Pointers to functions in arrays
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Thread: Pointers to functions in arrays

  1. #1
    Join Date
    Mar 2011
    Posts
    9

    Post Pointers to functions in arrays

    Hey so I'm experimenting with pointers to function and using them to call a desired function based on the number given.
    I have tried several things but none of them seem to work.

    What I want to do is have an array of pointers of functions defined in a base class (thats a mouth-full) and then call them in the top-level class based on the number I give. So I've made up some general piece of code that demonstrate what I've tried.
    hopefully you could help me with figuring this out.

    Thanks in advance.

    So here was just kind of my first attempt and then I looked online and tried starting from scratch and I just kept getting more confused.
    Hopefull this demonstrates what I want to do.
    Code:
    /*
    	Testing an array of functions
    
    	NOTES
    	+
    
    */
    #include <iostream>
    using namespace std;
    class testa{ //this class contains the member function that I want to run
    public:
    	testa();
    	int a(int x);
    	int b(int x);
    	int c(int x);
    	int d(int x);
    private:
    	int num;
    };
    testa::testa(){
    	num=0;
    }
    int testa::a(int x){
    	cout<<"a";
    	return x+1;
    }
    int testa::b(int x){
    	cout<<"b";
    	return x+1;
    }
    int testa::c(int x){
    	cout<<"c";
    	return x+1;
    }
    int testa::d(int x){
    	cout<<"d";
    	return x+1;
    }
    
    class testb: public testa{ //At first I tried declaring an instance of class testa but I ran into even more problems so I just made it public
    public:
    	testb();
    	void run_funcs(int x);
    private:
    	int (*F[4])(int x); //heres the array that I want to fill with functions
    };
    testb::testb(){
    	int (*F[4])(int x)={a, b, c, d}; //(this doesnt work either)  int (testa::*F[4])(int x)={a, b, c, d};
    }
    void run_funcs(int x){ //so this should print "abcd" to the screen
    	for(int i=0; i<4; i++){
    		//I feel like I should tell it what the included functions are from maybe?
    		//I tried declaring a testa a called g and then did:
    		//(g.*F[i])(x);
    		//and that didnt work either
    		(*F[i])(x);  
    	}
    }
    
    int main(){
    	int x=0;
    
    	run_funcs(x); 
    
    	//to pause the concole window(im working in an IDE)
    	char p;
    	cin>>p;
    return 0;
    }
    The errors I get from microsoft visual c++ express are:
    Code:
    1>------ Build started: Project: Pointer to function test, Configuration: Debug Win32 ------
    1>  test.cpp
    1>d:\my documents\visual studio 2010\projects\pointer to function test\pointer to function test\test.cpp(48): error C2440: 'initializing' : cannot convert from 'overloaded-function' to 'int (__cdecl *)(int)'
    1>          None of the functions with this name in scope match the target type
    1>d:\my documents\visual studio 2010\projects\pointer to function test\pointer to function test\test.cpp(48): error C2440: 'initializing' : cannot convert from 'overloaded-function' to 'int (__cdecl *)(int)'
    1>          None of the functions with this name in scope match the target type
    1>d:\my documents\visual studio 2010\projects\pointer to function test\pointer to function test\test.cpp(48): error C2440: 'initializing' : cannot convert from 'overloaded-function' to 'int (__cdecl *)(int)'
    1>          None of the functions with this name in scope match the target type
    1>d:\my documents\visual studio 2010\projects\pointer to function test\pointer to function test\test.cpp(48): error C2440: 'initializing' : cannot convert from 'overloaded-function' to 'int (__cdecl *)(int)'
    1>          None of the functions with this name in scope match the target type
    1>d:\my documents\visual studio 2010\projects\pointer to function test\pointer to function test\test.cpp(56): error C2065: 'F' : undeclared identifier
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

    Thanks again

  2. #2
    Join Date
    Aug 2008
    Posts
    902

    Re: Pointers to functions in arrays

    Oh boy, what are you really trying to accomplish with this?

    Take note that a function pointer is no good for pointing to a class function member.

