cppcoreguidelines-avoid-const-or-ref-data-members

This check warns when structs or classes have const-qualified or reference (lvalue or rvalue) data members. Having such members is rarely useful, and makes the class only copy-constructible but not copy-assignable.

Examples:

// Bad, const-qualified member
struct Const {
  const int x;
}

// Good:
class Foo {
 public:
  int get() const { return x; }
 private:
  int x;
};

// Bad, lvalue reference member
struct Ref {
  int& x;
};

// Good:
struct Foo {
  int* x;
  std::unique_ptr<int> x;
  std::shared_ptr<int> x;
  gsl::not_null<int> x;
};

// Bad, rvalue reference member
struct RefRef {
  int&& x;
};

The check implements rule C.12 of C++ Core Guidelines.

Further reading: Data members: Never const.