编译失败：strlen 不是 std 的成员

Question

我正在尝试编译这个非常简单的程序，我在其中实现了 C++ 字符串的简化版本。但是编译器找不到 std::strlen 函数，即使我包含了

//main.cpp
#include "str.h"

int main()
{
    return 0;
}

// str.h
#ifndef _STRING_H_
#define _STRING_H_

#include <vector>
#include <cstring>
#include <algorithm>
#include <iterator>

class Str {
public:
    typedef std::vector<char>::size_type size_type;

    Str() { }
    Str(size_type n, char c): data(n, c) { }
    Str(const char* cp) {
        std::copy(cp, cp+std::strlen(cp), std::back_inserter(data));
    }
    template <class In> Str(In b, In e) {
        std::copy(b, e, std::back_inserter(data));
    }

private:
    std::vector<char> data;
};

#endif

我正在使用 g++（自制软件）进行编译

g++ main.cpp -o main

这是结果日志

In file included from str.h:5,
                 from main.cpp:1:
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:75:11: error: '::memchr' has not been declared
   using ::memchr;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:76:11: error: '::memcmp' has not been declared
   using ::memcmp;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:77:11: error: '::memcpy' has not been declared
   using ::memcpy;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:78:11: error: '::memmove' has not been declared
   using ::memmove;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:79:11: error: '::memset' has not been declared
   using ::memset;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:80:11: error: '::strcat' has not been declared
   using ::strcat;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:81:11: error: '::strcmp' has not been declared
   using ::strcmp;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:82:11: error: '::strcoll' has not been declared
   using ::strcoll;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:83:11: error: '::strcpy' has not been declared
   using ::strcpy;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:84:11: error: '::strcspn' has not been declared
   using ::strcspn;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:85:11: error: '::strerror' has not been declared
   using ::strerror;
           ^~~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:86:11: error: '::strlen' has not been declared
   using ::strlen;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:87:11: error: '::strncat' has not been declared
   using ::strncat;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:88:11: error: '::strncmp' has not been declared
   using ::strncmp;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:89:11: error: '::strncpy' has not been declared
   using ::strncpy;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:90:11: error: '::strspn' has not been declared
   using ::strspn;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:91:11: error: '::strtok' has not been declared
   using ::strtok;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:92:11: error: '::strxfrm' has not been declared
   using ::strxfrm;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:93:11: error: '::strchr' has not been declared
   using ::strchr;
           ^~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:94:11: error: '::strpbrk' has not been declared
   using ::strpbrk;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:95:11: error: '::strrchr' has not been declared
   using ::strrchr;
           ^~~~~~~
/usr/local/Cellar/gcc/8.3.0/include/c++/8.3.0/cstring:96:11: error: '::strstr' has not been declared
   using ::strstr;
           ^~~~~~
In file included from main.cpp:1:
str.h: In constructor 'Str::Str(const char*)':
str.h:19:31: error: 'strlen' is not a member of 'std'
         std::copy(cp, cp+std::strlen(cp), std::back_inserter(data));
                               ^~~~~~
str.h:19:31: note: suggested alternative: 'strstr'
         std::copy(cp, cp+std::strlen(cp), std::back_inserter(data));
                               ^~~~~~
                               strstr

我是不是做错了什么？我认为我没有遗漏任何 headers 或名称空间标识符，因此它应该可以编译。

Answer 1

宏名_STRING_H_保留给C标准使用。定义这样的宏是未定义的行为。

你在 str.h 周围包含了 guard 是 _STRING_H_ 并且你定义了它。但是，同样的 include guard 很可能用在 string.h 标准 C 头文件中。因此，来自 string.h 的所有符号在您的程序中都将不可见。因为你可能会幸运地使用 glibc, as it uses _STRING_H, but I could find many implementations that use 字符串 _STRING_H_ 作为包括他们文件周围的守卫。

要解决此问题，请将您的包含防护从 _STRING_H_ 更改为示例 STR_H_ 或 STRING_H_，而无需前导下划线。记住，写程序的时候约reserved names by the C standard

编译失败：strlen 不是 std 的成员

Fail to compile: strlen is not a member of std

c++

std

strlen