How to create a Callback function by Using C/C++

Question

does C\C++ Support a mechanism for callback function ? and how to create it ? I've written several code, to create callback function in C++ but it failed ..

    #include<iostream>
    using namespace std;

    void callee()
    {
         printf("callee\n"); 
         printf("calleeeeeeeeee\n"); 
    }

    void callback(void* func)
    {
         printf("callback\n");     
    }

    int main()
    {
        void (*call)(void*);
        void (*call2)(void);
        call2 = &callee;
        call = &callback;
        call((void*)call2);    
        getchar();
        return 0;    
    }

Answer 1

Here is an example of using callbacks with interfaces.

//listener observer design pattern
class MonitoringElements{
public:

//information to monitor

};

class ICondition{
public:
    virtual ~ICondition(){}

    virtual bool didTrigger(const MonitoringElements& current) = 0;
    virtual void risingEdge() = 0;
};

class MonitorManager final{
public:


    void addCondition(ICondition* condition){
        m_conditions.push_back(condition);
    }

    //constantly monitoring every x seconds
    void monitorOnce(){
        update();
        for(int i = 0; i < m_conditions.size(); ++i)
            if(m_conditions.at(i)->didTrigger(m_monitorElements)){
                m_conditions.at(i)->risingEdge();
            }
    }

private:
    void update(){
        //read sensor values here
        //update m_monitorElements;
    }

    MonitoringElements m_monitorElements;
    std::vector<ICondition*> m_conditions;
};

Answer 2

I don't know C++, I wrote up some C code, hope it helps

#include <stdio.h>

void call( int, void ( *f )( int ) );
void called( int );

int main( int argc, char const *argv[] ){
    printf( "start\n" );
    call( 1, called );
    printf( "end\n" );
    getchar();
    return 0;
}

void call( int a, void ( *f )( int ) ){
    printf( "doing stuff\n" );
    printf( "a: %d\n", a );
    printf( "going to call back\n" );
    f( a * 2 );
}

void called( int b ){
    printf( "b: %d\n", b );
    printf( "b*2: %d\n", b*2 );
    printf( "call back function being called\n" );
}

Calling call back functions in a function is no more than having a function pointer and call the function whenever you finished your planed job.

I modified the code and made it up to be more clear to show how you would use call backs.
This is the result you will see, if you compile and run it:

start
doing stuff
a: 1
going to call back
b: 2
b*2: 4
call back function being called
end

Well, either the author edited the question or someone edited it for him, there's no longer the C tag. Ignore this answer if you don't want to see pure C code. I'm just gonna leave it here in case anyone could be interested.

Answer 3

Well, you never called "callee" - you executed "callback" with "call(void*)call2);", but in order for your "callee" function to execute, you need to run it from within your "callback" function. Here, this works:

#include <iostream>
#include <stdio.h>
using namespace std;

void callee()
{
     printf("callee\n");
     printf("calleeeeeeeeee\n");
}

void callback(void* func)
{
    void (*mCallbackFunc)(void);

    printf("callback\n");

    mCallbackFunc = (void(*)())func;
    mCallbackFunc();
}

int main()
{
    void (*call)(void*);
    void (*call2)(void);
    call2 = &callee;
    call = &callback;

    call((void*)call2);

    return 0;
}

Output:

callback
callee
calleeeeeeeeee