The mutable function adaptor allows using a non-const function object inside of a const-function object. In Fit, all the function adaptors use const call overloads, so if there is a function that has a non-const call operator, it couldn’t be used directly. So, mutable_ allows the function to be used inside of the call operator.

NOTE: This function should be used with caution since many functions are copied, so relying on some internal shared state can be error-prone.


template<class F>
mutable_adaptor<F> mutable_(F f)


F must be: