Force the compiler to ignore some lines in the program

Question

Suppose that I have 10,000 lines of C++ code. 200 lines of this code are for testing purpose (for example, check the program and show an error message).

Is there an way in C++ to ignore or consider some lines of the code (maybe with preprocessor keywords)?

Answer 1

Short answer:

Use macros and #ifdef checking. For example:

#ifdef MY_CONTROL_MACRO ... #endif 

the code within this scope will only be compiled if you already defined the MY_CONTROL_MACRO macro.

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More stuff:

1. To define such a macro, you can

   • Add #define MY_CONTROL_MACRO to your code. Or,
   • For VS, add MY_CONTROL_MACRO to Project > Properties > C/C++ > Preprocessor > Preprocessor Definitions. Or,
   • For GCC, compile your code with option -DMY_CONTROL_MACRO.

2. You can check out here for more info.

   This block is called a conditional group. controlled text will be included in the output of the preprocessor if and only if MACRO is defined. We say that the conditional succeeds if MACRO is defined, fails if it is not.

   The controlled text inside of a conditional can include preprocessing directives. They are executed only if the conditional succeeds. You can nest conditional groups inside other conditional groups, but they must be completely nested. In other words, ‘#endif’ always matches the nearest ‘#ifdef’ (or ‘#ifndef’, or ‘#if’). Also, you cannot start a conditional group in one file and end it in another.

3. You can also use the advanced ifdef-else-endif style:

   #ifdef MY_CONTROL_MACRO     ... // this part will be valid if MY_CONTROL_MACRO is defined #else     ... // this part will be valid if MY_CONTROL_MACRO is NOT defined #endif 

Answer 2

Surround the code with "#ifdef...#endif", and then use the compiler options to set the flag:

#ifdef MYTEST_ONLY_FUNCTIONALITY_ENABLED ... #endif 

You can then use the compiler options to include this code. For example, in GCC:

-DMYTEST_ONLY_FUNCTIONALITY_ENABLED 

Though, to be honest, I think this approach is generally not very maintainable in large projects and, if possible, it is generally better to simply move the test-only code to a completely separate library (without this conditional logic) and simply link that code into your test binary rather than your non-test binary. That also avoids having to compile each of the other libraries in both debug and non-debug modes.