在C99标准中，可以查看到说明：
在6. Language -> 6.10 Preprocessing directives -> 6.10.1 Conditional inclusion里，第4点：
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.
在C++11标准中，也可以查看到说明：
在16. Preprocessing directives -> 16.1 Conditional inclusion里，第4点：
After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers and keywords146, except for true and false, are replaced with the pp-number 0, and then each preprocessing token is converted into a token.
就是说，没有定义的宏，会转换为0值。
关于pp-number：
在C99标准中，6. Language -> 6.4 Lexical elements -> 6.4.8 Preprocessing numbers里，
pp-number:
digit
. digit
pp-number digit
pp-number identifier-nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
pp-number指的是预处理过程中的数字符号。可以是数字开头，或小数点开头。可以使用计数法，比如使用e+, e-, E+, E-, p+, p-, P+ 或P-的字符序列，对应相应的计数方法。
关于解析过程
宏定义在预处理阶段展开，如果没有定义，在预处理命令中使用此宏，则此宏的值为0；在普通代码中展开此宏的话，会出现编译错误。例子如下：
未定义的宏的值为0：
#include <stdio.h>
int main()
{
#if VALUE == 0
printf("hello\n");
#endif
printf("goodbye!\n");
return 0;
}
$ g++ -o testMacro testMacro.cpp
$ ./testMacro
hello
goodbye!
如果在代码中使用未定义宏会编译出错：
#include <stdio.h>
int main()
{
#if VALUE == 0
int abc = VALUE;
printf("hello\n");
#endif
printf("goodbye!\n");
return 0;
}
$ g++ -o testMacro testMacro.cpp
testMacro.cpp: In function ‘int main()’:
testMacro.cpp:7:11: error: ‘VALUE’ was not declared in this scope
7 | int abc = VALUE;
| ^~~~~
参考：
1，c++ - What is the value of an undefined constant used in #if? - Stack Overflow