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Hello all,
Today I am approaching the wonderful community of Code Project, with a question about rewriting functions as macros in C/C++.

I'm currently learning C (and C++), and, as an activity to help me learn more, I'm writing a program that'll take a file, and read it, and then display the data in a way that is useful to the user. The project might be fairly useless, but so far, it has been worth it. (I've been able to learn File I/O, and I took the liberty of learning the basics of pointers, so that I would understand more about how File I/O works in C/C++)

Now, to get to the point of this question:
I wrote a function, that takes a string, and "explodes" it, based on a token (yes, I know there is tokstr, but again, this is for learning). I've been reading about macros lately, and I've gotten mixed-opinions about them.

On one hand, people say "yes, use macros [if you plan on reusing the code]", and on the other, people say "no, macros should be avoided in C++", so I'm sort of trying to figure out, should I invest time in writing a macro to replace this function?

Secondly, I'd like to know (even if I'm not going to write a macro for this function), what should I refrain from doing in a macro? I've also read that you should surround code in your macro(s), with a do/while loop, is there a specific reason?

I do apologize for the long question, but hopefully I'll be able to get some help.

Happy St. Patrick's Day,

Gigabyte Giant
Sergey Alexandrovich Kryukov 17-Mar-15 16:42pm
Why? why?!
Gigabyte Giant 17-Mar-15 16:43pm
Why what?
Sergey Alexandrovich Kryukov 17-Mar-15 16:56pm
If you have a function, why converting it to a macro?
Gigabyte Giant 17-Mar-15 16:58pm
I'd like to lay emphasis on the multiple times I said "this is for learning"... I'm trying to learn C/C++, therefor I want to soak up all the knowledge I can about it... I have collected numerous books about the languages, but I still have questions.
Sergey Alexandrovich Kryukov 17-Mar-15 21:42pm
I really appreciate that. Thank you for this explanation.
Albert Holguin 18-Mar-15 12:05pm
Learn to use macros... but also learn about the problems they cause. With that said, happy learning! :)

1 solution

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Solution 1

Why macros are good?

1. avoid repetitive typing (less risk of mistakes).

Examples: MFC, boost and ATL/WTL use macros for this reason.

2. trick existing code to be compiled with wildly different behavior.

Examples: Mock facilities for testing; swapping out definitions of new and delete for heap debugging.

3. generated code - source code not meant for hand-editing.

Why macros are bad?

1. Adversely impacts debugging.
a. single-step through a macro that expands to a dozen lines.
b. set a break point on a macro definition.

2. Hiding - macros can hide things from you - to make debugging and code review more difficult.

Example: #define true false

3. Trick existing code to be compiled with wildly different behavior.

4. Macros are not confined to a namespace.

Back in the day - before inline functions - a function defined as a macro was a way to have inline code instead of a plain function call. With the __inline keyword, this use of a macro is obsolete.


Use a macro only if absolutely needed and only if there is no other mechanism. Instead, use const, enum, and typedef for types and constants / literals.

Your interest in re-writing a function as a macro has much value as a learning exercise but no more. You will also find a few surprises once you start defining macros using other macros.

After you're done learning macros, you can eventually play with C++ templates - which are more more powerful than macros - but can be confined to a namespace.

Good luck.
Gigabyte Giant 17-Mar-15 14:02pm
Definitely interesting! +5 stars.
I'll refrain from converting future functions to macros, however, out of curiosity, what should I avoid doing when converting a function to a macro? How much will I need to change the syntax? Can I still use loops?
bling 17-Mar-15 15:15pm
Mutli-line macros require a backslash at the end of each line - except the last line so ...

void something_useful(const char *something, int x, int y)
int sum = x + y;
printf("Say %s sum is %d\n", something, sum);

... becomes ...

#define SAY(something, x, y) \
{ \
int sum = x + y; \
printf("Say %s sum is %d\n", something, sum); \

Notice that you also lose the compile time type safety in this example.
Gigabyte Giant 17-Mar-15 18:54pm
I see! Thanks for your help, accepting as solution!
Sergey Alexandrovich Kryukov 17-Mar-15 16:42pm
Nice explanation, a 5.
manchanx 17-Mar-15 17:31pm
+5. Well explained.

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

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