关于extern "C"的纳闷
关于extern "C"的疑惑
编译环境 vs2005
//a.h
extern "C"
{
const int g_i;
}
//a.cpp
extern "C"
{
const int g_i = 21;
}
编译报错：
错误 1 error C2734: “g_i”: 如果不是外部的，则必须初始化常量对象
但是如果修改为：
//a.h
extern "C" const int g_i;
//a.cpp
extern "C" const int g_i = 21;
编译通过。
两者做法应该是一样的呀，不明白，望解答！
------解决方案--------------------
编译器翻译问题，vs就这么规定的，换成g++就ok
------解决方案--------------------
两种做法就是在这一点上是不一样的呀！
这是非此即彼，可以推导的。
虽然，要找到出处，就只有你自己去翻标准啦，标准上是有明确说明的。
------解决方案--------------------
但是就楼主的例子来说，在头文件里声明变量，按惯例应该要声明为extern的。
------解决方案--------------------
Mixed-Language Programming with C++
Home
------解决方案--------------------
Overview
------解决方案--------------------
How Do I
C++ uses the same calling convention and parameter-passing techniques as C, but naming conventions are different because of C++ decoration of external symbols. By causing C++ to drop name decoration, the extern "C" syntax makes it possible for a C++ module to share data and routines with other languages.
The following example declares prn as an external function using the C naming convention. This declaration appears in C++ source code.
extern "C"
{
void prn();
}
To call functions written in Fortran (or MASM), declare the function as you would in C and use a "C" linkage specification. For example, to call the Fortran function FACT from C++, declare it as follows:
extern "C" { int __stdcall FACT( int n ); }
The extern "C" syntax can be used to adjust a call from C++ to other languages, or to change the naming convention of C++ routines called from other languages. However, extern "C" can be used only from within C++. If the C++ code does not use extern "C" and cannot be changed, you can call C++ routines only by determining the name decoration and generating it from the other language. You can always determine the decoration by using the DUMPBIN utility. Use this approach only as a last resort, because the decoration scheme is not guaranteed to remain the same between versions.
Use of extern "C" has some restrictions:
You cannot declare a member function with extern "C".
You can specify extern "C" for only one instance of an overloaded function; all other instances of an overloaded function have C++ linkage.
For more information on the extern "C" linkage specification, see Linkage Specifications in C++ Language Reference.
------解决方案--------------------
定义常量的时候必须初始化
extern "C" const int g_i;
也应该报错才对。
难道 extern "C" const int g_i; 被等价于了
extern "C"
{
extern const int g_i;
}
------解决方案--------------------
Compiler Error C2734
'identifier' : const object must be initialized if not extern
The identifier is declared const but not initialized or extern.
The following sample generates C2734:
// C2734.cpp
const int j; // C2734
extern const int i; // OK, declared as extern
编译环境 vs2005
//a.h
extern "C"
{
const int g_i;
}
//a.cpp
extern "C"
{
const int g_i = 21;
}
编译报错：
错误 1 error C2734: “g_i”: 如果不是外部的，则必须初始化常量对象
但是如果修改为：
//a.h
extern "C" const int g_i;
//a.cpp
extern "C" const int g_i = 21;
编译通过。
两者做法应该是一样的呀，不明白，望解答！
------解决方案--------------------
编译器翻译问题，vs就这么规定的，换成g++就ok
------解决方案--------------------
两种做法就是在这一点上是不一样的呀！
这是非此即彼，可以推导的。
虽然，要找到出处，就只有你自己去翻标准啦，标准上是有明确说明的。
------解决方案--------------------
但是就楼主的例子来说，在头文件里声明变量，按惯例应该要声明为extern的。
------解决方案--------------------
Mixed-Language Programming with C++
Home
------解决方案--------------------
Overview
------解决方案--------------------
How Do I
C++ uses the same calling convention and parameter-passing techniques as C, but naming conventions are different because of C++ decoration of external symbols. By causing C++ to drop name decoration, the extern "C" syntax makes it possible for a C++ module to share data and routines with other languages.
The following example declares prn as an external function using the C naming convention. This declaration appears in C++ source code.
extern "C"
{
void prn();
}
To call functions written in Fortran (or MASM), declare the function as you would in C and use a "C" linkage specification. For example, to call the Fortran function FACT from C++, declare it as follows:
extern "C" { int __stdcall FACT( int n ); }
The extern "C" syntax can be used to adjust a call from C++ to other languages, or to change the naming convention of C++ routines called from other languages. However, extern "C" can be used only from within C++. If the C++ code does not use extern "C" and cannot be changed, you can call C++ routines only by determining the name decoration and generating it from the other language. You can always determine the decoration by using the DUMPBIN utility. Use this approach only as a last resort, because the decoration scheme is not guaranteed to remain the same between versions.
Use of extern "C" has some restrictions:
You cannot declare a member function with extern "C".
You can specify extern "C" for only one instance of an overloaded function; all other instances of an overloaded function have C++ linkage.
For more information on the extern "C" linkage specification, see Linkage Specifications in C++ Language Reference.
------解决方案--------------------
定义常量的时候必须初始化
extern "C" const int g_i;
也应该报错才对。
难道 extern "C" const int g_i; 被等价于了
extern "C"
{
extern const int g_i;
}
------解决方案--------------------
Compiler Error C2734
'identifier' : const object must be initialized if not extern
The identifier is declared const but not initialized or extern.
The following sample generates C2734:
// C2734.cpp
const int j; // C2734
extern const int i; // OK, declared as extern