How do I define an in-class constant?

If you want a constant that you can use in a compile time constant expression, say as an array bound, use constexpr if your compiler supports that C++11 feature, otherwise you have two other choices:

class X { constexpr int c1 = 42; // preferred static const int c2 = 7; enum { c3 = 19 }; array<char,c1> v1; array<char,c2> v2; array<char,c3> v3; // ... };

You have more flexibility if the constant isn’t needed for use in a compile time constant expression:

class Z { static char* p; // initialize in definition const int i; // initialize in constructor public: Z(int ii) :i(ii) { } }; char* Z::p = "hello, there";

You can take the address of a static member if (and only if) it has an out-of-class definition:

class AE { // ... public: static const int c6 = 7; static const int c7 = 31; }; const int AE::c7; // definition int f() { const int* p1 = &AE::c6; // error: c6 not an lvalue const int* p2 = &AE::c7; // ok // ... }