[c++] What is __declspec and when do I need to use it?

I have seen instances of __declspec in the code that I am reading. What is it? And when would I need to use this construct?

I know it's been eight years but I wanted to share this piece of code found in MRuby that shows how __declspec() can bee used at the same level as the export keyword.

/** Declare a public MRuby API function. */
#if defined(MRB_BUILD_AS_DLL)
#if defined(MRB_CORE) || defined(MRB_LIB)
# define MRB_API __declspec(dllexport)
#else
# define MRB_API __declspec(dllimport)
#endif
#else
# define MRB_API extern
#endif

Essentially, it's the way Microsoft introduces its C++ extensions so that they won't conflict with future extensions of standard C++. With __declspec, you can attribute a function or class; the exact meaning varies depending on the nature of __declspec. __declspec(naked), for example, suppresses prolog/epilog generation (for interrupt handlers, embeddable code, etc), __declspec(thread) makes a variable thread-local, and so on.

The full list of __declspec attributes is available on MSDN, and varies by compiler version and platform.

It is mostly used for importing symbols from / exporting symbols to a shared library (DLL). Both Visual C++ and GCC compilers support __declspec(dllimport) and __declspec(dllexport). Other uses (some Microsoft-only) are documented in the MSDN.

The canonical examples are __declspec(dllimport) and __declspec(dllexport), which instruct the linker to import and export (respectively) a symbol from or to a DLL.

// header
__declspec(dllimport) void foo();


// code - this calls foo() somewhere in a DLL
foo();

(__declspec(..) just wraps up Microsoft's specific stuff - to achieve compatibility, one would usually wrap it away with macros)

Another example to illustrate the __declspec keyword:

When you are writing a Windows Kernel Driver, sometimes you want to write your own prolog/epilog code sequences using inline assembler code, so you could declare your function with the naked attribute.

__declspec( naked ) int func( formal_parameters ) {}

Or

#define Naked __declspec( naked )
Naked int func( formal_parameters ) {}

Please refer to naked (C++)