am trying to write a custom allocator for a particular class. My experience with allocators is basically none, so this might be obvious.

I wish my allocations to be dependent on certain parameters. Roughly speaking, I wish to have multiple memory managers, each managing a few objects. When creating the objects, I shall know which manager the object should belong to. Having read up a fair bit on overloading operator new, I am lost as to how I can implement what I need.

This page says that there is a version of operator new that accepts user defined parameters, but it seems this version would not be called while allocating objects using new. Could somebody point out how to construct anything other than a global allocator? On a related note, how can I call parameterized constructors if I am using overloaded class-specific operator new?

Answer 1

You can create a customized allocator that takes in extra parameters by defining an overload for operator new like this:

void* operator new (size_t size, /* extra parameters here */) {
    /* Do something, then return a pointer to at least 'size' bytes. */
}

You can then use your allocator by writing

MyObject* obj = new (/* extra params */) MyObject(/* constructor args */);

For example, here's a silly custom allocator that prints out a message when you allocate something:

void* operator new(size_t numBytes, const std::string& message) {
    std::cout << "Custom message: " << message << std::endl;

    /* Use the default allocator to get some bytes. */
    return ::operator new(numBytes);
}

You could call it by writing

std::complex<double>* obj = new ("Hello, world!") std::complex<double>(0.0, 0.0);