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After reading this and this I still feel confused about this kind of expressions:

static constexpr int = 0;

AFAIK, in C++:

  • static ensures life-time memory address along whole execution and safe initialization with concurrent threads
  • constexpr ensures time-compilation evaluation as rvalue, which means it shall have no memory address

They look contradictory to me. static ensures the variable will have a long-time memory address whereas constexpr ensures the opposite assumption. Surprisingly, the discussion in the first link mentions this:

constexpr int x = 3; const int* p = &x;

How can we even obtain the memory address of x if it is an rvalue?

Could anyone explain it?

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  • Could you be more explicit whether your are you asking about local scope static or namespace scope static? They have a different meaning and your stated understanding doesn't apply to both. Commented Nov 16, 2020 at 10:02
  • I don't believe constexpr and value category (evaluation as rvalue) has any connection. Commented Nov 16, 2020 at 10:06
  • My question comes from seeing many class members declared static constexpr, I am not sure which kind that should be. In any case, could you briefly explain both? Commented Nov 16, 2020 at 10:06
  • Your first link specifically contradicts a constexpr variable having no address. Commented Nov 16, 2020 at 10:08
  • @Victor Static class members are even more different form other static. You should specify your question. Commented Nov 16, 2020 at 10:10

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static has a number of meanings. In classes (per your comment), it means that the member is associated with the class, and not a specific instance (object) of that class.

For a constexpr, that makes a lot of sense. That's typically initialized by a value known to the compiler, and not from ctor arguments.

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2 Comments

Have I understood you correctly? static ensures that there is only one variable for all objects whereas constexpr ensures that its initialization is done at compilation time. Thanks.
@Victor: Yes, exactly. (Also, there's one static variable even if there are zero objects).

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