CODEWITHSUNDEEP
c++public
hong pei
hong pei

Reputation: 929

How the accessbility of a class or function in C++ is controlled

In language like C#. you can put public or internal in front of a class to control the access level of a class. How this is done in an C++ DLL?

Upvotes: 0

Views: 310

Answers (3)

Arunprasad Rajkumar
Arunprasad Rajkumar

Reputation: 1444

Unlike C#(or any other modern languages) C++ language standard doesn't talks anything about Dynamic Libraries/Shared Objects.

So it is all specific to the language/compiler implementors to define their own way of exporting class definitions across DLL/SO.

Refer http://gcc.gnu.org/wiki/Visibility for more info.

#if defined _WIN32 || defined __CYGWIN__
  #ifdef BUILDING_DLL
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllexport))
    #else
      #define DLL_PUBLIC __declspec(dllexport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #else
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllimport))
    #else
      #define DLL_PUBLIC __declspec(dllimport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #endif
  #define DLL_LOCAL
#else
  #if __GNUC__ >= 4
    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
    #define DLL_LOCAL  __attribute__ ((visibility ("hidden")))
  #else
    #define DLL_PUBLIC
    #define DLL_LOCAL
  #endif
#endif

extern "C" DLL_PUBLIC void function(int a);
class DLL_PUBLIC SomeClass
{
   int c;
   DLL_LOCAL void privateMethod();  // Only for use within this DSO
public:
   Person(int _c) : c(_c) { }
   static void foo(int a);
};

From the above snippet you can observe that exporting class is directly dependent on the compiler specific extension.

CL/MSVC - __declspec(dllexport), __declspec(dllimport
GCC - __attribute__ ((visibility ("default")))

GCC also provide **-fvisibility=[hidden|default] to have a more control over the symbols which is been exported over DLL/SO.

Upvotes: 0

Jerry Coffin
Jerry Coffin

Reputation: 490108

Presumably you're basically asking how to export a class from a DLL.

In that case, most compilers for Windows (e.g., VC++, MinGW) use __declspec(dllexport) to do the job.

To mirror that, the client codes need to declare the class as __declspec(dllimport). Typically you end up with something like:

#ifdef BUILD_DLL
#define DLL __declspec(dllexport)
#else
#define DLL __declspec(dllimport)
#endif

DLL class whatever { 
    // ...
};

...in the header, then the make file for building the DLL will define BUILD_DLL:

cflags += /DBUILD_DLL

Upvotes: 1

jandresrodriguez
jandresrodriguez

Reputation: 794

By nesting one class inside another:

class A
{
public:
    class B {};
protected:
    class C {};
private:
    class D {};
};

Upvotes: 0

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