Unnecessary curly braces in C++?

When doing a code review for a colleague today I saw a peculiar thing. He had surrounded his new code with curly braces like this:

Constructor::Constructor()
{
   existing code

   {
      New code: do some new fancy stuff here
   }

   existing code
}

What is the outcome, if any, from this? What could be the reason for doing this? Where does this habit come from?

Edit:

Based on the input and some questions below I feel that I have to add some to the question, even though that I already marked an answer.

The environment is embedded devices. There is a lot of legacy C code wrapped in C++ clothing. There are a lot of C turned C++ developers.

There are no critical sections in this part of the code. I have only seen it in this part of the code. There are no major memory allocations done, just some flags that are set, and some bit twiddling.

The code that is surrounded by curly braces is something like:

{
   bool isInit;
   (void)isStillInInitMode(&isInit);
   if (isInit) {
     return isInit;
   }
}

(Don't mind the code, just stick to the curly braces... ;) ) After the curly braces there are some more bit twiddling, state checking, and basic signaling.

I talked to the guy and his motivation was to limit the scope of variables, naming clashes, and some other that I couldn't really pick up.

From my POV this seems rather strange and I don't think that the curly braces should be in our code. I saw some good examples in all the answers on why one could surround code with curly braces, but shouldn't you separate the code into methods instead?

Solution 1:

It's sometimes nice since it gives you a new scope, where you can more "cleanly" declare new (automatic) variables.

In C++ this is maybe not so important since you can introduce new variables anywhere, but perhaps the habit is from C, where you could not do this until C99. :)

Since C++ has destructors, it can also be handy to have resources (files, mutexes, whatever) automatically released as the scope exits, which can make things cleaner. This means you can hold on to some shared resource for a shorter duration than you would if you grabbed it at the start of the method.

Solution 2:

One possible purpose is to control variable scope. And since variables with automatic storage are destroyed when they go out of scope, this can also enable a destructor to be called earlier than it otherwise would.

Solution 3:

The extra braces are used to define the scope of the variable declared inside the braces. It is done so that the destructor will be called when the variable goes out of scope. In the destructor, you may release a mutex (or any other resource) so that other could acquire it.

In my production code, I've written something like this :

void f()
{
   //some code - MULTIPLE threads can execute this code at the same time

   {
       scoped_lock lock(mutex); //critical section starts here

       //critical section code
       //EXACTLY ONE thread can execute this code at a time

   } //mutex is automatically released here

  //other code  - MULTIPLE threads can execute this code at the same time
}

As you can see, in this way, you can use scoped_lock in a function and at the same time, can define its scope by using extra braces. This makes sure that even though the code outside the extra braces can be executed by multiple threads simultaneously, the code inside the braces will be executed by exactly one thread at a time.

Solution 4:

As others have pointed out, a new block introduces a new scope, enabling one to write a bit of code with its own variables that don't trash the namespace of the surrounding code, and doesn't use resources any longer than necessary.

However, there's another fine reason for doing this.

It is simply to isolate a block of code that achieves a particular (sub)purpose. It is rare that a single statement achieves a computational effect I want; usually it takes several. Placing those in a block (with a comment) allows me tell the reader (often myself at a later date):

• This chunk has a coherent conceptual purpose
• Here's all the code needed
• And here's a comment about the chunk.

e.g.

{  // update the moving average
   i= (i+1) mod ARRAYSIZE;
   sum = sum - A[i];
   A[i] = new_value;
   sum = sum + new_value;
   average = sum / ARRAYSIZE ;  
}

You might argue I should write a function to do all that. If I only do it once, writing a function just adds additional syntax and parameters; there seems little point. Just think of this as a parameterless, anonymous function.

If you are lucky, your editor will have a fold/unfold function that will even let you hide the block.

I do this all the time. It is great pleasure to know the bounds of the code I need to inspect, and even better to know that if that chunk isn't the one I want, I don't have to look at any of the lines.

Solution 5:

One reason could be that the lifetime of any variables declared inside the new curly braces block is restricted to this block. Another reason that comes to mind is to be able to use code folding in the favourite editor.