它是标准的#if承担不确定的符号常量为0？ [英] Is it standard for #if to assume undefined symbolic constants as 0?

问题描述

在此 C 程序

#include<stdio.h>
int main()
{

        #if UnDefinedSymbolicConstant==0
                printf("UnDefinedSymbolicConstant is equal to 0\n ");
        #else
                printf("UnDefinedSymbolicConstant is not equal to 0\n");

        #endif
        return 0;
}

UnDefinedSymbolicConstant 尚未的#define 的任何地方，它仍然被假设为0 并给出了<一个HREF =htt​​p://ideone.com/dIzNx>输出 GCC-4.3.4 为：

UnDefinedSymbolicConstant has not been #define anywhere, still it is being assumed as 0 and gives the output on gcc-4.3.4 as:

UnDefinedSymbolicConstant is equal to 0

那么，
这是标准的行为，或只在GCC是这样的吗

推荐答案

是的，这是由标准规定在6.10.1：

Yes, this is specified by the standard in 6.10.1:

由于宏扩展和定义的一元运算符替换所有已执行后，所有剩余的标识符（包括那些词法相同的关键字）被替换为PP-数0

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

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