Why don’t C++ class definitions take up memory until an object is created?

A class definition mainly contains the executable code for the member functions and possibly a vtable if you have virtual functions, along with any static members. So, in reality, that code is already in memory. However, if the linker finds out that certain parts are never used, it might skip them. So while the class exists, it doesn’t always occupy memory unless it’s needed.

When you run a program, all of its instructions are loaded into memory. The class definition is essentially broken down into instructions for creating an object, and that info lives in the code segment of your program. This segment is fixed in size and read-only, separating it from areas where dynamic variables are allocated.

Just to clarify, while the class info technically exists, it doesn't use stack memory unless you're creating an object. It's more about how the creation code is generated at compile time, which means the definition itself doesn’t have to hang around.