16 Preprocessing directives [cpp]

The expression that controls conditional inclusion shall be an integral constant expression except that identifiers (including those lexically identical to keywords) are interpreted as described below146 and it may contain zero or more defined-macro-expressions and/or has-include-expressions as unary operator expressions.

A defined-macro-expression evaluates to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.

The third and fourth forms of has-include-expression are considered only if neither of the first or second forms matches, in which case the preprocessing tokens are processed just as in normal text.

The header or source file identified by the parenthesized preprocessing token sequence in each contained has-include-expression is searched for as if that preprocessing token sequence were the pp-tokens in a #include directive, except that no further macro expansion is performed. If such a directive would not satisfy the syntactic requirements of a #include directive, the program is ill-formed. The has-include-expression evaluates to 1 if the search for the source file succeeds, and to 0 if the search fails.

The #ifdef and #ifndef directives, and the defined conditional inclusion operator, shall treat __has_include as if it were the name of a defined macro. The identifier __has_include shall not appear in any context not mentioned in this section.

Each preprocessing token that remains (in the list of preprocessing tokens that will become the controlling expression) after all macro replacements have occurred shall be in the lexical form of a token ([lex.token]).

Preprocessing directives of the forms

# if		constant-expression new-line groupopt
# elif		constant-expression new-line groupopt

check whether the controlling constant expression evaluates to nonzero.

Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined.

After all replacements due to macro expansion and evaluations of defined-macro-expressions and has-include-expressions have been performed, all remaining identifiers and keywords, except for true and false, are replaced with the pp-number 0, and then each preprocessing token is converted into a token. [ Note: An alternative token ([lex.digraph]) is not an identifier, even when its spelling consists entirely of letters and underscores. Therefore it is not subject to this replacement.  — end note ]

The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of [expr.const] using arithmetic that has at least the ranges specified in [support.limits]. For the purposes of this token conversion and evaluation all signed and unsigned integer types act as if they have the same representation as, respectively, intmax_t or uintmax_t ([cstdint]). [ Note: Thus on an implementation where std::numeric_limits<int>::max() is 0x7FFF and std::numeric_limits<unsigned int>::max() is 0xFFFF, the integer literal 0x8000 is signed and positive within a #if expression even though it is unsigned in translation phase 7 ([lex.phases]).  — end note ] This includes interpreting character literals, which may involve converting escape sequences into execution character set members. Whether the numeric value for these character literals matches the value obtained when an identical character literal occurs in an expression (other than within a #if or #elif directive) is implementation-defined. [ Note: Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same value in these two contexts:

#if 'z' - 'a' == 25
if ('z' - 'a' == 25)

 — end note ] Also, whether a single-character character literal may have a negative value is implementation-defined. Each subexpression with type bool is subjected to integral promotion before processing continues.

Preprocessing directives of the forms

# ifdef		identifier new-line groupopt
# ifndef		identifier new-line groupopt

check whether the identifier is or is not currently defined as a macro name. Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively.

Each directive's condition is checked in order. If it evaluates to false (zero), the group that it controls is skipped: directives are processed only through the name that determines the directive in order to keep track of the level of nested conditionals; the rest of the directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose control condition evaluates to true (nonzero) is processed; any following groups are skipped and their controlling directives are processed as if they were in a group that is skipped. If none of the conditions evaluates to true, and there is a #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped.147

[ Example: This demonstrates a way to include a library optional facility only if it is available:

#if __has_include(<optional>)
#  include <optional>
#  define have_optional 1
#elif __has_include(<experimental/optional>)
#  include <experimental/optional>
#  define have_optional 1
#  define experimental_optional 1
#else
#  define have_optional 0
#endif

 — end example ]