检查预处理器中的__declspec宏 [英] Check a __declspec macro in preprocessor

问题描述

如果我有 SOME_MACRO ，它被定义为 __ declspec（dllimport）或 __ declspec （dllexport），有没有一种方法可以在编译时检查正在使用哪个？

If I have SOME_MACRO which is defined as either __declspec(dllimport) or __declspec(dllexport), is there a way to check at compile time which one is being used?

Ie像这样的东西：

#if SOME_MACRO == __declspec(dllimport)
// do something
#else
// do something else
#endif

---

UPD。
看得到的答案，我想我应该更具体地说明为什么需要这个。

---

UPD. Looking at the answers I'm getting I guess I should be more specific in why I need this.

我正在尝试编译一个相当大的3rd方库，该库在其代码的大部分包含该函数的地方声明为 dllexport 。但是，其中有一个组件是 dllimport 。

I'm trying to compile a rather big 3rd party library, which has a function declared as dllexport in most parts of their code where it's included. There's however one component in which it's a dllimport.

我需要为<$ c稍作修改声明$ c> dllimport 案例。这两个声明之间的切换不是很简单，这是由于 #ifdef 指令的树很深而导致的，这些指令分布在多个文件中。原则上，我可以从这些说明中提取此信息，但是要确保我正确地进行了操作，则必须尝试在几种不同的配置下编译整个库（每次编译需要几个小时）。

I need to modify the declaration slighly for the dllimport case. The switch between the two declarations is not very simple, it is a result of quite a deep tree of #ifdef instructions spread across several files. In principle I could dig this info out form these instructions, but to be sure I did it correctly I'd have to try and compile the whole library under several different configurations (each compilation taking a couple hours).

另一方面，如果有一种简单的方法来检查其 SOME_MACRO 是否被评估为可以导入或导出，我可以在一个很小的范围内进行测试

If on the other hand there was a simple way check whether their SOME_MACRO is evaluated to import or export, I could test this on a small program quickly and safely put that inside the library.

推荐答案

您不能使用

#if SOME_MACRO == __declspec(dllimport)

__ declspec（dllimport）不是预处理器表达式的有效令牌。

__declspec(dllimport) is not a valid token for a preprocessor expression.

您最好的选择是使用另一个预处理器宏，例如：

Your best option is to use another preprocessor macro, such as:

// Are we building the DLL?
#if defined(BUILD_DLL)
   // Yes, we are.
   #define SOME_MACRO __declspec(dllexport)
#else
   // No. We are using the DLL
   #define SOME_MACRO __declspec(dllimport)
#endif

现在，您可以使用：

#if defined(BUILD_DLL)

包含条件代码取决于您是否构建DLL或使用DLL。

to include conditional code depending on whether you are building the DLL or using the DLL.

实际上，这最终会涉及更多点。

Practically speaking, that ends to be a little bit more involved.

1. 大多数项目具有多个DLL。 BUILD_DLL 无法正常工作。每个生成的DLL将需要 BUILD_xxx_DLL 。假设您有两个DLL，即实用程序和核心。

1. Most projects have more than one DLL. BUILD_DLL is not going to work. You will need BUILD_xxx_DLL for each DLL you build. Let's say you have two DLLs, utility and core. and an application that depends on both.

您可能还需要创建一个静态库。

You may also need to create a static library.

在实用程序库的每个公共.h文件中，您都将需要以下内容。

You will need something like the following in every public .h file of the utility library.

#if defined(BUILD_UTILITY_STATIC)
   #define UTLIITY_EXPORT
#elif defined(BUILD_UTILITY_DLL)
   #define UTLIITY_EXPORT__declspec(dllexport)
#else
   #define UTLIITY_EXPORT__declspec(dllimport)
#endif

当然，您不需要在许多.h文件中重复相同的代码。您将创建一个.h文件，其中包含上述文件，并且 #include 包含在所有其他.h文件中。

Of course, you don't want to have to repeat the same code in lots of .h files. You will create a .h file that contains the above and #include that in all other .h files.

生成Utility.dll时，需要定义 BUILD_UTILITY_DLL 并保留 BUILD_UTILITY_STATIC 未定义。

When building utility.dll, you will need to define BUILD_UTILITY_DLL and leave BUILD_UTILITY_STATIC undefined.

构建utllity.lib（静态库）时，需要定义 BUILD_UTILITY_STATIC 并保留 BUILD_UTILITY_DLL 未定义。

When building utllity.lib (static library), you will need to define BUILD_UTILITY_STATIC and leave BUILD_UTILITY_DLL undefined.

utility.dll的用户将离开 BUILD_UTILITY_STATIC 以及 BUILD_UTILITY_DLL 未定义。

Users of utility.dll will leave BUILD_UTILITY_STATIC as well as BUILD_UTILITY_DLL undefined.

utility.lib（静态库）的用户将需要定义 BUILD_UTILITY_STATIC 并保留 BUILD_UTILITY_DLL 未定义。

Users of utility.lib (static library) will need to define BUILD_UTILITY_STATIC and leave BUILD_UTILITY_DLL undefined.

对于core.dll和core.lib，您将需要一个类似的文件。

You will need a similar file for core.dll and core.lib.

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