Thread: macros: can i test if a function was defined?

can i test if a function was defined for i see what function to use?
if i call a not defined function, i will get 2 compilers errors. so for resolve the problem, i must use macros.
i'm trying do some code but without success

Code:

#ifdef clsPointer->MouseClick            
                 clsPointer->MouseClick();
        #else
            MouseClick();
        #endif

these code isn't completed, but for show what i mean.
if i define the clsPointer->MouseClick, the compiler must use the clsPointer->MouseClick, but if sin't, the compiler must use MouseClick.
so what i miss on these code?

No, you can't. Not in the way you want, anyway.

You could have an array of function pointers that its element is NULL when a function doesn't exist, like a C++ map. But of course there's no way to generate it automagically, it'd have to be maintained manually by the user.

For example:

Code:

void foo()
{
    // ....
}

void bar()
{
    // ....
}

typedef void (*funcPtr)();

std::map<std::string, funcPtr> myFunctions;
myFunctions["foo"] = foo;
myFunctions["bar"] = bar;

if (myFunctions.find("bar") != myFunctions.end()) {
    myFunctions["bar"]();
}

EDIT: Wait... now that I think about it, aren't virtual methods saved as functions pointers inside an object? If you could check whether a virtual method existed or not, that would be great, but I don't know.

No, you can't do this with macros, because macros are evaluated before function definitions are evaluated.

There are several facilities in c++ that allow choosing between a specialized behavior, and a general purpose behavior if the specialization does not exist. These work such that at the call site calls the most general function, but a specialized implementation is invoked based on the type of th object calling it.

For example

Code:

class DefaultContext {};
struct ClsContext { //I'm only using a struct here to keep the example short. 
    Cls * clsPtr;
};

void mouseClick(DefaultContext)
{
   mouseClick();
}

void mouseClick(ClsContext context)
{
   context.clsPrt->mouseClick();
}

int main()
{
  auto context = someFactoryToGetContext();
  mouseClick(context); // calls the correct function depending on the context type.
}