C++ vs C: 文件范围内const变量默认linkage 为 internal

======== x.cpp =========

void print_it();

char const hello[] = "asdf";

int main(int argc, char *argv)

{

    print_it();

    return 0;

}

======= y.cpp =========

#include

extern char const hello[1] ;

void print_it()

{

  printf("string = %s\n", hello );

}

链接时出现下面的错误:

y.obj : error LNK2019: unresolved external symbol "char const * const hello" (?hello@@3QBDB) referenced in function "void __cdecl print_it(void)" (?print_it@@YAXXZ)

原因就是C++中, const变量的默认linkage 规则变了. 要消除上面的错误, 要么使用extern "C", 使它回到C语言的规则中, 要么在x.cpp中 hello的定义处显式地加上extern.

这是在C++ templates一书中, 8.3.3 小节中一句话的怀疑引起的:

template

class Message;

extern char const hello[] = "Hello, World!";

Message* hello_msg;

Note the need for the extern keyword because otherwise a const array variable would have internal linkage.

因为记得文件范围内的函数和变量默认都是外部链接.