Implementing a callback for in in C++?

Question

As the title asks, I find myself unable to implement a straightforward (ha) callback function in C++.

Now if you're thinking "I'm sure I've seen a question/answer for that before", you're absolutely correct, namely: Callback functions in c++

The reason I am posting anew is that I cannot get anything to work. I've tried applying the 1st answer that I fully understand [1], directly to my code; but I get 30 errors.

The first problem of these 30 errors, I don't understand. It's relating to the typedef declaration:

  typedef std::tr1::function<int (const GameCharacter&)> HealthCalcFunc;

It appears that 'tr1' is missing:

..\..gLib\..sor.h(47) : error C2039: 'function' : is not a member of 'std::tr1'

The following errors are reminiscent of a missing ';' gone missing or similar:

..\..gLib\..sor.h(47) : error C2143: syntax error : missing ';' before '<'
..\..gLib\..sor.h(47) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
..\..gLib\..sor.h(47) : error C2238: unexpected token(s) preceding ';'
..\..gLib\..sor.h(51) : error C2061: syntax error : identifier 'DataProcessorFunc'
..\..gLib\..sor.h(104) : error C2146: syntax error : missing ';' before identifier 'callbackFunc'

Moving down the page, there is another answer [2] that appears even more obvious. So I've created a fresh project with these two classes but again the errors beat me; I find myself unable to progress.

...\gamecharactertest\class1.h(17) : error C2146: syntax error : missing ';' before identifier 'F1'
    ...\gamecharactertest\class1.h(17) : error C2182: 'CALLBACK' : illegal use of type 'void'
jom:    ...\GameCharacterTest\Makefile.Release [release\class1.obj] Error 2
    ...\gamecharactertest\class1.h(17) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    ...\gamecharactertest\class1.h(17) : warning C4183: 'F1': missing return type; assumed to be a member function returning 'int'
    ...\gamecharactertest\class1.h(19) : error C2143: syntax error : missing ':' before '}'
class1.cpp(11) : error C2143: syntax error : missing ';' before 'C1::F1'
class1.cpp(11) : error C2182: 'CALLBACK' : illegal use of type 'void'
class1.cpp(12) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    [...]
    ...\gamecharactertest\class2.h(16) : error C2143: syntax error : missing ')' before '<tag>::*'
    ...\gamecharactertest\class2.h(16) : error C2657: 'C1::*' found at the start of a statement (did you forget to specify a type?)
    ...\gamecharactertest\class2.h(16) : error C2059: syntax error : ')'
    ...\gamecharactertest\class2.h(16) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    ...\gamecharactertest\class2.h(16) : warning C4183: 'pfnCallBack': missing return type; assumed to be a member function returning 'int'
    ...\gamecharactertest\class2.h(21) : error C2061: syntax error : identifier 'pfnCallBack'
class2.cpp(10) : error C2061: syntax error : identifier 'pfnCallBack'
class2.cpp(14) : error C2065: 'pFn' : undeclared identifier

If someone could be so kind as to point out where I have screwed up, I would be eternally grateful. Many thanks in advance.

[1] https://stackoverflow.com/a/2298291/2903608

[2] https://stackoverflow.com/a/24899454/2903608

--------------------- Edit --------------------

I have amended my question and I'm adding this in reply to this skyking's reply. It's very good, I think I'm just failing at the final hurdle though.

I have a header file (class1.h) containing "class A" as illustrated in his reply:

#ifndef CLASS1_H
#define CLASS1_H

#include <QCoreApplication>

class A {
    void callback(int value) {
        printf("callback: got %d\n", value);
    }
};
#endif // CLASS1_H

and its associated (class1.cpp) containing its sole method:

#include "class1.h"

void do_something(int value, A* obj, void (A::*cb)(int)) {
    (obj->*cb)(value);
}

and in my file (mainly.cpp) I've placed the function definitions and the main function:

#include <QCoreApplication>
#include "class1.h"

using namespace std;

void callback(int value) {
     printf("callback: got %d\n", value);
}

void do_something(int value, void (*cb)(int)) {
     // do something
     cb(value); // call the callback
}

void do_anotherthing(int value, std::function<void(int)> const& cb) {
    cb(value);
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    A theA = A();
    do_anotherthing(45, theA.callback);

    // Original exercise, works fine
    do_something(42, callback);
    return a.exec();
}

So when I do the initial step, calling do_something(..) that works fine. But the final step fails. Noted that to avoid confusions over names, I have renamed skyking's two last definition of do_something() to be called to do_anotherthing(). However, I have the following errors which I don't know how to sort out:

main.cpp(15) : error C2039: 'function' : is not a member of 'std'
main.cpp(15) : error C2061: syntax error : identifier 'function'
main.cpp(16) : error C3861: 'cb': identifier not found

main.cpp(24) : error C3867: 'A::callback': function call missing argument list; use '&A::callback' to create a pointer to member
main.cpp(24) : error C2660: 'do_anotherthing' : function does not take 2 arguments

Finally, I would just like to take a moment to express my immense gratitude for the knowledge and speed of the replies on the occasions I needed to post a question here at SO. Thank you all very much indeed.

Answer 1

I think one could question the claim that you fully understand the answer(s).

To get an understanding of it I think you should probably start in the right end of the problem by cutting away all the bells-and-whistles that distracts from the core issue. Let's start with a plain C-ish solution:

void callback(int value) {
     printf("callback: got %d\n", value);
}

void do_something(int value, void (*cb)(int)) {
     // do something
     cb(value); // call the callback
}

int main() {
    do_something(42, callback);
    return 0;
}

nothing fancy - the callback is simply a pointer to a function that is then called.

Next step is maybe to watch what happens if you want to pass a method to do_something instead of a plain vanilla function. It will fail because a method is not of the same type as a function (because they cannot be used the same way, a method needs an object to be called while a function doesn't). This means that you would have to use a pointer to member of (a) class instead:

class A {
    void callback(int value) {
        printf("callback: got %d\n", value);
    }
};

void do_something(int value, A* obj, void (A::*cb)(int)) {
    (obj->*cb)(value);
}

However this has the problem that you have restricted youself to having a method of class A as the callback - you couldn't no longer use a plain function as callback. Here std::function comes to the rescue since it uses a wrapper that can wrap any kind of callables in a single object

void do_something(int value, std::function<void(int)> const& cb) {
    cb(value);
}

then you can call it with any kind of callbacks.