Question

I’ve found out that the C++ compiler for AVR uCs doesn’t support the new and delete operators, but also that there is a quick fix:

void * operator new(size_t size) 
{ 
  return malloc(size); 
} 

void operator delete(void * ptr) 
{ 
  free(ptr); 
}

I’m assuming that it would now be possible to call new ClassName(args);.

However, I am not really sure how this works. For example, what actually returns a size_t here? I thought that constructors don’t return anything…

Could it be that new is now supposed to be used differently (in conjunction with sizeof())?

Answer 1

new T(args); is roughly equivalent to the following.

void* storage = operator new(sizeof(T)); // obtain raw storage
call_constructor<T>(storage, args); // make an object in it

(Here call_constructor is supposed to call the constructor^† of T making storage be the this pointer within that constructor.)

The operator new part obtains the requested amount of raw storage, and the constructor call is the one that actually makes an object, by invoking the constructor.

The code in the question only replaces the operator new part, i.e. the retrieval of storage. Both the sizeof part and the constructor invocation are done automatically by the compiler when you use new T(args).

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† The language has a way to express this direct constructor invocation called “placement new“, but I omitted it for clarity.