[C++][Language][Design] function as first class citizen in C++

Reference:
https://yinwang0.wordpress.com/2013/12/24/oop/
Prof. Peter Norvig on design pattern [1998]

https://github.com/boostcon/cppnow_presentations_2015/blob/master/files/functions_want_to_be_free.pdf

• Focus on the goal, not the syntax
• maximize encapsulation
• Not to write member functions
• Note: Or, if it's pure, consider writing it as callable object, which introduce a layer of 'namespace' and take advantage of EBO(Empty base class optimizations), [no_unique_address], refer to: [C++][accu2018] Tricks library implementation to know), i.e no constructor call overhead, class as a layer of namespace.
• Prefer non-friend free functions
• Isolates changes in implementation
• Forces implementation via the public interface
• Write them as algorithm instead member function.

How?

• Implement functions in terms of other functions
• Everything left over cannot be a normal function

Header:
<utility>

Keep using std::move_if_noexcept in mind.

With testing equivalence (keep Regular type in mind):

• operator==(lhs, rhs)
• operator<(lhs, rhs)
• use above 2 to do the rest of test.

Swap:

• Member swap should be deprecated
• Free function swap should rarely be specialized
• std::exchange

template<typename T>
void swap(T & lhs, T & rhs) {
rhs = std::exchange(lhs, std::move(rhs));
}

Erase:

• std::move (Defined in header <algorithm>)
• <iterator>

repeat_n:

• Range library.

operators that must be members:

• operator=
• operator[]
• operator()
• operator->

Sum up:

• If it must be a member, make it a member
• Virtual functions
• Member operators
• Constructors
• If it can be a non-friend function, make it free
• Only if no loss of efficiency
• Remember insert (carefully designed)
• Otherwise, maximize consistency

ISO Reference:

n4165.pdf

Comment on N4165, N4174