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    C++
     
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    General topics
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    Preprocessor
     
    Der Präprozessor unterstützt die bedingte Kompilierung von Teilen der Quelldatei. Dieses Verhalten wird durch #if, #else, #elif, #ifdef, #ifndef und #endif Richtlinien gesteuert .
    Original:
    The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef and #endif directives.
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    Inhaltsverzeichnis

    [Bearbeiten]Syntax

    #ifexpression
    #elifexpression
    #ifdefexpression
    #ifndefexpression
    #elseexpression
    #endifexpression

    [Bearbeiten]Erklärung

    Die bedingte Vorverarbeitung Block beginnt mit #if, #ifdef oder #ifndef Richtlinie, dann schließt gegebenenfalls eine beliebige Anzahl von #elif Richtlinien, dann schließt gegebenenfalls höchstens eine #else Richtlinie und ist mit #endif Richtlinie beendet. Alle inneren bedingte Vorverarbeitung Blöcke werden getrennt verarbeitet .
    Original:
    The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately.
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    Jeder #if, #elif, #else, #ifdef und #ifndef Richtlinien Steuercode Block bis zum ersten #elif, #else, #endif Richtlinie ohne Zugehörigkeit zu einem inneren bedingte Vorverarbeitung Blöcken .
    Original:
    Each of #if, #elif, #else, #ifdef and #ifndef directives control code block until first #elif, #else, #endif directive not belonging to any inner conditional preprocessing blocks.
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    #if, #ifdef und #ifndef Richtlinien testen Sie die angegebene Bedingung (siehe unten), und wenn es den Wert true ergibt, stellt die kontrollierte Code-Block. In diesem Fall nachfolgenden #else und #elif Richtlinien werden ignoriert. Andernfalls, wenn die angegebene Bedingung falsch ist, wird die kontrollierte Codeblock übersprungen und die nachfolgende #else oder #elif Richtlinie (falls vorhanden) verarbeitet wird. Im ersteren Fall wird der Code-Block durch die Richtlinie #else gesteuert bedingungslos zusammengestellt. Im letzteren Fall wirkt die #elif Richtlinie als ob es #if Richtlinie war: Kontrollen für den Zustand, kompiliert oder überspringt die kontrollierte Codeblock basierend auf dem Ergebnis, und im letzteren Fall verarbeitet nachfolgenden #elif und #else Richtlinien. Die bedingte Vorverarbeitungsblock wird durch #endif Direktive beendet .
    Original:
    #if, #ifdef and #ifndef directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent #else and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif directive acts as if it was #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif and #else directives. The conditional preprocessing block is terminated by #endif directive.
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    [Bearbeiten]Bedingungsauswertung

    [Bearbeiten]#if, #elif

    Die expression ist ein konstanter Ausdruck mit nur Literale und Kennungen, definiert mit #define Richtlinie. Jeder Bezeichner, die nicht wörtlich, nicht definierten Verwendung #define Richtlinie ausgewertet 0 .
    Original:
    The expression is a constant expression, using only Literale and identifiers, defined using #define directive. Any identifier, which is not literal, non defined using #define directive, evaluates to 0.
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    Der Ausdruck kann unäre Operatoren in Form enthalten definedidentifier oder defined (identifier) die 1 zurück, wenn die identifier wurde unter Verwendung von #define Richtlinie und 0 anders. Wenn die expression ausgewertet Wert ungleich Null, wird die kontrollierte Codeblock enthalten und übersprungen anders. Wenn eine verwendete Kennung ist keine Konstante, wird es mit ​0​ ersetzt .
    Original:
    The expression may contain unary operators in form defined identifier or defined (identifier) which return 1 if the identifier was defined using #define directive and 0 otherwise. If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with ​0​.
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    [Bearbeiten]#ifdef, #ifndef

    Prüft, ob die Kennung definiert wurde mit #define Richtlinie .
    Original:
    Checks if the identifier was defined using #define directive.
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    #ifdefidentifier entspricht im Wesentlichen #if defined(identifier) .
    Original:
    #ifdef identifier is essentially equivalent to #if defined( identifier).
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    #ifndefidentifier entspricht im Wesentlichen #if !defined(identifier) .
    Original:
    #ifndef identifier is essentially equivalent to #if !defined( identifier).
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    [Bearbeiten]Beispiel

    #define ABCD 2#include <iostream>   int main(){   #ifdef ABCDstd::cout<<"1: yes\n";#elsestd::cout<<"1: no\n";#endif   #ifndef ABCDstd::cout<<"2: no1\n";#elif ABCD == 2std::cout<<"2: yes\n";#elsestd::cout<<"2: no2\n";#endif   #if !defined(DCBA) && (ABCD < 2*4-3)std::cout<<"3: yes\n";#endif}

    Output: 

    1: yes 2: yes 3: yes
    Von „https://de.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&oldid=27744“
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