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
65
#define A
#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
#undef A
end example]