    Code:
    class Base
    {
    public:
    	int FunctionA(int x)
    	{
    		std::cout << "A";
    		return x + 1;
    	}
    	int FunctionB(int x)
    	{
    		std::cout << "B";
    		return x + 1;
    	}
    };
    
    class Derived : public Base
    {
    public:
    	Derived()
    	{
    		Functions[0] = &Base::FunctionA;
    		Functions[1] = &Base::FunctionB;
    	}
    	void AllFunctions(int x)
    	{
    		for(int i = 0; i < 2; ++i)
    		{
    			(this->*Functions[i])(x);
    		}
    	}
    private:
    	int(Base::*Functions[2])(int);
    };

  3. #3
    Join Date
    Mar 2011
    Posts
    9

    Re: Pointers to functions in arrays

    This is extremely helpful.
    Just learned about the this pointer.
    I'm going to have to do a bit more reading.

    Thank you for such a thorough example.

  4. #4
    Lindley is offline Elite Member Power Poster
    Join Date
    Oct 2007
    Location
    Fairfax, VA
    Posts
    10,891

    Re: Pointers to functions in arrays

    You may find it easier to use a std::function (if your compiler supports it) rather than a function pointer.

  5. #5
    Join Date
    Mar 2011
    Posts
    9

    Re: Pointers to functions in arrays

    hmmm.
    std::function?
    I cant seem to find an exact definition or use on the internet. Does it have to do with templates?
    Or is it just a simple function used in main on the class?
    basically: is function, here, a keyword?


    And I hope this question isn't too easy.
    Last edited by wsad597; June 3rd, 2011 at 06:34 PM. Reason: added last line

  6. #6
    Join Date
    Jun 2008
    Posts
    592

    Re: Pointers to functions in arrays

    std::function and std::bind are upcoming features for c++0x.
    look at boost's library for info since c++0x versions are based off of boost.
    http://en.wikipedia.org/wiki/C%2B%2B...nction_objects
    http://www.boost.org/doc/libs/1_46_1...bind/bind.html

    simple examples
    Code:
    class cButton
    {
    public:
        cButton( cWindow& Window );
        function< void () > OnClicked;
    };
    Code:
    #include "cGui.h"
    
    using namespace std;
    
    cWindow Window;
    cButton Button( Window );
    
    void Button_Clicked()
    {
        Button.SetName( TEXT( "Clicked" ) );
    }
    
    int main()
    {
        Button.OnClicked = Button_Clicked;
        Button.SetName( TEXT( "Click" ) );
        Button.SetSize( 125, 25 );
        Window.SetInnerSize( 125, 25 );
        Window.Show();
        return RunApp();
    }
    Code:
    #include "cGui.h"
    
    using namespace std;
    
    class cForm
    {
        cWindow Window;
        cButton Button;
    
        void Button_Clicked()
        {
            Button.SetName( TEXT( "Clicked" ) );
        }
    
    public:
        cForm() : Window(), Button( Window )
        {
            Button.OnClicked = bind( &cForm::Button_Clicked, this );
            Button.SetName( TEXT( "Click" ) );
            Button.SetSize( 125, 25 );
            Window.SetInnerSize( 125, 25 );
            Window.Show();
        }
    };
    
    int main()
    {
        cForm Form;
        return RunApp();
    }
    0100 0111 0110 1111 0110 0100 0010 0000 0110 1001 0111 0011 0010 0000 0110 0110 0110 1111 0111 0010
    0110 0101 0111 0110 0110 0101 0111 0010 0010 0001 0010 0001 0000 0000 0000 0000
    0000 0000 0000 0000

  7. #7
    Join Date
    Aug 2008
    Posts
    902

    Re: Pointers to functions in arrays

    If you have a non-ancient version of GCC or MSVC 2010, just include <functional> to get std::function. If you have MSVC 2008, it's called std::tr1::function instead. If you have an even older compiler, you can use boost::function.

    Code:
    #include <functional>
    
    class MyClass
    {
    public:
        int SomeMemberFunction(int x);
        static int SomeStaticMemberFunction(int x);
    };
    
    int SomeFreeFunction(int x);
    
    using std::function;
    using std::bind;
    using std::placeholders::_1;
    
    int main()
    {
        MyClass c1;
    
        function<int(int)> Func1 = SomeFreeFunction;
        function<int(int)> Func2 = MyClass::SomeStaticMemberFunction;
        function<int(int)> Func3 = bind(&MyClass::SomeMemberFunction, c1, _1);
    
        Func1(42);
        Func2(42);
        Func3(42);
    }

  8. #8
    Join Date
    Mar 2011
    Posts
    9

    Re: Pointers to functions in arrays

    Whoa!
    Okay, thats really interesting. Ya that is probably a more straightforward solution.

    Thank you.

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