When.md

When

Examples are in examples/When.

/*Primary templates*/
template <typename ExprType, typename Case1Type, typename Return1Type, typename... Args>
auto when(ExprType&& expr, Case1Type&& case1, Return1Type&& return1, Args&&... args);     //1

template<typename Return1Type, typename ...Args>
auto when(bool case1, Return1Type&& return1, Args&&...args);                              //2

• 1 when that has an expression/pattern to be matched
• 2 when that doesn't has an expression/pattern to be matched and works as boolean switches

Globals

struct _Anything
{};
constexpr inline _Anything _;

Which defines operator== and returns true conditionlessly as a place-holder for supporting pattern matching.

!Important Node!

Because in C++, you have to return the same time from a function, and because lambda expressions are unique types, I did a little trick to get it done. Basically I check if the returned objects are not the same type and are not arrays, when will return a std::function object to wrap up the lambdas.

when(expression,
   case1, return1,  // --
                    //   | check if they are same type or not?
   case2, return2,  // --
   ...
);

Therefore, as a side effect, if you are not intended to get a function, you must ensure the return statements are the same type after std::remove_reference_t. Use casting if they are not.