9.5.3 Declaration directives

The declaration directives are used to define or undefine conditional

compilation symbols.

pp-declaration::

whitespaceopt # whitespaceopt define whitespace conditional-symbol

pp-new-line

whitespaceopt # whitespaceopt undef whitespace conditional-symbol

pp-new-line

pp-new-line::

whitespaceopt single-line-commentopt new-line

The processing of a #define directive causes the given conditional

compilation symbol to become defined,

starting with the source line that follows the directive. Likewise, the

processing of an #undef directive causes

the given conditional compilation symbol to become undefined, starting with

the source line that follows the

directive.

Any #define and #undef directives in a source file must occur before the

first token (§9.4) in the source file;

otherwise a compile-time error occurs. In intuitive terms, #define and

#undef directives must precede any

.real code. in the source file.

[Example: The example:

#define Enterprise

#if Professional || Enterprise

#define Advanced

#endif

namespace Megacorp.Data

{

#if Advanced

class PivotTable {...}

#endif

}

is valid because the #define directives precede the first token (the

namespace keyword) in the source file.

end example]

[Example: The following example results in a compile-time error because a

#define follows real code:

#define A

namespace N

{

#define B

#if B

class Class1 {}

#endif

}

end example]

A #define may define a conditional compilation symbol that is already

defined, without there being any

intervening #undef for that symbol. [Example: The example below defines a

conditional compilation symbol A

and then defines it again.

Chapter 9 Lexical structure

#define A

For compilers that allow conditional compilation symbols to be defined as

compilation options, an alternative

way for such redefinition to occur is to define the symbol as a compiler

option as well as in the source. end

example]

A #undef may .undefine. a conditional compilation symbol that is not

defined. [Example: The example below

defines a conditional compilation symbol A and then undefines it twice;

although the second #undef has no

effect, it is still valid.

#define A

#undef A

end example]