CODEWITHSUNDEEP
c++c++11
Giovanni Botta
Giovanni Botta

Reputation: 9816

constexpr static member vs variable

I stumbled upon some C++11 code that looks like this:

// some_file.h
namespace blah {
  class X {
   public:
    constexpr const static std::initializer_list<uint64> SOME_LIST =
      {1,2,3};
  };
}

// some_file.cpp
#include "some_file.h"
namespace blah {
  constexpr const std::initializer_list<uint64> X::SOME_LIST;
}

Which compiles fine. I assume the definition in the cpp file is used to avoid symbol duplication in each file that includes the header (please correct me if I'm wrong).

I then tried the following:

// my_file.h
namespace bleh {
  constexpr const static char SOME_CONSTANT[] = "yay";
}

// my_file.cpp
#include "my_file.h"
namespace bleh {
  // if I add this or any other variation, compilation breaks!
  //constexpr const static char SOME_CONSTANT[];
}

The code above does not work if I add an explicit definition in the .cpp file. So I am wondering: is there symbol duplication in the second case? If so, is there a way to define the variable without an enclosing class?

Upvotes: 3

Views: 520

Answers (1)

typ1232
typ1232

Reputation: 5607

The static keywords mean two different things here:

When you declare a variable or function at file scope (global and/or namespace scope), the static keyword specifies that the variable or function has internal linkage. When you declare a variable, the variable has static duration and the compiler initializes it to 0 unless you specify another value.

When you declare a data member in a class declaration, the static keyword specifies that one copy of the member is shared by all instances of the class. A static data member must be defined at file scope. An integral data member that you declare as const static can have an initializer.

C++ needs you to define static class members somewhere, because the class symbol is global (and so is your member). This can't be done in the header because of multiple definitions.

In the second case every compilation unit uses its own variable and there is no global symbol.

Upvotes: 2

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