Preprocessors - C Interview Questions VII

Q. Is it better to use a macro or a function?

The answer depends on the situation you are writing code for. Macros have the distinct advantage of being more efficient (and faster) than functions, because their corresponding code is inserted directly into your source code at the point where the macro is called. There is no overhead involved in using a macro like there is in placing a call to a function. However, macros are generally small and cannot handle large, complex coding constructs. A function is more suited for this type of situation.

Additionally, macros are expanded inline, which means that the code is replicated for each occurrence of a macro. Your code therefore could be somewhat larger when you use macros than if you were to use functions.

Thus, the choice between using a macro and using a function is one of deciding between the tradeoff of faster program speed versus smaller program size. Generally, you should use macros to replace small, repeatable code sections, and you should use functions for larger coding tasks that might require several lines of code.

Q. What is the best way to comment out a section of code that contains comments?

Most C compilers offer two ways of putting comments in your program. The first method is to use the /* and */ symbols to denote the beginning and end of a comment. Everything from the /* symbol to the */ symbol is considered a comment and is omitted from the compiled version of the program. This method is best for commenting out sections of code that contain many comments. For instance, you can comment out a paragraph containing comments like this:

/*

This portion of the program contains

a comment that is several lines long

and is not included in the compiled

version of the program.

*/

The other way to put comments in your program is to use the // symbol. Everything from the // symbol to the end of the current line is omitted from the compiled version of the program. This method is best for one-line comments, because the // symbol must be replicated for each line that you want to add a comment to. The preceding example, which contains four lines of comments, would not be a good candidate for this method of commenting, as demonstrated here:

// This portion of the program contains

// a comment that is several lines long

// and is not included in the compiled

// version of the program.

You should consider using the /* and */ method of commenting rather than the // method, because the // method of commenting is not ANSI compatible. Many older compilers might not support the // comments.