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Home » Tip Bank » C++
Language: C++
Expertise: Beginner
Oct 24, 2000

The const pass-by-value

Question:

void foo(const int x) {
  // not altering x
}

Herb Sutter in Exceptional C++, pg.177 says, "const pass-by-value is unuseful and misleading at best." I disagree, at a glance you know x isn't going to change throughout the function I feel that's useful to know. It's 50/50 amongst people in our office. Where do you stand?

Answer:
const in pass-by-value is indeed redundant. The function can't make changes to the original variable anyway, because it has a local copy thereof. Now one can argue that even in that case, the use of const documents the fact that the function doesn't change its local copy. However, do we really care about this? If you are the implementer of the function, you probably do but you certainly don't need to document this fact, as it is merely an implementation detail, not a part of the interface. Users of this function certainly don't care; all they need to know is that the original variable can't be altered, and the use of pass-by-value, with or without const, already guarantees that. To conclude, the use of const in pass-by-value is an example of over-specification that exposes an implementation detail. In general, exposing implementation details is bad object-oriented programming practice.

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