1. learn.microsoft.com

    In this article. The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is kept in the translation unit.. Grammar. conditional: if-part elif-parts opt else-part opt endif-line if-part:
  2. stackoverflow.com

    #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?
  3. geeksforgeeks.org

    Jul 19, 2024#if, #else and #elif: All these directives works together and control compilation of portions of the program using some conditions. If the condition with the #if directive results in a non zero value, then the group of line immediately after the #if directive will be executed otherwise if the condition with the #elif directive evaluates to a ...
  4. stackoverflow.com

    Preprocessor directives are lines included in the code of programs preceded by a hash sign (#). These lines are not program statements but directives for the preprocessor. The preprocessor examines the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements.
  5. learn.microsoft.com

    A preprocessor directive must be the only instruction on a line. Nullable context. The #nullable preprocessor directive sets the annotations and warning flags in the nullable context. This directive controls whether nullable annotations have effect, and whether nullability warnings are given. Each flag is either disabled or enabled.
  6. en.cppreference.com

    Jun 30, 2023Explanation. The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif, #elifdef, or #elifndef (since C++23) directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately. Each of #if, #ifdef, #ifndef, #elif, # ...
  7. cs.auckland.ac.nz

    This directive checks to see if the identifier is not currently defined. 8.2.4 The #else Directive. The #else directive has the following syntax: #else newline. This directive delimits alternative source text to be compiled if the condition tested for in the corresponding #if, #ifdef, or #ifndef directive is false. An #else directive is optional.
  8. techonthenet.com

    Description. In the C Programming Language, the #else directive provides an alternate action when used with the #if, #ifdef, or #ifndef directives. The preprocessor will include the C source code that follows the #else statement when the condition for the #if, #ifdef, or #ifndef directive evaluates to false.
  9. sanfoundry.com

    But these construct affect program execution at run time. #if, #elif and #else are preprocessor directives which work alike if, else if and else construct but during pre-compilation of the program. They are used to make the compiler see what fragment of code to be compiled or skipped. Therefore, these constructs form conditional compilation.
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  1. Here's a suggestion, based largely on comments posted to your question:

    #if defined(CALC_MODE)
        typedef MyCalcClass ChosenClass;
    #elif defined(USER_MODE)
        typedef MyUserClass ChosenClass;
    #else
        #error "Define CALC_MODE or USER_MODE"
    #endif

    --Adrian Mole

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