在C++中，if前面加#是什么意思啊该如何解决
在C++中，if前面加#是什么意思啊？？？
在C++中，if前面加#是什么意思啊？？？
------解决方案--------------------
#if 是预编译。#if如果不是真的话， 其后面的代码是不会被编译的。跟#ifdef 差不多，只是#if后面跟的是条件，而 #ifdef后面跟的是一个宏，判断其是否定义。
------解决方案--------------------
#if 预处理命令。它的条件表达式必须是一个整数常量预处理表达式。
一般情况下用的比较少
The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is retained in the translation unit.
Grammar
conditional :
if-part elif-partsopt else-partopt endif-line
if-part :
if-line text
if-line :
#if constant-expression
#ifdef identifier
#ifndef identifier
elif-parts :
elif-line text
elif-parts elif-line text
elif-line :
#elif constant-expression
else-part :
else-line text
else-line :
#else
endif-line :
#endif
Each #if directive in a source file must be matched by a closing #endif directive. Any number of #elif directives can appear between the #if and #endif directives, but at most one #else directive is allowed. The #else directive, if present, must be the last directive before #endif.
The #if, #elif, #else, and #endif directives can nest in the text portions of other #if directives. Each nested #else, #elif, or #endif directive belongs to the closest preceding #if directive.
All conditional-compilation directives, such as #if and #ifdef, must be matched with closing #endif directives prior to the end of file; otherwise, an error message is generated. When conditional-compilation directives are contained in include files, they must satisfy the same conditions: There must be no unmatched conditional-compilation directives at the end of the include file.
Macro replacement is performed within the part of the command line that follows an #elif command, so a macro call can be used in the constant-expression.
The preprocessor selects one of the given occurrences of text for further processing. A block specified in text can be any sequence of text. It can occupy more than one line. Usually text is program text that has meaning to the compiler or the preprocessor.
The preprocessor processes the selected text and passes it to the compiler. If text contains preprocessor directives, the preprocessor carries out those directives. Only text blocks selected by the preprocessor are compiled.
The preprocessor selects a single text item by evaluating the constant expression following each #if or #elif directive until it finds a true (nonzero) constant expression. It selects all text (including other preprocessor directives beginning with #) up to its associated #elif, #else, or #endif.
If all occurrences of constant-expression are false, or if no #elif directives appear, the preprocessor selects the text block after the #else clause. If the #else clause is omitted and all instances of constant-expression in the #if block are false, no text block is selected.
The constant-expression is an integer constant expression with these additional restrictions:
Expressions must have integral type and can include only integer constants, character constants, and the defined operator.
The expression cannot use sizeof or a type-cast operator.
The target environment may not be able to represent all ranges of integers.
The translation represents type int the same as type long, and unsigned int the same as unsigned long.
The translator can translate character constants to a set of code values different from the set for the target environment. To determine the properties of the target environment, check values of macros from LIMITS.H in an application built for the target environment.
在C++中，if前面加#是什么意思啊？？？
------解决方案--------------------
#if 是预编译。#if如果不是真的话， 其后面的代码是不会被编译的。跟#ifdef 差不多，只是#if后面跟的是条件，而 #ifdef后面跟的是一个宏，判断其是否定义。
------解决方案--------------------
#if 预处理命令。它的条件表达式必须是一个整数常量预处理表达式。
一般情况下用的比较少
The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. If the expression you write (after the #if) has a nonzero value, the line group immediately following the #if directive is retained in the translation unit.
Grammar
conditional :
if-part elif-partsopt else-partopt endif-line
if-part :
if-line text
if-line :
#if constant-expression
#ifdef identifier
#ifndef identifier
elif-parts :
elif-line text
elif-parts elif-line text
elif-line :
#elif constant-expression
else-part :
else-line text
else-line :
#else
endif-line :
#endif
Each #if directive in a source file must be matched by a closing #endif directive. Any number of #elif directives can appear between the #if and #endif directives, but at most one #else directive is allowed. The #else directive, if present, must be the last directive before #endif.
The #if, #elif, #else, and #endif directives can nest in the text portions of other #if directives. Each nested #else, #elif, or #endif directive belongs to the closest preceding #if directive.
All conditional-compilation directives, such as #if and #ifdef, must be matched with closing #endif directives prior to the end of file; otherwise, an error message is generated. When conditional-compilation directives are contained in include files, they must satisfy the same conditions: There must be no unmatched conditional-compilation directives at the end of the include file.
Macro replacement is performed within the part of the command line that follows an #elif command, so a macro call can be used in the constant-expression.
The preprocessor selects one of the given occurrences of text for further processing. A block specified in text can be any sequence of text. It can occupy more than one line. Usually text is program text that has meaning to the compiler or the preprocessor.
The preprocessor processes the selected text and passes it to the compiler. If text contains preprocessor directives, the preprocessor carries out those directives. Only text blocks selected by the preprocessor are compiled.
The preprocessor selects a single text item by evaluating the constant expression following each #if or #elif directive until it finds a true (nonzero) constant expression. It selects all text (including other preprocessor directives beginning with #) up to its associated #elif, #else, or #endif.
If all occurrences of constant-expression are false, or if no #elif directives appear, the preprocessor selects the text block after the #else clause. If the #else clause is omitted and all instances of constant-expression in the #if block are false, no text block is selected.
The constant-expression is an integer constant expression with these additional restrictions:
Expressions must have integral type and can include only integer constants, character constants, and the defined operator.
The expression cannot use sizeof or a type-cast operator.
The target environment may not be able to represent all ranges of integers.
The translation represents type int the same as type long, and unsigned int the same as unsigned long.
The translator can translate character constants to a set of code values different from the set for the target environment. To determine the properties of the target environment, check values of macros from LIMITS.H in an application built for the target environment.