Are there advantages to declaring stack variables constant in C++

Question

It's not clear to me what benefits there are of declaring your stack variables as constant in C++, I was hoping somebody might explain the benefits and purpose for this technique.

For example:

void func(const std::string& arg)
{
   if(someCondition)
   {
      const std::string foo ("some string plus " + arg);
      std::out << foo << std::endl;
      someFunction(foo);
      // dozens more lines of code...
   }
   // bla bla bla...
}

Thanks!

Answer 1

Yes.

1. It helps the reader to understand your intent. Clearly foo is given an initial value and never changed thereafter.

2. It helps the compiler to optimise your code, for both speed and space. The compiler can perform certain optimisations such as hoisting values out of loops if there is a promise that the value cannot change. This is particularly the case if the value has an alias, which would otherwise defeat optimisation.

3. It occasionally catches bugs, when you accidentally write code to modify something you didn't mean to.

On the down side, you may not be able to pass the value into a function that takes non-const arguments.