If you're using OpenGL and you need to create a window, you could use GLUT or SDL to create the window. They're both C APIs so you could write your code in C++.
If you need a Mac GUI and are determined to avoid Objective-C, you could use Carbon, but Carbon has several problems.
1. Xcode 3.2 doesn't ship with Carbon project templates so you would have to install Xcode 3.1 to get access to the Carbon project templates.
2. You can't write 64-bit GUI applications with Carbon.
3. More people use Cocoa than Carbon so getting answers to Carbon questions will be difficult.
A third option is to use Cocoa and Objective-C for your GUI and C++ for the rest of your code. By giving your Objective-C code the extension .mm, the compiler will treat the files as Objective-C++, which lets you write programs that use both C++ and Objective-C.