Question

如果我在名为ppmformat.h的文件中声明一个函数

//file ppmformat.h
namespace imaging
{
Image * ReadPPM(const char * filename);
} //namespace imaging

...并在ppmformat.cpp

中定义它

static imaging::Image * imaging::ReadPPM(const char *filename)
{
 ....
}

我收到以下错误：

'imaging :: Image * imaging :: ReadPPM（const char *）'被声明为'extern'，后来被称为'static'[-fpermissive]

    //ppmformat.h
    #ifndef _PPM
    #define _PPM

    #include "Image.h"
    namespace imaging
    {
    /*! Reads a PPM image file and returns a pointer to a newly allocated                  Image object containing the image.
    *
    *   \param filename is the null-terminated string of the name of the  file to open.
    *
    *  \return a pointer to a new Image object, if the read operation was successful, nullptr otherwise.
    */
    Image * ReadPPM(const char * filename);
    } //namespace imaging

    #endif

   //ppmformat.cpp
   #include <iostream>
   #include<string.h>
   #include<stdio.h>
   #include<stdlib.h>
   #include <string>
   #include <fstream>
   #include "ppmformat.h"

   using namespace std;

   imaging::Image * imaging::ReadPPM(const char *filename)
   {
   ......
   }

   //Image.h

   #ifndef _IMAGE
   #define _IMAGE

   #include "Color.h"
   #include <iostream>

   namespace imaging
   {
    class Image
    {
    ....
    }
   }

  //Image.cpp
  #include <iostream>
  #include "Color.h"
  #include "Image.h"
  ....
  //end of Image.cpp

  //Color.h
  #ifndef _COLOR
  #define _COLOR
  namespace imaging
  { 
    Class Color
    {
    ....
    }
  }

Answer 1

声明具有不同链接的同一实体的代码无效。

C ++11§7.11/ 8（dcl.std / 8）：

“对特定实体的连续声明所暗示的联系应予以同意。也就是说，在给定的范围内，声明相同变量名称或函数名称相同重载的每个声明都应表示相同的联系。但是，给定的一组重载函数中的每个函数都可以具有不同的链接。

对于实践中，g ++ 5.1.4拒绝编译它，而不幸的是，Visual C ++ 2015将其作为语言扩展接受，并带有警告：

[C:\my\forums\so\257]
> g++ foo.cpp
foo.cpp: In function 'imaging::Image* imaging::ReadPPM(const char*)':
foo.cpp:9:62: error: 'imaging::Image* imaging::ReadPPM(const char*)' was declared 'extern' and later 'static' [-fpermissive]
 static imaging::Image * imaging::ReadPPM(const char *filename)
                                                              ^
foo.cpp:5:13: note: previous declaration of 'imaging::Image* imaging::ReadPPM(const char*)'
     Image * ReadPPM(const char * filename);
             ^
foo.cpp: At global scope:
foo.cpp:9:54: warning: unused parameter 'filename' [-Wunused-parameter]
 static imaging::Image * imaging::ReadPPM(const char *filename)
                                                      ^

[C:\my\forums\so\257]
> cl foo.cpp
foo.cpp
foo.cpp(10): warning C4211: nonstandard extension used: redefined extern to static
foo.cpp(9): warning C4100: 'filename': unreferenced formal parameter
foo.cpp(9): warning C4505: 'imaging::ReadPPM': unreferenced local function has been removed

[C:\my\forums\so\257]
> _

如果编译器接受此警告，则最好将该警告变为硬错误。使用Visual C ++，可以选择/we4211。

错误'extern'和后来的'static'

1 个答案: