flipcode - Using Macros To Keep Data In Sync

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Using Macros To Keep Data In Sync
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If you've ever needed to keep multiple arrays of constant information in sync then this COTD tip is for you.

For example - here is a snippet from an imaginary profiler which needs to keep an enum, an array of strings for names and an array of budgets in sync:

In ProfileItem.h....

enum ProfileItem
{
	ProfileItem_Render,
	ProfileItem_Simulation,

	//Add new profile item here.

	ProfileItem_Max
};

And in ProfileItem.cpp...

const char * profileName[ProfileItem_Max]  = 
{
	"Render",
	"Simulation"

	//Add description for profile item here.
};

const int Budget[ProfileItem_Max] = 
{
	10,
	12

	//Add budget for profile item here.
};

The above code can easily go out of sync unless everyone on a team is very careful and always adds new items to all three areas in exactly the same order.

An alternative to the above is to use the following technique:

In ProfileItemX.h....

	XM(Render, 10)
	XM(Simulation,12)
	//Add new profile item and budget here...use format XM(item, budget)

Now in ProfileItem.h....

enum ProfileItem
{
#define XM(profileItem, budget) ProfileItem_##profileItem,
#include "ProfileItemX.h"
#undef XM
	ProfileItem_Max
};

And in ProfileItem.cpp...

const char * profileName[ProfileItem_Max]  = 
{
#define XM(profileItem, budget) #ProfileItem,
#include "ProfileItemX.h"
#undef XM
};

const int Budget[ProfileItem_Max] = 
{
#define XM(profileItem, budget) ##budget,
#include "ProfileItemX.h"
#undef XM
};

Now you and your team only need to change one line of code to add a new profile item and macro's keep everything in sync, leaving much less chance for error.

Thanks to Matt Curtis for introducing me to this technique.

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