| ▲ | squirrellous 5 days ago | |
This might be an unpopular opinion - I think const by-value parameters in C++ shouldn’t exist. Const reference and mutable values are enough for 99% cases, and the other 1% is r-value refs. Regarding const by-value parameters, they should never appear in function declarations (without definition) since that doesn’t enforce anything. In function definitions, you can use const refs (which have lifetime extension) to achieve the same const-correctness, and const refs are better for large types. Admittedly this further proves the point that c++ is needlessly complicated for users, and I agree with that. | ||
| ▲ | quuxplusone 4 days ago | parent [-] | |
Absolutely correct. Basically, C++ has value semantics — you pass arguments of type X like `void f(X x)`, and you return them like `X f()`, and that's good enough for a first approximation. (This is the only thing C lets you do.) The second refinement is that you can use `const X&` as an optimization of `X`. (Perfectly safe for parameters; somewhat treacherous for return values.) Passing by `X&` without the const, or by `const X` without the ampersand, are both typos, and you should regularly use tooling to find and fix that kind of typo. https://quuxplusone.github.io/blog/2019/01/03/const-is-a-con... And that's it, for business-logic code. If you're writing your own resource-management type, you'll need to know about `X(X&&)` and `X& operator=(X&&)`, but ordinary business-logic code never does. "What about `X&` for out-parameters?" Pass out-parameters by pointer. It's important and helpful to indicate their out-parameter-ness at the call-site, which is exactly what passing by pointer does. (And the pointer value itself will be passed by value, just like in C.) "What about return by const value, like Scott Meyers recommended 20–30 years ago?" No, don't do that. It disables the ability to move-assign or move-construct from the return value, which means it's a pessimization. Scott found this out, retracted that advice in 2009, and correctly issued the opposite advice in his 2014 book. https://quuxplusone.github.io/blog/2019/01/03/const-is-a-con... At work I use a Clang patched with "-Wqual-class-return-type" to report return-by-const-value typos — since, again, `const X getter()` is almost always a typo for `const X& getter()`. You can use that compiler too: https://godbolt.org/z/7177MTfb8 | ||