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  A Simple Association Template for C++
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At my previous employer, we used visual development tools to take UML and generate source code to implement some of the more complex relationships that UML can define but C++ doesn't have built in to the language. When I moved to my new employer, I found myself particularly missing simple associations, which are, essentially, a two-way pointer. So I set out to write up code to model them in C++ using templates.

A simple association is a simple, but powerful design tool and having an efficient implementation can be quite helpful. This template implements a simple (one to one) association. It can be used with other container classes to implement multiple or complex associations. I have included a sample main program which creates some dummy classes and tests things out. I figured that this code is something that not too many C++ developers have ever considered doing, but might find very useful. Enjoy.

Greg Taylor
gtaylor@falcon.csc.calpoly.edu

Currently browsing [assoc.zip] (2,870 bytes) - [assoc.h] - (3,586 bytes)

// assoc.h
//
// A simple association template
//
// Provides a simple 1 to 1 association between two container classes.  If
// one end of the association is set or broken, the other end is updated to
// match.  Assertions check that the application programmer doesn't attempt
// to double-set an association and other failure states.
//
// Instanciation:
// Provide a member in each container class of type 
//   assoc<myContainer type, associatedContainer type>
//
// This member must be constructed with its parameterized constructor,
// which takes a single parameter, a pointer to the container class (this).
//
// Public methods:
//
// link(assoc<other,my> *otherEnd) - This method sets up the linkage between
//   the two container classes.  Its argument is a pointer to the association
//   member in the other class.  Calling this method on one end of the 
//   association will set up both ends, so it only need be called once per
//   association.
//
// unlink(assoc<other,my> *otherEnd) - De-associates the two container classes.
//   There are two versions of this method, one with a parameter, which is 
//   simply the same pointer that was used to set up the association (a pointer
//   to the other end) and one without.  The parameterized version is safer to
//   use simply because it hs more assertion error checks, which may help to 
//   catch bugs earlier.
//
// Lastly, there is an overloaded type-cast for the association class to the
// type of the associated container.  This is useful for preserving the abstracted
// concept of the association.  It will return NULL if no association is currently
// set. The 'get' method returns the same thing.

#ifndef SIMPLE_ASSOCIATIONTEMPLATE
#define SIMPLE_ASSOCIATIONTEMPLATE

#include <assert.h>

#ifndef NULL #define NULL 0 #endif

template <class myContainer, class otherContainer> class assoc { private: myContainer *containedIn; assoc<otherContainer, myContainer> *linkedTo;

void linkToMe(assoc<otherContainer, myContainer> *otherEnd) { assert(otherEnd != NULL); assert(linkedTo == NULL); linkedTo = otherEnd; }

void unlinkFromMe(assoc<otherContainer, myContainer> *otherEnd) { assert(otherEnd != NULL); assert(linkedTo == otherEnd);

linkedTo = NULL; }

myContainer *getContainer() { return containedIn; }

public: assoc(myContainer *myOwner) { assert(myOwner != NULL); containedIn = myOwner; linkedTo = NULL; }

~assoc() { if (linkedTo != NULL) { linkedTo->unlinkFromMe(this); } } void link(assoc<otherContainer, myContainer> *target) { assert(target != NULL); assert(linkedTo == NULL);

linkedTo = target; target->linkToMe(this); }

void unlink(assoc<otherContainer, myContainer> *target) { assert(target == linkedTo); assert(linkedTo != NULL);

target->unlinkFromMe(this); linkedTo = NULL; }

void unlink() { // The unlink method with a parameter is prefered because it does additional // error checking. Either method will accomplish the same task, however. assert(linkedTo != NULL);

linkedTo->unlinkFromMe(this); linkedTo = NULL; }

operator otherContainer *() { if (linkedTo == NULL) return NULL; return linkedTo->getContainer(); }

otherContainer *get() { if (linkedTo == NULL) return NULL; return linkedTo->getContainer(); }

friend class assoc<otherContainer, myContainer>; };

#endif

Currently browsing [assoc.zip] (2,870 bytes) - [AssocTestMain.cpp] - (2,293 bytes)

// Simple association tester and example usage code
#include "assoc.h"
#include <stdio.h>

// two dummy classes that might have an association to each other. class Edge;

class Vertex { public: float x, y, z ; assoc<Vertex,Edge> edge;

Vertex() : edge(this) { x = 0.0f; y = 0.0f; z = 0.0f; } };

class Edge { public: float angle;

assoc<Edge,Vertex> vertex;

Edge() : vertex(this) { angle = 0; } };

// Test/example usage 'main' for simple associations. int main () { Vertex v1; Edge e1; Vertex *pv; Edge *pe;

// Set up an edge and vertices v1.x = 24.0f; v1.y = 14.0f; v1.z = 4.0f; e1.angle = 0.5;

// Associate v1 and e1 v1.edge.link(&e1.vertex);

// Test association v1/e1 directly printf("This should be %g => %g\n", v1.x, e1.vertex.get()->x); printf("and this should be %g => %g\n", e1.angle, v1.edge.get()->angle);

// That interface can be clumbsy so it can be cleaner to use the // overloaded cast operation. float i = ((Edge *)v1.edge)->angle;

// Or for the cleanest referencing, use pointers pv = e1.vertex; pe = v1.edge;

// Print out verification of expected results printf("This should be %g => %g\n", v1.y, pv->y); printf("and this should be %g => %g\n", e1.angle, pe->angle);

// Now test clearing the association pe->vertex.unlink(&pv->edge);

if (e1.vertex == NULL && v1.edge == NULL) printf("We successfully cleared the association\n"); else printf("Test FAILED.\n");

// Re-create the association. v1.edge.link(&e1.vertex);

// Now break it another way v1.edge.unlink();

// Test again if (e1.vertex == NULL && v1.edge == NULL) printf("We successfully cleared the association\n"); else printf("Test FAILED.\n");

// Now test the destructor Vertex *v2 = new Vertex;

v2->z = 3.1415f; v2->edge.link(&e1.vertex); pv = e1.vertex;

// Print out verification of expected results printf("This should be %g => %g\n", v2->z, pv->z); printf("and this should be %g => %g\n", e1.angle, pe->angle);

delete v2;

if (e1.vertex == NULL) printf("We successfully cleared the association\n"); else printf("Test FAILED.\n");

return 0; }

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