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Conditional inclusion

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C++
 
C++言語
一般的なトピック
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General topics
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フロー制御
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Flow control
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条件付き実行文
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Conditional execution statements
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Iteration statements
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式
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Expressions
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インラインアセンブリ
 
プリプロセッサ
 
プリプロセッサは、ソースファイルの一部の条件付きコンパイルをサポートしています。この動作は#if、#else、#elif、#ifdef、#ifndefと#endifディレクティブによって制御されます.
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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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目次

[編集] 構文

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

[編集] 説明

条件付きプリブロックは#ifで始まり、#ifdefまたは#ifndef指令、その後必要に応じて#elifディレクティブは、任意の数を含んで、その後必要に応じて、最大で1つの#elseディレクティブが含まれており、#endifディレクティブで終了します。内側のどの条件、前処理ブロックは別々に処理されます.
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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#ifの各々は、#elif、#else、#ifdefと#ifndefディレクティブの制御コード·ブロックまで初回#elif、#else、任意の内部条件、前処理ブロックに属さない#endifディレクティブ.
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と#ifndefディレクティブは、指定した条件をテストします（下記参照）、それが真と評価された場合に、制御されたコードブロックをコンパイルします。その場合には、後続の#elseと#elifディレクティブは無視されます。指定した条件がfalseと評価された場合、それ以外の場合、制御コード·ブロックはスキップされ、後続の#elseまたは#elifディレクティブが（もしあれば）処理されます。前者の場合には、#elseディレクティブで制御コード·ブロックは無条件にコンパイルされます。条件をチェックし、コンパイルしたり結果に基づいて、制御されたコードブロックをスキップし、後者の場合には、後続の#elifと#ifディレクティブを処理します後者の場合には、#elif·ディレクティブは、それが#else指令であるかのように動作します。条件付きプリブロックは#endifディレクティブで終了する.
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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[編集] 条件の評価

[編集] #if, #elif

expressionはリテラルディレクティブを使用して定義された唯一 #defineと識別子を使用して、定数式です。リテラルではありません任意の識別子は、 #defineディレクティブを使用して定義された非、 0と評価された.
Original:
The expression is a constant expression, using only リテラル and identifiers, defined using #define directive. Any identifier, which is not literal, non defined using #define directive, evaluates to 0.
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式はフォームに単項演算子が含まれている場合がありdefinedidentifierまたはdefined (identifier)1がidentifier指令と'そうでなければ0を使用して定義されている場合 #defineを返している。 expressionがゼロ以外の値に評価された場合、制御コード·ブロックが含まれ、それ以外の場合はスキップされます。任意に使用される識別子が定数でない場合、それは​0​に置き換えられます.
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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[編集] #ifdef, #ifndef

小切手識別子が #defineディレクティブを使用して定義されている場合.
Original:
Checks if the identifier was defined using #define directive.
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#ifdefidentifier#if defined(identifier)と基本的に同等です.
Original:
#ifdef identifier is essentially equivalent to #if defined( identifier).
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#ifndefidentifier#if !defined(identifier)と基本的に同等です.
Original:
#ifndef identifier is essentially equivalent to #if !defined( identifier).
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[編集] 例

このコードを実行します
#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
}

出力: 

1: yes
2: yes
3: yes
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