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Flow control
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Bedingte Ausführung Aussagen
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Conditional execution statements
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Exceptions
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Types
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decltype specifier (C++11)
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Inline Montage
 
 
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

#if expression
#elif expression
#ifdef expression
#ifndef expression
#else expression
#endif expression

[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 defined identifier 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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#ifdef identifier entspricht im Wesentlichen #if defined( identifier) .
Original:
#ifdef identifier is essentially equivalent to #if defined( identifier).
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#ifndef identifier 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 ABCD
    std::cout << "1: yes\n";
#else
    std::cout << "1: no\n";
#endif
 
#ifndef ABCD
    std::cout << "2: no1\n";
#elif ABCD == 2
    std::cout << "2: yes\n";
#else
    std::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