下面是MSDN中的解释，你可以仔细看看
The #if, #elif, #else, and #endif Directives
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.
Syntax
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.
The expression must not perform any environmental inquiries and must remain insulated from implementation details on the target computer.
The preprocessor operator defined can be used in special constant expressions, as shown by the following syntax:
Syntax
defined( identifier )
defined identifier
This constant expression is considered true (nonzero) if the identifier is currently defined; otherwise, the condition is false (0). An identifier defined as empty text is considered defined. The defined directive can be used in an #if and an #elif directive, but nowhere else.
In the following example, the #if and #endif directives control compilation of one of three function calls:
#if defined(CREDIT)
credit();
#elif defined(DEBIT)
debit();
#else
printerror();
#endif
The function call to credit is compiled if the identifier CREDIT is defined. If the identifier DEBIT is defined, the function call to debit is compiled. If neither identifier is defined, the call to printerror is compiled. Note that CREDIT and credit are distinct identifiers in C and C++ because their cases are different.
The conditional compilation statements in the following example assume a previously defined symbolic constant named DLEVEL.
#if DLEVEL > 5
#define SIGNAL 1
#if STACKUSE == 1
#define STACK 200
#else
#define STACK 100
#endif
#else
#define SIGNAL 0
#if STACKUSE == 1
#define STACK 100
#else
#define STACK 50
#endif
#endif
#if DLEVEL == 0
#define STACK 0
#elif DLEVEL == 1
#define STACK 100
#elif DLEVEL > 5
display( debugptr );
#else
#define STACK 200
#endif
The first #if block shows two sets of nested #if, #else, and #endif directives. The first set of directives is processed only if DLEVEL > 5 is true. Otherwise, the statements after #else are processed.
The #elif and #else directives in the second example are used to make one of four choices, based on the value of DLEVEL. The constant STACK is set to 0, 100, or 200, depending on the definition of DLEVEL. If DLEVEL is greater than 5, then the statement
#elif DLEVEL > 5
display(debugptr);
is compiled and STACK is not defined.
A common use for conditional compilation is to prevent multiple inclusions of the same header file. In C++, where classes are often defined in header files, constructs like the following can be used to prevent multiple definitions:
// EXAMPLE.H - Example header file
#if !defined( EXAMPLE_H )
#define EXAMPLE_H
class Example
{
...
};
#endif // !defined( EXAMPLE_H )
The preceding code checks to see if the symbolic constant EXAMPLE_H is defined. If so, the file has already been included and need not be reprocessed. If not, the constant EXAMPLE_H is defined to mark EXAMPLE.H as already processed.
Microsoft Specific
Conditional compilation expressions are treated as signed long values, and these expressions are evaluated using the same rules as expressions in C++. For example, this expression:
#if 0xFFFFFFFFL > 1UL
is true.
