Question

If you have the following function:

static const map<ushort, ulong> MakeMap()
{
  map<ushort, ulong> mymap;
  for(int i=0; i<myTableSize; i++)
  {
    mymap[myTable[i].x] = myTable[i].y;
  }
  return mymap;
}

and somewhere use you have:

static const map<ushort, ulong> numMap = MakeMap();

will the compiler actually set numMap to the address returned from MakeMap or will it actually make a copy of the map? Also, is this even safe to do?

Thanks for your feedback!

Answer 1

The return type is not static const map<ushort, ulong>. It’s only const map<ushort, ulong>. The keyword static applies to the function. That is, it’s the function which is static which means the function has internal linkage^* and cannot be called from other translation unit^*.

^{* Go through the links to know about them.}

Now coming back to your question, first of all, const in the return type doesn’t make sense. The following is better:

//remove the static also if you don't want it to have internal linkage
static map<ushort, ulong> MakeMap();

And then you can still write:

 const map<ushort, ulong> numMap = MakeMap();

If you’re using good compiler, then most likely it will optimize on the return value. Read about:

• Return Value Optimization