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# if RTC
/* some code */
# endif
Should the macro RTC be defined with a value? My compiler is not throwing an error. Do all compilers do the same? Isn't defining the macro safer?
425
A macro is a set of instructions used to execute repetitive tasks. You can record a set of commands and then play them back with one or two keystrokes. That means that you can save A LOT of time when doing routine and repetitive tasks.
A macro is an action or a set of actions that you can use to automate tasks. Macros are recorded in the Visual Basic for Applications programming language. You can always run a macro by clicking the Macros command on the Developer tab on the ribbon.
Macros are used to make a sequence of computing instructions available to the programmer as a single program statement, making the programming task less tedious and less error-prone. (Thus, they are called "macros" because a "big" block of code can be expanded from a "small" sequence of characters.)
Using Excel Macros can speed up work and save you a lot of time. One way of getting the VBA code is to record the macro and take the code it generates. However, that code by macro recorder is often full of code that is not really needed. Also macro recorder has some limitations.
In a preprocessing directive such as this, if the macro is not defined, it is treated as 0.
That is guaranteed by the language.
You can rely on there not being a compilation failure.
Here's the C++ wording:
[cpp.cond]/11: After all replacements due to macro expansion and evaluations of defined-macro-expressions, has-include-expressions, and has-attribute-expressions have been performed, all remaining identifiers and keywords, except fortrueandfalse, are replaced with the pp-number0, and then each preprocessing token is converted into a token. [..]
120
No, it doesn't have to be defined. If the identifier is undefined at the end of an #if expansion, it evaluates to 0.
From ANSI C90:
After all replacements due to macro expansion and the
definedunary operator have been performed, all remaining identifiers are replaced with the pp-number0.
This, however, may not be present in compilers from before C was standardized. I once saw a GCC warning about this (I think it was on MinGW) but I can't find a source for it right now.
Conclusion: All standards-conformant C compilers should not throw an error upon encountering this. You do not need to define the macro before using it in #if.
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