C++ semantics of `static const` vs `const`

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In C++ specifically, what are the semantic differences between for example:

static const int x = 0 ;

and 

const int x = 0 ;

for both static as a linkage and a storage class specifier (i.e. inside and outside a function).

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2 Answers

C++17 standard draft on const implies static at file scope

This is the quote for what was mentioned at: https://stackoverflow.com/a/3709257/895245

C++17 n4659 standard draft 6.5 "Program and linkage":

3 A name having namespace scope (6.3.6) has internal linkage if it is the name of

  • (3.1) — a variable, function or function template that is explicitly declared static; or,
  • (3.2) — a non-inline variable of non-volatile const-qualified type that is neither explicitly declared extern nor previously declared to have external linkage; or
  • (3.3) — a data member of an anonymous union.

Annex C (informative) Compatibility, C.1.2 Clause 6: "basic concepts" gives the rationale why this was changed from C:

6.5 [also 10.1.7]

Change: A name of file scope that is explicitly declared const, and not explicitly declared extern, has internal linkage, while in C it would have external linkage.

Rationale: Because const objects may be used as values during translation in C++, this feature urges programmers to provide an explicit initializer for each const object. This feature allows the user to put const objects in source files that are included in more than one translation unit.

Effect on original feature: Change to semantics of well-defined feature.

Difficulty of converting: Semantic transformation.

How widely used: Seldom.

See also: Why does const imply internal linkage in C++, when it doesn't in C?

What you likely want to do instead on headers

Explained in detail at: What does 'const static' mean in C and C++?

  • pre C++17: extern in header, definition in cpp file
  • post C++17: inline variable on header
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