Sell me on const correctness

If you use const rigorously, you'd be surprised how few real variables there are in most functions. Often no more than a loop counter. If your code is reaching that point, you get a warm feeling inside...validation by compilation...the realm of functional programming is nearby...you can almost touch it now...

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.

Here's a piece of code with a common error that const correctness can protect you against:

void foo(const int DEFCON)
{
   if (DEFCON = 1)     //< FLAGGED AS COMPILER ERROR! WORLD SAVED!
   {
       fire_missiles();
   }
}

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?

From experience, this is a total myth. It happens when non const-correct sits with const-correct code, sure. If you design const-correct from the start, this should NEVER be an issue. If you make something const, and then something else doesn't complile, the compiler is telling you something extremely important, and you should take the time to fix it properly.