[英]Why does 'x' have internal rather than external linkage?
Based on my reading of the C++ 2011 spec, I would think that the following code would create a variable 'x' with external linkage in file1.cc. 基于我对C ++ 2011规范的阅读,我认为以下代码将在file1.cc中创建一个带有外部链接的变量“x”。 I would think that I would be able to access that variable from main.cc and therefore that the program would print 'x'. 我认为我可以从main.cc访问该变量,因此该程序将打印'x'。 However, I instead get a linker error for an undefined reference to 'x' from main.cc. 但是,我从main.cc获取了对'x'的未定义引用的链接器错误。 Why does 'x' from file1.cc have internal linkage? 为什么file1.cc中的'x'有内部链接? I think the compiler is interpreting section 3.5.3 as giving 'x' internal linkage in file1.cc. 我认为编译器将第3.5.3节解释为在file1.cc中给出'x'内部链接。 However I have not "explicitly declared" 'x' to be 'const', as that section would require. 但是我没有“明确声明”'x'是'const',因为该部分需要。 I am using g++ version 4.6.3. 我正在使用g ++版本4.6.3。
main.cc: main.cc:
#include <iostream>
typedef const char CC;
extern CC x[];
int main(void) {
std::cout << x[0] << std::endl;
}
file1.cc: file1.cc:
typedef const char CC;
CC x[] = "abc";
The const
makes all the difference. const
使一切变得不同。 In C++ const
variables declared at file scope implicitly have internal linkage. 在C ++中,在文件范围内声明的const
变量隐式具有内部链接。 This is because in C++ const values can be used as compile-time constants (which leave nothing to link to). 这是因为在C ++中,const值可以用作编译时常量(不留任何东西链接)。
See this answer . 看到这个答案 。
You can add extern
to your definition in file1.cc to explicitly specify external linkage for x
: 您可以在file1.cc中为您的定义添加extern
,以明确指定x
外部链接:
extern CC x[] = "abc";
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