Why const keyword re-evaluated in while code block?

Question

Code:

void Test()
{
    float dt = 0.3;
    float playTime = 1;
    while ( playTime > 0.0 )
    {
      const float delta = std::min( playTime, dt );
      std::cout << delta << std::endl;
      playTime -= delta;
    }
}

Result:

0.3
0.3
0.3
0.1

In c++, const keyword means doesn't change. So, we use the keyword const to make vars constant.

But in this case,
Why const var, delta can be changed every time being called?

Since, while loop start new code block(in stack area),
So, I think variable, delta exist already for while looping execution.

But `delta` is never changed in your code example. At every loop iteration you create a new `delta` variable initializing it with a value returned by `std::min`. — user7860670, Nov 20 '17 at 07:32
You should probably get [a good beginners book or two](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) and learn about *scoping* and *life-time*. — Some programmer dude, Nov 20 '17 at 07:34
Thank @dude for your advice. I'm not a novice. And I am CS majored and also got a PL class. But, I just confused the keyword const at this point. :) — Jinbom Heo, Nov 20 '17 at 07:44
I hope you didn't take it the wrong way, but it's not always easy to know. :) — Some programmer dude, Nov 20 '17 at 07:45
@JinbomHeo - Even seasons professionals should not learn C++ by trial and error. A good book is mandatory to get off to a good start — StoryTeller - Unslander Monica, Nov 20 '17 at 07:46

score 4 · Accepted Answer · answered Nov 20 '17 at 07:38

Because you get a new variable with every iteration of the loop.

The scope of the variable delta is restricted to the body of the loop. That means it will be destroyed and re-created with every iteration. The const simply means that its value cannot change while its alive, but since you recreate it with every iteration, its value may indeed change between iterations, just never within the course of a single iteration.

Note that it is possible in C++ to declare a variable in a way that it persists throughout the lifetime of a program, even if its scope is left. That is what the static keyword is for. Change your delta declaration to static const float delta = std::min( playTime, dt ); and it will retain the same value throughout.

In this particular case though, it's probably better to just move the declaration before the loop, as static will also persist the variable through multiple calls to the function Test.

score 3 · Answer 2 · answered Nov 20 '17 at 07:34

The scope of delta is the block of the while loop. It comes to life at the start of it, and dies at the end. That happens every time the block is entered and exited, i.e. every iteration.

The const qualifier only means you cannot modify it while it's alive. It doesn't change the extent of its lifetime. In essence, it's a "different" delta each time the block is executed.

If you want it to be fixed for each and every iteration, you must move it to an enclosing scope (such as the function body), and initialize is there.

Why const keyword re-evaluated in while code block?

2 Answers2