在向前声明类时，我是否也应该使用属性定义？

Question

When I'm forward declaring classes and I have a define like:

#define API __declspec(dllexport)

Should I declare the function with it, or without it? I know that I need to when I'm fully declaring the class (like, with a body and stuff), but I want to know if I should use it on forward declarations aswell.

Answer 1

In general my answer will be 'No'.

__declspec(dllexport) and __declspec(dllimport) attributes are needed only to know the fact "we need to export this" and "we need to import this". And in places where only forward declaration is sufficient this fact doesn't matter because it means that in this place it's enough to know just that such class exists.

One of the most common use cases would be like this:

// --- Library ---
// Library.h
class LIBRARY_API LibraryClass
{
     void SomeMethod();
}

// Library.cpp
#include "Library.h" // we include the declaration together with __declspec(dllimport)
void LibraryClass::SomeMethod() { /*do something*/ }


// --- Some module which uses Library ---
// Some class .h file
class LibraryClass; // forward declaration
class SomeClass
{
    std::unique_ptr<LibraryClass> mp_lib_class; 
    // forward decl of LibraryClass is enough 
    // - compiler does not need to know anything about this class

    // ...   
}

// Some class .cpp file
#include <Library/Library.h> 
// here we include LibraryClass declaration with __declspec(dllimport)

//...
SomeClass::SomeClass()
    : mp_lib_class(std::make_unique<LibraryClass>())
        // here we need to actually call LibraryClass code
        // - and we know that it's imported because of #include
{}

And if you are forward declaring LibraryClass inside of a Library where it's actually defined - putting __declspec(dllexport) will not change anything because class is exported no matter of these forward declarations.