c++ - 如何仅包含一次文件/防止来自 C++ 程序的旧 C API 中的函数重定义

Question

我有一个 C++ 程序，它必须使用基于 C 的 API 来完成特定任务。如果我在我的程序中只包含一个 cpp，我完全没有问题。但是，如果我包含在头文件中（我需要在其中使用某些类型），我会得到链接器的重新定义错误。

这是由于在外部库（带有已编译 dll 的 C 标头）标头中创建了许多 typedef。我正在寻找任何可能的解决方案来解决这个问题。

当我搜索时，我似乎只能找到标头保护的东西（和 #pragma 一次），但这些并不是问题的解决方案，因为我发现的是单独编译单元中的多个定义，然后在链接时发生冲突。

我正在考虑 extern 但发现我无法将标头中定义的任何类型用作 C++ 类中的成员，这主要是我想要做的。

Output for building on Warning Level 4
Warning 1   warning C4273: 'getwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  615 1   project-test
Warning 2   warning C4273: 'putwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  617 1   project-test
Warning 3   warning C4273: 'getwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  615 1   project-test
Warning 4   warning C4273: 'putwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  617 1   project-test
Warning 5   warning C4100: 'data' : unreferenced formal parameter   c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 6   warning C4100: 'event' : unreferenced formal parameter  c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 7   warning C4100: 'widget' : unreferenced formal parameter c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 8   warning C4100: 'data' : unreferenced formal parameter   c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 9   warning C4100: 'event' : unreferenced formal parameter  c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 10  warning C4100: 'widget' : unreferenced formal parameter c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 11  warning C4505: 'g_bit_nth_lsf' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   267 1   project-test
Warning 12  warning C4505: 'g_bit_nth_msf' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   269 1   project-test
Warning 13  warning C4505: 'g_bit_storage' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   271 1   project-test
Warning 14  warning C4505: 'g_string_append_c_inline' : unreferenced local function has been removed    c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gstring.h  130 1   project-test
Warning 15  warning C4505: 'g_trash_stack_push' : unreferenced local function has been removed  c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  44  1   project-test
Warning 16  warning C4505: 'g_trash_stack_pop' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  46  1   project-test
Warning 17  warning C4505: 'g_trash_stack_peek' : unreferenced local function has been removed  c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  47  1   project-test
Warning 18  warning C4505: 'g_trash_stack_height' : unreferenced local function has been removed    c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  48  1   project-test
Error   19  error LNK2005: "long __cdecl abs(long)" (?abs@@YAJJ@Z) already defined in FitsFile.obj  C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   20  error LNK2005: "__int64 __cdecl abs(__int64)" (?abs@@YA_J_J@Z) already defined in FitsFile.obj  C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   21  error LNK2005: "struct _ldiv_t __cdecl div(long,long)" (?div@@YA?AU_ldiv_t@@JJ@Z) already defined in FitsFile.obj   C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   22  error LNK2005: "struct _lldiv_t __cdecl div(__int64,__int64)" (?div@@YA?AU_lldiv_t@@_J0@Z) already defined in FitsFile.obj  C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   23  error LNK2005: _getwchar already defined in FitsFile.obj    C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   24  error LNK2005: _putwchar already defined in FitsFile.obj    C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\project-test\main.obj  project-test
Error   25  error LNK1169: one or more multiply defined symbols found   C:\Users\Ian\SkyDrive\Documents\Uni\FINPRO\projects\project-test\Debug\project-test.exe project-test

Output for building on Warning Level 4 (when only including for one place)
Warning 1   warning C4273: 'getwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  615 1   project-test
Warning 2   warning C4273: 'putwchar' : inconsistent dll linkage    c:\program files (x86)\microsoft visual studio 11.0\vc\include\stdio.h  617 1   project-test
Warning 3   warning C4100: 'data' : unreferenced formal parameter   c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 4   warning C4100: 'event' : unreferenced formal parameter  c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 5   warning C4100: 'widget' : unreferenced formal parameter c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  8   1   project-test
Warning 6   warning C4100: 'data' : unreferenced formal parameter   c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 7   warning C4100: 'event' : unreferenced formal parameter  c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 8   warning C4100: 'widget' : unreferenced formal parameter c:\users\ian\skydrive\documents\uni\finpro\projects\project-test\project-test\main.cpp  15  1   project-test
Warning 9   warning C4505: 'g_bit_nth_lsf' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   267 1   project-test
Warning 10  warning C4505: 'g_bit_nth_msf' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   269 1   project-test
Warning 11  warning C4505: 'g_bit_storage' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gutils.h   271 1   project-test
Warning 12  warning C4505: 'g_string_append_c_inline' : unreferenced local function has been removed    c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gstring.h  130 1   project-test
Warning 13  warning C4505: 'g_trash_stack_push' : unreferenced local function has been removed  c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  44  1   project-test
Warning 14  warning C4505: 'g_trash_stack_pop' : unreferenced local function has been removed   c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  46  1   project-test
Warning 15  warning C4505: 'g_trash_stack_peek' : unreferenced local function has been removed  c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  47  1   project-test
Warning 16  warning C4505: 'g_trash_stack_height' : unreferenced local function has been removed    c:\users\ian\skydrive\documents\uni\finpro\gtk\include\glib-2.0\glib\gtrashstack.h  48  1   project-test

Answer 1

抽象间接层背后的 C 功能：

原始结构（如果我理解正确，这就是你现在所拥有的）：

file1.h -> #include included_c_api.h
file2.h -> #include included_c_api.h CAUSES errors.

新结构：

new_c_api.h -> wrapper over included_c_api.h functionality (w/ your own symbols)
new_c_api.c[pp] -> #include included_c_api.h
file1.h -> #include new_c_api.h
file2.h -> #include new_c_api.h

如果included_c_api.h 中暴露的功能非常大，这个解决方案可能不切实际。

Answer 2

寻找两件事：

a）您不使用包含警卫。作为一般规则，所有标题都应具有以下结构。这可确保只有一个标头副本包含在 C++ 文件中。

#ifndef MYPROJECT_DIR_FILE_H
#define MYPROJECT_DIR_FILE_H

... all declarations and definitions ...

#endif

b) 确保头文件中定义的所有函数都是内联的。否则，多个 C++ 文件将具有相同函数的副本，这会使链接器感到困惑。inline请求丢弃除一份函数之外的所有副本。

Answer 3

您不会说问题出在变量还是函数上。他们有不同的解决方案：

变量：头文件必须有一个变量的“extern”声明，只有一个包含定义的“C”文件（与extern声明相同，但前面没有extern一词）。

函数：仅当您在标头中有函数体时才会发生这种情况 - 在这种情况下，您应该将“内联”放在函数定义的前面，以便函数代码嵌入到使用它的每个目标模块中，而链接器不会根本不必处理它。函数原型不会导致重复定义错误。

在链接 C 代码和 C++ 代码时，您还需要正确命名修饰 - 这基本上意味着当您使用 C++ 编译器编译这些声明时，您应该使用以下代码包围所有适用于“C”对象的声明：

extern "C" { 

... 'C' declarations

}

你通常会看到

#if defined(__cplusplus)
extern "C" { 
#endif 

在头文件的顶部（底部的类似代码），这些文件被设计为包含在 C 和 C++ 文件中。这在这里得到了很好的介绍-但听起来您目前没有名称修改问题，尽管我希望您会在项目完成之前...