Sell me on const correctness

Question

So why exactly is it that it's always recommended to use const as often as possible? It seems to me that using const can be more of a pain than a help in C++. But then again, I'm coming at this from the python perspective: if you don't want something to be changed, don't change it. So with that said, here are a few questions:

1. It seems like every time I mark something as const, I get an error and have to change some other function somewhere to be const too. Then this causes me to have to change another function somewhere else. Is this something that just gets easier with experience?

2. Are the benefits of using const really enough to compensate for the trouble? If you don't intend on changing an object, why not just not write code that doesn't change it?

I should note that at this point in time, I'm most focused on the benefits of using const for correctness and maintainability purposes, although it is also nice to have an idea of the performance implications.

Answer 1

This is the definitive article on "const correctness": https://isocpp.org/wiki/faq/const-correctness.

In a nutshell, using const is good practice because...

1. It protects you from accidentally changing variables that aren't intended be changed,
2. It protects you from making accidental variable assignments, and

3. The compiler can optimize it. For instance, you are protected from

   if( x = y ) // whoops, meant if( x == y )
   

At the same time, the compiler can generate more efficient code because it knows exactly what the state of the variable/function will be at all times. If you are writing tight C++ code, this is good.

You are correct in that it can be difficult to use const-correctness consistently, but the end code is more concise and safer to program with. When you do a lot of C++ development, the benefits of this quickly manifest.