Thread: C++: lambdas - what is the best for inicializate the lambda?

Code:

#define event(EventName, ... ) std::function<void(__VA_ARGS__ )> EventName

if i do:

Code:

event(Create) =NULL;

on form procedure:

Code:

static LRESULT CALLBACK WndProcForm(HWND HandleWindow, UINT msg, WPARAM wParam, LPARAM lParam)
        {
            static bool blnInstCreated=false;
            static POINT PreviousLocation, Location;
            static bool Tracking = false;
            static MouseButtons MBButtons=MouseButtons::None;
            static bool blControl = false;
            static bool blShift = false;
            static bool blResize = false;
            static int xPos = 0;
            static int yPos = 0;
            const UINT_PTR timerid=0;
            const UINT_PTR KeyBoardTimer=2;
            const UINT_PTR JoystickTimer=1;
            static HMENU menuhandle=NULL;
            static bool blnDrag = false;
            static bool KeyPressed=false;
            static bool JoystickIsConneted=false;
            static bool blnMouseDown=false;//testing the mouse down



            form *inst = (form *)GetWindowLongPtr(HandleWindow, GWLP_USERDATA);

            if(inst!=NULL && inst->Create )
            {
                static bool i=true;
                 if(i==true)
                 {
                    blnInstCreated=true;
                    i=false;
                    inst -> Create();
                 }
            }

the Create is called.
but if i do:

Code:

event(Create) =[](){;};

the Create isn't called... why?
(i use a macro for be more easy)

Don't use a macro, use auto.

Code:

    auto f = [](){ cout << "hello world" << endl; };
    f();

Super easy.

Were you trying to initialize a std::function with a lambda? I guess you could do that but lambdas are a compiler-generated type so as whiteflags said, just use auto because the actual type of a lambda is only of concern to the compiler.

I think C++17 is implementing std::is_callable as well so you can build some pretty nice looking templated code.

but i'm asking 1 thing for an 'empty' lambda... it's:

Code:

[](){;}

or

Code:

[](){}

or

Code:

NULL

?????

Define an "empty" lambda. A lambda is a callable object. Being empty could mean something such as a non-existent object or a lambda that has an empty body.

Originally Posted by MutantJohn

Define an "empty" lambda. A lambda is a callable object. Being empty could mean something such as a non-existent object or a lambda that has an empty body.

i'm sorry, but from that 3 ways, what is the more correct?

My comments on your code are simple.

Code:

[](){} and [](){;}

should be the same, because the ; statement is a no-op.

What isn't the same is NULL. From what I can tell that originally worked for you because std::function has a nullptr_t constructor:

Code:

function( std::nullptr_t );

thanks for all to all... thank you

Originally Posted by Elkvis

Don't use NULL. Use nullptr. nullptr is guaranteed to be a pointer, and will not accidentally convert to int.

thank you for correct me