Why do I have to put the data in my class declarations?

You don’t. If you don’t want data in an interface, don’t put it in the class that defines the interface. Put it in derived classes instead. See, Why do my compiles take so long?.

Sometimes, you do want to have representation data in a class. Consider class complex:

template<class Scalar> class complex { public: complex() : re(0), im(0) { } complex(Scalar r) : re(r), im(0) { } complex(Scalar r, Scalar i) : re(r), im(i) { } // ... complex& operator+=(const complex& a) { re+=a.re; im+=a.im; return *this; } // ... private: Scalar re, im; };

This type is designed to be used much as a built-in type and the representation is needed in the declaration to make it possible to create genuinely local objects (i.e. objects that are allocated on the stack and not on a heap) and to ensure proper inlining of simple operations. Genuinely local objects and inlining is necessary to get the performance of complex close to what is provided in languages with a built-in complex type.