Is there any benefit to declaring a const rather than a variable inside a function?

Question

If I have a function which only exists for a short amount of time, does making the list of colors a constant make a difference?

string getrandcolor(){
    const string colors[5] = {"red", "blue", "green", "yellow", "purple"};
    return colors[rand() % 5];
}

Note: The actual list of colors contains hundreds of colors not just the small sample I have shown, not sure if this makes a difference either.

Answer 1

It prevents you from accidentally overwriting variables that you didn't mean to change. "Oops!"-protection is probably const's most important function.

Hypothetically a compiler could divine some kind of optimization from knowing a variable isn't supposed to change, but my testing has never found one that actually does something meaningful in the situation you describe.

const static does have an important difference in your particular code sample. Because your colors[] array is local, it must be constructed afresh each time the getrandcolor() function is called. That means the string constructor gets run five times each time you call getrandcolor(), which is very wasteful. const static means that the data is constructed only once — the first time the function is run — and put into a shared memory space.

Answer 2

In terms of performance? No, probably not. It looks like your array could be static too, and then perhaps yes.

In terms of code style? Possibly. Though the addition of const makes your code a little verbose, it also makes clear that the data is not to be modified. This is both documenting and safe.

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Ideally, all objects in C++ would be constants by default, and you'd have to write mutable to make them variables. It's all backwards!