Question

I’m relatively new to C++ and working on a fairly large C++ project at work. I notice handfuls of functions that take double pointers as parameters for objects that the function will instantiate on the heap. Example:

int someFunc(MyClass** retObj) {
    *retObj = new MyClass();

    return 0;
}

I’m just not sure why double pointers are always used, in this project, instead of just a single pointer? Is this mostly a semantic cue that it’s an out/return parameter, or is there a more technical reason that I’m not seeing?

Answer 1

The double pointer pattern is used so that the newly allocated MyClass can be passed to the caller. For example

MyClass* pValue;
someFunc(&pValue);
// pValue now contains the newly allocated MyClass

A single pointer is insufficient here because parameters are passed by value in C++. So the modification of the single pointer would only be visible from within someFunc.

Note: When using C++ you should consider using a reference in this scenario.

int someFunc(MyClass*& retObj) {
  retObj = new MyClass();
  return 0;
}

MyClass* pValue;
someFunc(pValue);

This allows you to pass the argument by reference instead of by value. Hence the results are visible to the caller.