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Posted 26 Nov 2010

C++0x Dynamic Message Passing

, 26 Nov 2010 CPOL
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C++0x Dynamic Message Passing Ala Objective-C


Sometimes it is useful to have dynamic message passing as in Objective-C. The small header presented below allows any class to contain a dynamic message map that can be used to add methods dynamically to an object.


Objective-C is deemed more flexible than C++ because it allows a form of dynamic dispatch known as message passing.

Message passing is a form of dynamic dispatch that does not require implementation of a specific interface type. When sending a message to a target object, it is unknown to the compiler if the message can be handled by the object or not. It is only during the execution of the program that conformance to an interface is discovered.

Message passing is quite flexible because it does not require a lot of planning. Objective-C has been praised for this feature as more flexible than C++. This article demonstrates how easy it is to do the same in standard C++ (c++0x).

Using the Code

Using the code is very easy. The following steps have to be followed:

  • Include the header "mp_object.hpp" in your project.
  • Inherit from class mp_object.
  • Add dynamic methods to your object by using the method 'add_method'.

In order to add methods to an object, you need a prototype function. A prototype function can be any free-standing function. The following code is an example of how to declare prototypes and add dynamic methods to your code:

#include <iostream>
#include "mp_object.hpp"
using namespace std;

//prototype method

void draw() {}

//prototype method

void setColor(int color) {}


class rect : public mp_object {

    rect() {
        add_method(::draw, &rect::draw);
        add_method(::setColor, &rect::setColor);


    void draw() {
        cout << "rect\n";

    //set color

    void setColor(int color) {
        cout << "rect color = " << color << "\n";


class circle : public mp_object {

    circle() {
        add_method(::draw, &circle::draw);
        add_method(::setColor, &circle::setColor);


    void draw() {
        cout << "circle\n";

    //set color

    void setColor(int color) {
        cout << "circle color = " << color << "\n";

You can invoke an object dynamically like this:

int main() {
    rect r;
    circle c;


    r.invoke(setColor, 10);
    c.invoke(setColor, 20);

    return 0;

The output of the above is:

rect color = 10
circle color = 20

Implementation Details

The implementation is very simple, really. The class mp_object has a hash map of prototype function pointers to method pointers. When a method is invoked, the appropriate method pointer is retrieved from the prototype function and executed.

In order to minimize memory usage, the class mp_object uses copy on-write: a method map is shared between many instances, until it is modified. If modified, the method map is copied and the change is applied to the copy.

The mp_object code is 39 lines of code, thanks to C++0x. If a previous version of C++ was used, then a lot of boilerplate code would be needed for doing essentially the same thing.

The code can easily be expanded with new capabilities; for example, querying an object if it supports a specific method or a form of reflection (combined with macros). These are beyond the scope of this short article, though.


  • 26th November, 2010: Initial post


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Achilleas Margaritis
Software Developer (Senior)
Greece Greece
No Biography provided

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Comments and Discussions

GeneralMy vote of 2 Pin
Wessam Fathi14-Dec-10 4:33
memberWessam Fathi14-Dec-10 4:33 
GeneralGood idea, also I suggest putting the "message" functions in a separate namespace Pin
wtwhite9-Dec-10 1:49
memberwtwhite9-Dec-10 1:49 
GeneralRe: Good idea, also I suggest putting the "message" functions in a separate namespace Pin
Achilleas Margaritis9-Dec-10 2:10
memberAchilleas Margaritis9-Dec-10 2:10 
GeneralMy vote of 5 Pin
Jim Crafton2-Dec-10 9:22
memberJim Crafton2-Dec-10 9:22 
GeneralReally good start Pin
Jim Crafton2-Dec-10 9:16
memberJim Crafton2-Dec-10 9:16 
GeneralRe: Really good start Pin
Achilleas Margaritis3-Dec-10 2:28
memberAchilleas Margaritis3-Dec-10 2:28 
GeneralMy vote of 2 Pin
xComaWhitex29-Nov-10 9:57
memberxComaWhitex29-Nov-10 9:57 
GeneralMy vote of 4 Pin
alex_dacosta29-Nov-10 2:20
memberalex_dacosta29-Nov-10 2:20 
GeneralMy vote of 1 Pin
gbb2127-Nov-10 10:36
membergbb2127-Nov-10 10:36 

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