I would like help with my basic callback function pointer library

This is a discussion on I would like help with my basic callback function pointer library within the C++ Programming forums, part of the General Programming Boards category; Hi all, I am attempting to wrap my brain around function pointers. I created two files, exeCallback.h and exeCallback.cpp. I ...

  1. #1
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    Nov 2012
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    Casa Grande, AZ
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    I would like help with my basic callback function pointer library

    Hi all,
    I am attempting to wrap my brain around function pointers. I created two files, exeCallback.h and exeCallback.cpp. I am receiving the error: "expression must have (pointer-to-) function type". The error resides at:
    Code:
    foo(2);
    After reading that error, I edited the line that was giving me the error to
    Code:
    exeCallback->foo(2);
    .
    Code:
    exeCallback->foo(2);
    throws two new errors that are, expected an identifier and expected a type specifier. What am I not doing to get my basic callback library to work successfully?

    exeCallback.h code:
    Code:
    /*
    File: exeCallback.h
    
    Header file for exeCommand Library.
    */
    
    #ifndef EXECALLBACK_H
    #define EXECALLBACK_H
    
    #include "mbed.h"
    
    #include <functional>
    
    #include <map>
    
    class exeCallback
    {
    public:
        exeCallback();
        
        void my_int_func(int x);
        
        void (exeCallback::*foo)(int);
    
    private:
    };
    
    #endif
    exeCallback.cpp file code:
    Code:
    /*
    File: exeCallback.cpp
    
    Execute functions in other Sensor libraries/classes
    
    Constructor
    */
    
    #include "mbed.h"
    #include "ConfigFile.h"
    #include "msExtensions.h"
    #include "cfExtensions.h"
    #include "exeCallback.h"
    
    exeCallback::exeCallback()
    {
        
        foo = &exeCallback::my_int_func;
    
        foo(2);
    
    }
    
    void exeCallback::my_int_func(int x)
    {
        printf( "%d\n", x );
    }

  2. #2
    Registered User
    Join Date
    Jun 2005
    Posts
    6,290
    You're not playing with pointers to functions. You're playing with pointers to members (it just so happens the member you're playing with is a member function). They are different things.

    To use a pointer to member it is necessary to explicitly tell the compiler what object the member is for. So ... foo(2) needs to be
    Code:
            (this->*foo)(2);
    or
    Code:
           (*this.*foo)(2);
    Right 98% of the time, and don't care about the other 3%.

  3. #3
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    Casa Grande, AZ
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    Quote Originally Posted by grumpy View Post
    You're not playing with pointers to functions. You're playing with pointers to members (it just so happens the member you're playing with is a member function). They are different things.

    To use a pointer to member it is necessary to explicitly tell the compiler what object the member is for. So ... foo(2) needs to be
    Code:
            (this->*foo)(2);
    or
    Code:
           (*this.*foo)(2);
    Thank you. Your explaination helped.
    Code:
    (this->*foo)(2);
    worked successfully.

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