syntax - C++ const keyword - use liberally?

Question

In the following C++ functions:

void MyFunction(int age, House &purchased_house)
{
    ...
}


void MyFunction(const int age, House &purchased_house)
{
    ...
}

Which is better?

In both, 'age' is passed by value. I am wondering if the 'const' keyword is necessary: It seems redundant to me, but also helpful (as an extra indication the variable will not be changing).

Does anyone have any opinion as to which (if any) of the above are better?

Answer 1

First, it's just an implementation detail, and if you put const there, don't put it in the declaration set (header). Only put it in the implementation file:

// header
void MyFunction(int age, House &purchased_house);

// .cpp file
void MyFunction(const int age, House &purchased_house);
{
    ...
}

Whether or not a parameter is const in a definition is purely an implementation detail, and should not be part of the interface.

I've not seen this sort of thing often, and i also don't do this. Having the parameter const would confuse me more often than help, because i would immediately pattern-match-fail it to "const int &age" :) The matter of course is entirely different from having const at another level:

// const is a *good* thing here, and should not be removed,
// and it is part of the interface
void MyFunction(const string &name, House &purchased_house);
{
    ...
}

In this case, the const will affect whether the function can change the caller's argument. Const in this meaning should be used as often as possible, because it can help ensuring program correctness and improve self-documenting the code.