C 预处理器 : what is the motivation behind treating undefined macro as 0?

Question

ISO/IEC 9899:202x (E) 工作草案 - 2020 年 2 月 5 日 C17..C2x N2479:

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers (including those lexically identical to keywords) are replaced with the pp-number 0, and then each preprocessing token is converted into a token.

换句话说:

An identifier that is not defined as a macro is converted to 0 before the expression is evaluated.

这导致 UNDEFINED == 0 为真:

#if UNDEFINED == 0
#pragma message "UNDEFINED == 0 is true"
#endif
$ gcc t0.c -E
t0.c:3:9: note: '#pragma message: UNDEFINED == 0 is true'

但是，如何在概念上(逻辑上、数学上、哲学上等)UNDEFINED 等于 0(导致 UNDEFINED == 0是 true) 吗？

例如，在 IEEE 754 中，NaN 可以被视为 UNDEFINED(不是数字，因此，没有数字，数字未定义，即未定义) 和 NaN == 0 是 false:

#include <stdio.h>
#include <math.h>
int main( void )
{
    printf("NaN == 0 is %d\n", NAN == 0);
    return 0;
}
$ gcc t0.c && ./a.exe
NaN == 0 is 0

问题:在 C 预处理器中将未定义宏视为 0 的动机是什么？

Answer 1

Question: What is the motivation behind treating in C preprocessor undefined macro as 0?

强调预处理器。预处理器将未定义的宏视为0，但编译器不将其视为0。比如下面的编译失败:

#undef UNDEFINED
#if UNDEFINED == 0  // ok, preprocessor evaluates this as '0 == 0'
int n = UNDEFINED;  // error, compiler sees this as 'int n = <undeclared-identifier>;'
#endif

与其他特质一样，“为什么”问题的真正答案是“因为 C 语言标准是这样说的”。虽然很难追查几十年前的原始动机，但这种行为实际上确实简化了某些结构，例如以下足以检查 C99 合规性，而不必担心 standard macro __STDC_VERSION__ 是否由实现定义:

#if __STDC_VERSION__ >= 199901L
// safely assume C99 or later
#else
// either __STDC_VERSION__ is not defined
// or it is earlier than C99
#endif