\rSec1[ub.cpp]{\ref{cpp}: Preprocessing directives}

\rSec2[ub.cpp.cond]{Conditional inclusion}

\ubxref{cpp.cond.defined} \\

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.

\begin{example}

\begin{codeblock}

#define A

#define B defined(A)

#if B // undefined behavior, \tcode{defined} is generated by macro replacement for controlling expression

#endif

\end{codeblock}

\end{example}

\rSec2[ub.cpp.include]{Source file inclusion}

\ubxref{cpp.include.one.of.two.forms} \\

If the an include directive after replacement does not match one of the two forms

outlined [cpp.indlude]\iref{cpp.include} then the behavior is undefined

\begin{example}

\begin{codeblock}

#include " // undefined behavior, does not match one of the allowable forms

\end{codeblock}

\end{example}

\rSec2[ub.cpp.replace]{Macro replacement}

\ubxref{cpp.replace.macro.pptoken} \\

If there are sequences of preprocessing tokens within the list of arguments to a function-like macro

that would otherwise act as preprocessing directives, the behavior is undefined.

\begin{example}

\begin{codeblock}

#define M(d, n) d = n

void f(int x) {

M(x,

#ifdef D // undefined behavior, preprocessing directive

16

#else // undefined behavior, preprocessing directive

32

#endif // undefined behavior, preprocessing directive

);

}

int main() {

int x;

f(x);

}

\end{codeblock}

\end{example}

\rSec2[ub.cpp.stringize]{The \# operator}

\ubxref{cpp.stringize.invalid.char} \\

If an invalid character results from using the stringize operator the behavior is undefined.

\begin{example}

\begin{codeblock}

#define s_lit(x) #x

s_lit( \ ) // stringizes to "\tcode{\textbackslash}"", which is not a valid token

\end{codeblock}

\end{example}

\rSec2[ub.cpp.concat]{The \#\# operator}

\ubxref{cpp.concat.invalid.preprocessing.token} \\

If an invalid preprocessing token results from using the concat operator the behavior is undefined.

\begin{example}

\begin{codeblock}

#define cat(x, y) x##y

void f() {

cat(/, /) // undefined behavior // is not a valid preprocessing token

}

\end{codeblock}

\end{example}

\rSec2[ub.cpp.line]{Line control}

\ubxref{cpp.line.zero.or.overflow} \\

When using the \tcode{\#line} directive, if the digit sequence

specifies zero or a number greater than 2147483647, the behavior is undefined.

\begin{example}

\begin{codeblock}

#line 4294967295 // undefined behavior, number greater than 2147483647

int main() { return notdefined; }

\end{codeblock}

\end{example}

\ubxref{cpp.line.pptoken.not.match} \\

When using the \grammarterm{pp-tokens} variant of the \tcode{\#line} directive, if the directive resulting after all replacements does not match

one of the two previous forms, the behavior is undefined.

\begin{example}

\begin{codeblock}

#define cat(a, b) a##b

#line cat(10, e20)

\end{codeblock}

\end{example}

\rSec2[ub.cpp.predefined]{Predefined macro names}

\ubxref{cpp.predefined.define.undef} \\

If any of the pre-defined macro names in [cpp.predefined], or the identifier defined, is the subject of a \tcode{\#define}

or a \tcode{\#undef} preprocessing directive, the behavior is undefined.

\begin{example}

\begin{codeblock}

#define __cplusplus 300012L

\end{codeblock}

\end{example}