Cheers,
WingIDE, a development environment for Python, has a nice concept of "Source assistant", as displayed below:
Basically, it's the reference that appears in C::B when you open the regular brackets.
However, it also displays the declaration of whatever is under the cursor.
From what I observed:
1) If you're text-cursor-pointing on an already completed line of code, it displays the declaration of whatever symbol is under the cursor. For example, if you're pointing on a function call, it displays the entire declaration of the function. If you're pointing on a comment, it displays the entire comment (not sure why you'd want that but that's what it does). If you're pointing on a variable, it displays the type of the variable. If you're pointing on an element of a struct, it displays the type of the struct element.
2) If you opened brackets and are passing arguments to a function call, it highlights the name and type of the next thing you need to pass.
It's obviously a standard part of the UI, meaning it doesn't pop up and disappear. It'd be perfect as a dockable window.
Code::Blocks already seems to have a fine code preprocessor, so it shouldn't be a problem to figure out what's the type of symbol under the text cursor.
So what do you think? Anyone interested in extending the UI a bit? It wouldn't have to be displayed by default, but it seems to work nicely in WingIDE for Python, so it might work nicely in Code::Blocks for C++, too.