What are they for and how to use them?

    3 answers 3

    # placed before the macro argument turns it into a string. For example, the code:

    #define QUOTE(a) #a int main() { cout << QUOTE(2 + 2) << " = " << 4 << endl; }

    after the preprocessor work will turn into

     int main() { cout << "2 + 2" << " = " << 4 << endl; }

    A useful example of use is the ASSERT debug macro. Example of implementation:

     #define ASSERT(c) \ if (!c) { \ std::cerr << "Assertion failed in " << __FILE__ \ << "(line " << __LINE__ << ") : " << #c << std::endl; \ exit(1); \ }

    If you give it a false condition to it, for example ASSERT (2 * 2! = 4), then it will output something like:

     Assertion failed in test.cpp(line 12) : 2 * 2 != 4

    and complete the program.

    ## is needed to peck macro arguments with other parts of the macro or with each other. It is rarely used, mainly for code generation. Wikipedia example:

     #define MYCASE(item,id) \ case id: \ item##_##id = id;\ break switch(x) { MYCASE(widget,23); }

    After preprocessing turns into:

     switch (x) { case 23: widget_23 = 23; break; }

      At the beginning of the line, # means the actual beginning of the preprocessor directive. In the middle of a directive, this is what it means. The fact is that if the macro is inside the quotation marks, no replacement is made. If we need to enclose a macro change in quotes, we can write:

       #define macro(a) puts(#a)

      Line

       macro (text);

      will be deployed like this

       puts ("text");

      Two ## signs will produce, when deployed, a concatenation of the actual macro parameters:

       #define macro(a, b) a##b macro (name, 1)

      unfold in

       name1

      See K & R ch. 4.11.2

        Something like this .

        Preprocessor directives:

         #define CONSTANT value //ΠΎΠΏΡ€Π΅Π΄Π΅Π»Π΅Π½ΠΈΠ΅ константы #include file_name //Π²ΠΊΠ»ΡŽΡ‡Π΅Π½ΠΈΠ΅ Π² ΠΏΡ€ΠΎΠ΅ΠΊΡ‚ Ρ„Π°ΠΉΠ»Π° file_name #pragma option //Π·Π°Π΄Π°Π΅ΠΌ машинно-зависимыС ΠΎΠΏΡ†ΠΈΠΈ(ΠΎΠΏΡ†ΠΈΠΈ компилятора) #define CONSTANT(paaram1) definition //опрСдСляСм макрос #ifdef CONSTANT //Π²ΠΊΠ»ΡŽΡ‡Π°Π΅Ρ‚ дальнСйший ΠΊΠΎΠ΄, Ссли константа ΠžΠŸΠ Π•Π”Π•Π›Π•ΠΠ #ifndef CONSTANT //Π²ΠΊΠ»ΡŽΡ‡Π°Π΅Ρ‚ дальнСйший ΠΊΠΎΡ‚ Ρ‚ΠΎΠ»ΡŒΠΊΠΎ Ссли константа НЕ ΠΎΠΏΡ€Π΅Π΄Π΅Π»Π΅Π½Π° #else //ΠΈΡΠΏΠΎΠ»ΡŒΠ·ΡƒΠ΅Ρ‚ΡΡ с двумя ΠΏΡ€Π΅Π΄Ρ‹Π΄ΡƒΡ‰ΠΈΠΌΠΈ. Π’ΠΊΠ»ΡŽΡ‡Π°Π΅Ρ‚ ΠΈΠ»ΠΈ ΠΈΡΠΊΠ»ΡŽΡ‡Π°Π΅Ρ‚ дальнСйший //ΠΊΠΎΠ΄ Π² зависимости ΠΎΡ‚ выполнСния условия ΠΏΡ€Π΅Π΄. Ρ€Π°Π·Π΄Π΅Π»Π° #endif //ΠΊΠΎΠ½Π΅Ρ† условно Π²ΠΊΠ»ΡŽΡ‡Π°Π΅ΠΌΡ‹Ρ…/ΠΈΡΠΊΠ»ΡŽΡ‡Π°Π΅ΠΌΡ‹Ρ… Ρ„Ρ€Π°Π³ΠΌΠ΅Π½Ρ‚ΠΎΠ² ΠΊΠΎΠ΄Π°

        Like that...

        UPD: The ## operator is used very infrequently, so you don’t have to think about it ...

        • It is used infrequently because they do not think about it. And we are talking about operators and not about directives. - Alexey Kotov
        • We are talking about their use! Can anyone describe the operator without giving examples of use? Some kind of nonsense ... UPD: not just to describe, but understandably describe! - 3JIoi_Hy6