If statement vs if-else statement, which is faster? [closed]

Answer 1

The answer from CompuChip shows that for int they both are optimized to the same assembly, so it doesn't matter.

What if value is a matrix ?

I will interpret this in a more general way, i.e. what if value is of a type whose constructions and assignments are expensive (and moves are cheap).

then

T value = init1;
if (condition)
   value = init2;

is sub-optimal because in case condition is true, you do the unnecessary initialization to init1 and then you do the copy assignment.

T value;
if (condition)
   value = init2;
else
   value = init3;

This is better. But still sub-optimal if default construction is expensive and if copy construction is more expensive then initialization.

You have the conditional operator solution which is good:

T value = condition ? init1 : init2;

Or, if you don't like the conditional operator, you can create a helper function like this:

T create(bool condition)
{
  if (condition)
     return {init1};
  else
     return {init2};
}

T value = create(condition);

Depending on what init1 and init2 are you can also consider this:

auto final_init = condition ? init1 : init2;
T value = final_init;

But again I must emphasize that this is relevant only when construction and assignments are really expensive for the given type. And even then, only by profiling you know for sure.