Can I initialize a `constexpr static` member outside the class?

Question

I'm working with a variable-width communications format. The structs to handle it look something like this:

struct Header
{
  int msgType = -1, len;

  Header() { len = sizeof(*this); }
};

struct A : public Header
{
  int x; char y;

  A() { msgType = 1; len = sizeof(*this); }
};

// Further structs B, C, ... declared along the same lines

I would like to have a constexpr static member Header::MAX_SIZE which gives the max size of any of these derived classes, e.g. so I can allocate a buffer which is guaranteed to hold any such packet. So I'd like to do something like

struct Header
{
  int msgType = -1, len;

  constexpr static std::size_t MAX_SIZE;

  Header() { len = sizeof(*this); }
};

// ... declaration of subclasses ...

inline Header::MAX_SIZE = std::max({ sizeof(A), sizeof(B), sizeof(C) });

I need the definition to come outside of the class because it depends on sizeof(A), etc., which in turn depend on the definition of Header.

It seems like this sort of thing should be unobjectionable: I'm giving the definition of the member in the same source file, and it can be computed at compile time. But I haven't found any way to tell the compiler to actually do this.

You can do this readily if you'll settle for const rather than constexpr. Or is that just not good enough? — Adrian Mole
– Adrian Mole, Commented Apr 28, 2021 at 12:49
I wanted to use it at compile time. I guess I can just declare this at namespace scope instead of inside the class, but this seems unnecessary. — Daniel McLaury
– Daniel McLaury, Commented Apr 28, 2021 at 12:50
Hmmm. I think a compile-time (constexpr) member of a base class whose value depends on the definitions of derived classes is opening the door to all sorts of recursive circularities. C++20 may have something that addresses this ... I haven't got round to fully digesting that Standard, yet. — Adrian Mole
– Adrian Mole, Commented Apr 28, 2021 at 12:56
I suppose you'd have to disallow using it from code in between the declaration and the definition. — Daniel McLaury
– Daniel McLaury, Commented Apr 28, 2021 at 12:57

Davis Herring · Accepted Answer · 2021-04-28 15:56:44Z

12

constexpr goes on the initializing declaration of a variable, so just put it outside the class:

struct Header
{
  int msgType = -1, len;

  static const std::size_t MAX_SIZE;

  Header() { len = sizeof(*this); }
};

// ... declaration of subclasses ...

inline constexpr std::size_t Header::MAX_SIZE = std::max({ sizeof(A), sizeof(B), sizeof(C) });

Note that the implicit const must be spelled out in the declaration. The definition should go in the same header to avoid any translation unit seeing the declaration but not the inline, which is not allowed.

answered Apr 28, 2021 at 15:56

Davis Herring

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7 Comments

xuhdev Over a year ago

Do you really need the inline keyword here?

Davis Herring Over a year ago

@xuhdev: The intent is to require it since not doing so is a regression versus C++14.

xuhdev Over a year ago

Do you have a reference that your code snippet is equivalent to initializing a constexpr static member inside the struct definition? I found this example en.cppreference.com/w/cpp/language/static (second last code block) and it looks a bit concerning that initializing a constexpr data member may not be allowed outside of the class definition.

Davis Herring Over a year ago

@xuhdev: I don’t understand: it’s declared constexpr and is initialized on that declaration. What inequivalence are you supposing might exist?

Spencer Over a year ago

@xuhdev Here's a godbolt example showing it working as intended: godbolt.org/z/zrEjf9cvE

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