Anonymous Lambda

Anonymous Lambda

In the following code, GetMessage return -1 on error

while(_hwnd && GetMessage(&msg, _hwnd, 0, 0)) {
	TranslateMessage(&msg);
	DispatchMessage(&msg);
}

In order to use the SHOUJIN_ASSERT_WIN32 inline, without moving GetMessage outside of the where, two features are needed:

• The macro must return the GetMessage return value
• There must be a way to tell the macro what values represent an error, since the default win32 *false means error” pattern is not applicable

Since this is happening in a macro, encapsulating any declared variables in a new {} scope is mandatory to avoid name conflicts.
How to have a block return a value? By defining an anonymous lambda and executing it right away!

#define SHOUJIN_ASSERT_WIN32(fn) \
	[&]() { \
		auto ret = (fn); \
		if(!ret) \
			shoujin::assert::AbortWin32(TEXT(__FILE__), TEXT(__FUNCTION__), __LINE__, TEXT(#fn)); \
		return ret; \
	}()

And for when we have to check for failure using a specific value? Macro do not simply support default arguments, and in most cases a specific value is not needed, so let’s just make a different macro such as the following, where isok_func is a lambda:

#define SHOUJIN_ASSERT_WIN32_FUNC(fn, isok_func) \
	[&]() { \
		auto ret = (fn); \
		if(!(isok_func(ret))) \
			shoujin::assert::AbortWin32(TEXT(__FILE__), TEXT(__FUNCTION__), __LINE__, TEXT(#fn)); \
		return ret; \
	}()

And the final usage is kind of long, but working fine

	auto isok_func = [](auto r) { return r != -1 /*GetMessage returns -1 on error*/; };
	while(_hwnd && SHOUJIN_ASSERT_WIN32_FUNC(GetMessage(&msg, _hwnd, 0, 0), isok_func)) {
#pragma warning(suppress : 6001) //Warning C6001 Using uninitialized memory - The anonymous lambda in SHOUJIN_ASSERT_WIN32 hides the fact that msg is initialized
		TranslateMessage(&msg);
		DispatchMessage(&msg);
	}
...