[C++] friend function definition inside a type can be found only through ADL if without forward declaration.

void foo(){
  auto f = &foo;
}


struct Test{

  void foo(){
    auto f = &Test::foo;
  }

  friend void bar(){
    // auto f = &bar; // Use of undeclared identifier 'baz'

    void bar();
    auto f = &bar;
  }

  friend void baz(Test t){
    // Find through ADL
    baz(Test{});
    // auto f = &baz; // Use of undeclared identifier 'baz'
  }

};


int main()
{
}

[C++11][note] C++11 Library Design

Excerpt from Aerix Consulting

Study notes:

1. Function Interface Design

Is my function? :

• easy to call correctly?
• hard to call incorrectly?
• efficient to call?
• with minimal copying?
• with minimal aliasing?
• without unnecessary resource allocation?
• easily composable with other functions?
• usable in higher-order constructs?

What’s the best way of getting data into and out of a function?

• Input Argument Categories
• Read-only: value is only ever read from, never modified or stored
• const l-ref (except small ones)
• Sink: value is consumed, stored, or mutated locally
• Goal: Avoid unnecessary copies, allow temporaries to be moved in.
• const l-ref and copy it into local variable
• r-ref. Take the r-value into function.
• What if the function takes more than 1 sink argument?
• Take sink arguments by value.
  Since if passing in r-value, the r-value will be created as the l-value argument.
  No extra copy constructor call needed. (extra reference: WANT SPEED? DON’T (ALWAYS) PASS BY VALUE.)
• Eric Niebler : Out Parameters, Move Semantics, and Stateful Algorithms
• Encapsulate an algorithm’s state in an object that implements the algorithm.
  

2. Class Design
Can my type be…?:

• …copied and assigned?
• …efficiently passed and returned?
• …efficiently inserted into a vector?
• …sorted?
• …used in a map? An unordered_map?
• …iterated over (if it’s a collection)?
• …streamed?
• …used to declare global constants?

• Regular Types
• Reference:
• rules of three:
• http://stackoverflow.com/a/4172724
• Basically, int-like types.
• Copyable, default constructable, assignable, equality-comparable, swappable, order-able
• They let us reason mathematically
• The STL containers and algorithms assume regularity in many places
• 
• Make your types regular (if possible)
• **Make your types’ move operations noexcept (if possible)
• Q: Is my type Regular?
  A: Check it at compile time!

template<typename T>
struct is_regular
: std::integral_constant< bool,
std::is_default_constructible<T>::value &&
std::is_copy_constructible<T>::value &&
std::is_move_constructible<T>::value &&
std::is_copy_assignable<T>::value &&
std::is_move_assignable<T>::value >
{};
struct T {};
static_assert(is_regular<T>::value, "huh?");

Movable Types:

• The moved-from state must be part of a class’s invariant.
• If above doesn’t make sense, the type isn’t movable.
• Every movable type must have a cheap(er)-to-construct, valid default state.

3. Module

Think:

• In C++11, what support is there for…
• … enforcing acyclic, hierarchical physical component dependencies? 
• … decomposing large components into smaller ones? 
• … achieving extensibility of components? 
• … versioning (source & binary) components?

New library version with interface-breaking changes:

• Put all interface elements in a versioning namespace from day one
• Make the current version namespace inline

Name Hijacking: Unintentional ADL finds the wrong overload:

• template arguments are participating ADL.

ADL:
If the class is a class template instantiation, then the types of the template type arguments and the classes and namespaces in which the template template arguments are declared are also included.

Solution:

1. Use a non-inline ADL-blocking namespace

2. Use global function objects instead of free functions(i.e: Global function object, aka. functor)

C++14 Variable Templates:
cpp14_variable_templates

template<typename T> struct lexical_cast_fn 
{ 
template<typename U> T operator()(U const &u) const 
{ //... } 
};

template<typename T> constexpr lexical_cast_fn<T> lexical_cast{};

int main()
{ 
lexical_cast<int>("42");
}

Ode To Function Objects:

• They are never found by ADL!
• If phase 1 lookup finds an object instead of a function, ADL is disabled!
• They are first class objects
• Easy to bind
• Easy to pass to higher-order functions like std::accumulate

Guideline:

• Put type definitions in an ADL-blocking (non-inline!)
  namespaces and export then with a using declaration, or…
• Prefer global constexpr function objects over named free functions (except for documented customization points)

[C++] ADL

Argument-dependent lookup

Be aware of template arguments' namespace also being considered into name look up.

[C++11] Inline namespace

1. Put all interface elements in a versioning namespace from day one 

2. Make the current version namespace inline

3. inline blocked ADL namespace could specialize template inside blocked ADL namespace's primary template class/func on the outside namespace.