On Tuesday, March 22, 2016 at 3:52:23 AM UTC-5, Bjorn Reese wrote:
On 03/21/2016 07:04 PM, Paul Fultz II wrote:
However, with lambdas it is the opposite. Lambdas default to const and then an explicit mutable keyword is needed for a mutable function object.
The const-by-default lambdas are the black swans of C++.
Other than working around bugs in MSVC, I have never used a mutable lambda in any of the codebases I have worked on. I find it quite surprising that const lambdas are considered a "black swan".
The fit documentation should at least provide a design rationale for its const-by-default decision which is addressed at those who are unfamiliar with the virtues of immutability.
Another concern with fit::mutable_ is its name. As you have already gone down the type-traits naming path with fit::decay, it may be an idea to rename fit::mutable_ to fit::remove_const.
No, fit::decay works something like this(ignoring unwrapping references): template<class T> std::decay_t<T> decay(T&& x) { return std::forward<T>(x); } It simply transforms the type according to the rules of the type trait std::decay. So, fit::remove_const would imply the same: template<class T> remove_const_t<T> decay(T&& x) { return const_cast<remove_const_t<T>>(x); } However, fit::mutable_ does not work that way at all. It is an adaptor, so it only accepts a function object, and returns a function object. Even more so,it does not remove const at all, not even from the function object call operator. Rather, fit::mutable_ creates a new function object with const added to the call operator. However, calling it fit::add_const would imply that it would prevent mutablilty which is not the case. So I think fit::mutable_ is a much better name, as it actually describes what is happening to the object using the same terminology that is used for lambdas. Plus, it the describes the C++ mechanism that is used to implement fit::mutable_ as well(that is the adaptor uses the mutable keyword).
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