9.5. const parameters

You might have noticed that the parameters for after and addTime are being passed by reference. Since these are pure functions, they do not modify the parameters they receive, so I could just as well have passed them by value.

The advantage of passing by value is that the calling function and the callee are appropriately encapsulated—it is not possible for a change in one to affect the other, except by affecting the return value.

On the other hand, passing by reference usually is more efficient, because it avoids copying the argument. Furthermore, there is a nice feature in C++, called const, that can make reference parameters just as safe as value parameters.

If you are writing a function and you do not intend to modify a parameter, you can declare that it is a constant reference parameter. The syntax looks like this:

void printTime (const Time& time) ...
Time addTime (const Time& t1, const Time& t2) ...

I’ve included only the first line of the functions. If you tell the compiler that you don’t intend to change a parameter, it can help remind you. If you try to change one, you should get a compiler error, or at least a warning.

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