Proper initialization of static constexpr array in

问题:

Static class members in C++ have caused a little confusion for me due to the standard's verbiage:

9.4.2 Static data members [class.static.data]

The declaration of a static data member in its class definition is not a definition...

However a constexpr is required to be initialized (AFAIK, couldn't find a quote from the standard) at its declaration (e.g., in the class definition).

Because of the restrictions on constexpr I had actually forgotten about the requisite for static members to be defined outside of the class, until I tried accessing a static constexpr array. This related question provides the correct way of defining the array member, but I'm interested as to the implications on this definition in a class template.

This is what I ended up with:

template<typename T>
class MyClass
{
private:
  static constexpr std::size_t _lut[256] = { /* ... */ };
  T _data;

public:
  static constexpr std::size_t GetValue(std::size_t n) noexcept
  {
    return _lut[n & 255];
  }

  // ...
};

template<typename T>
constexpr std::size_t MyClass<T>::_lut[256];

Is this the right syntax? Particularly the use of template in the definition feels awkward, but GCC seems to be linking everything appropriately.

As a follow-up question, should non-array static constexpr members be similarly defined (with template definition outside of class)?

回答1:

I think you want 9.4.2p3:

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [...] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

The definition of a template static data member is a template-declaration (14p1). The example given in 14.5.1.3p1 is:

template<class T> class X {
  static T s;
};
template<class T> T X<T>::s = 0;

However, as above a constexpr static or const static member whose in-class declaration specifies an initializer should not have an initializer in its namespace scope definition, so the syntax becomes:

template<class T> class X {
  const static T s = 0;
};
template<class T> T X<T>::s;

The difference with the non-array (i.e. integral or enumeration) static constexpr data member is that its use in lvalue-to-rvalue conversion is not odr-use; you would only need to define it if taking its address or forming a const reference to it.

回答2:

In case it helps anyone out, the following worked for me with GCC 4.7 using constexpr:

template<class T> class X {
  constexpr static int s = 0;
};
template<class T> constexpr int X<T>::s; // link error if this line is omitted

I'm not making any claims of whether this is "proper". I'll leave that to those more qualified.