問題描述

我看到的Qt源代碼是這樣的:

I saw the Qt source code like this:

class Q_CORE_EXPORT QBasicAtomicInt
{
public:
...
};

Q_CORE_EXPORT 宏定義如下:

define Q_DECL_IMPORT __declspec(dllimport)

那么 __declspec(dllimport) 到底是什么意思?

So what does __declspec(dllimport) really mean?

推薦答案

__declspec 是 Microsoft 特定的屬性，允許您指定存儲類信息.
_{(Nitpicker's Corner:但是，許多其他編譯器供應商(例如 GCC)現在支持此語言擴展，以便與針對 Microsoft 編譯器編寫的已安裝代碼庫兼容.有些甚至提供額外的存儲類屬??性.)}

可以指定的兩個存儲類屬性是 dllimport 和 dllexport.這些向編譯器表明函數或對象是從 DLL 導入或導出(分別).

Two of those storage-class attributes that can be specified are dllimport and dllexport. These indicate to the compiler that a function or object is imported or exported (respectively) from a DLL.

更具體地說，它們定義了 DLL 到客戶端的接口，而無需模塊定義 (.DEF) 文件.大多數人發現使用這些語言擴展比創建 DEF 文件要容易得多.

More specifically, they define the DLL's interface to the client without requiring a module-definition (.DEF) file. Most people find it much easier to use these language extensions than to create DEF files.

出于顯而易見的原因，__declspec(dllimport) 和 __declspec(dllexport) 通常是相互配對的.您使用 dllexport 將符號標記為從 DLL 導出，然后使用 dllimport 將該導出的符號導入另一個文件.

For obvious reasons, __declspec(dllimport) and __declspec(dllexport) are generally paired with one another. You use dllexport to mark a symbol as exported from a DLL, and you use dllimport to import that exported symbol in another file.

因此，并且因為在編譯 DLL 時和在使用 DLL 接口的客戶端代碼中通常使用相同的頭文件，定義一個在編譯時自動解析為適當屬性說明符的宏是一種常見模式-時間.例如:

Because of this, and because the same header file is generally used both when compiling the DLL and in client code that consumes the DLL's interface, it is a common pattern to define a macro that automatically resolves to the appropriate attribute specifier at compile-time. For example:

#if COMPILING_DLL
    #define DLLEXPORT __declspec(dllexport)
#else
    #define DLLEXPORT __declspec(dllimport)
#endif

然后用DLLEXPORT標記所有應該導出的符號.

And then marking all of the symbols that should be exported with DLLEXPORT.

據推測，這就是 Q_CORE_EXPORT 宏所做的，解析為 Q_DECL_IMPORT 或 Q_DECL_EXPORT.

Presumably, that is what the Q_CORE_EXPORT macro does, resolving to either Q_DECL_IMPORT or Q_DECL_EXPORT.

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