What is the purpose of using #ifdef and #if in C++?
#ifdef means if defined. If the symbol following #ifdef is defined, either using #define in prior source code or using a compiler command-line argument, the text up to the enclosing #endif is included by the preprocessor and therefore compiled. #if works similarly, but it evaluates the boolean expression following it.
c preprocessor - The role of #ifdef and #ifndef - Stack Overflow
Text inside an ifdef/endif or ifndef/endif pair will be left in or removed by the pre-processor depending on the condition. ifdef means "if the following is defined" while ifndef means "if the following is not defined".
C ifdef, else, endif directives - Stack Overflow
Jan 10, 2015 · #ifdef allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is. For example: For example: #ifdef TABLE_SIZE int table[TABLE_SIZE]; #endif
c++ - How to add a 'or' condition in #ifdef - Stack Overflow
Oct 26, 2021 · This doesn't really answer the question "How to add a 'or' condition in #ifdef?" Please avoid code only answers, especially if you try to address the question indirectly. – user694733
Difference between preprocessor directives #if and #ifdef
You can demonstrate the difference by doing: #define FOO 0 #if FOO // won't compile this #endif #ifdef FOO // will compile this #endif
Boolean in ifdef: is "#ifdef A && B" the same as "#if defined(A ...
Conditional Compilation. You can use the defined operator in the #if directive to use expressions that evaluate to 0 or 1 within a preprocessor line.
How can I use "else if" with the preprocessor #ifdef?
Aug 8, 2021 · #ifdef CALC_MODE typedef MyCalcClass ChosenClass; #elifdef USER_MODE typedef MyUserClass ChosenClass; #else static_assert(false, "Define CALC_MODE or USER_MODE"); #endif So I can do. #define CALC_MODE right before this. I can resign the use of static_assert if needed. How can I do this?
#ifdef vs #if - which is better/safer as a method for enabling ...
May 28, 2017 · Using #ifdef the above becomes much more complicated, especially getting the #else part to work. I work on code that uses conditional compilation extensively and we have a mixture of #if & #ifdef. We tend to use #ifdef/#ifndef for the simple case and #if whenever two or more symbols are being evaluation.
c - #define, #ifdef #undef #endif - Stack Overflow
Mar 7, 2017 · The ifdef condition is evaluated when the preprocessor gets to it. When you undef PROC_ADD inside the ifdef'd code, the preprocessor has already decided which section of code to include and which to ignore. Furthermore, yes: ifdef, undef, etc are processed at pre-processing time -- the compiler never even sees these so-called directives. This ...
c++ - Is it a bad practice to use #ifdef in code? - Stack Overflow
In that case, #ifdef is also handier than trying to figure out all the dependencies per project per platform, per build type. So, just use it to focus on the platform specific abstraction you need, and use abstractions so the client code is the same -- just like reducing the scope of a variable ;)