3

In C++17, is there any difference between declaring a global constant like this:

namespace ns
{
static constexpr const auto global_variable = 47;
}

Specifying the const modifier as well, and: 

namespace ns
{
static constexpr auto global_variable = 47;
}

Without specifying const? If yes, which are the differences and which version of the declaration is recommended in which scenarios?

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    In C++17 you may as well define a global constant inline rather than static. Or just in an anonymous namespace. Commented Dec 12, 2019 at 15:24
  • 1
    In the examples given, no. But static constexpr char* global_variable = "Happy"; applies the constexpr to the type char*, which is as-if it was char* const. And that's not what you really want. So there you need to do static constexpr char const* global_variable = "Happy";. Commented Dec 12, 2019 at 15:29
  • @DeiDei well it is already in a namspace, what are the benefits of making it inline instead of static in my example case? Commented Dec 12, 2019 at 15:36

2 Answers 2

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7

There is no difference, the constexpr specifier on a variable of object type implies const [dcl.constexpr]/9:

A constexpr specifier used in an object declaration declares the object as const. […]

Note that the static is redundant here as well because the const-qualified type already implies internal linkage [basic.link]/3.2:

A name having namespace scope has internal linkage if it is the name of

  • […]
  • non-inline variable of non-volatile const-qualified type that is neither explicitly declared extern nor previously declared to have external linkage […]
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You don't need to have const here, constexpr implies const.

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