Question

I’m in a discussion at work as to how to properly handle containers as parameters.

We have a function that takes in a container parameter, and wants to return the container filled ONLY with what the function puts into it:

class bar;

void foo(std::vector<bar> &bars) 
{
   //do stuff that fills bars.
   //exceptions may be thrown.
   //we may also legally return early 
   return;
}

On one side of the discussion, we have people that say we should bars.clear() first and then run the function.

For example:

void foo(std::vector<bar> &bars) 
{
   bars.clear();
   //do stuff that fills bars.
   //exceptions may be thrown.
   //we may also legally return early 
   return;
}

My own preference is to try to reach the strong exception guarantee as closely as I can, which means making a local container, filling that and swapping before returning, but otherwise leaving bars untouched.

For example:

void foo(std::vector<bar> &bars) 
{
   std::vector<bar> localBars;
   //do stuff that fills localBars.
   //exceptions may be thrown.
   //we may also legally return early 
   if (returnEarly)
   {
       bars.swap(localBars);
       return;
   }
   //do more stuff that may change localBars

   bars.swap(localBars);
   return;
}

The first example is the ‘classic’ method; of clearing your parameters before doing anything and going from there.

The second method, to me, sets up a strong exception guarantee (assuming nothing else the function does can change internal states), and avoids a clear() call.

Are there any advantages or disadvantages to picking one method over the other one?

Note that for this function, a strong exception guarantee isn’t required; if the function fails nothing in the parameters or anything else it does will matter by the time it gets up to the exception handler.

Answer 1

The two are really different.

Advantages of clear:

• reuse memory
• no out-dated information left in bars

Advantages of swap:

• strong exception guarantee

The question cannot be answered in general, what semantics/guarantees do you seek in your case ?