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c++arraysglobal-variablesconstantsextern

extern variable and array declare issue c++

I have a problem with an extern variable and an array declaration with it. How to declare an array with global variable which located not in the declarable file.

file1.cpp

const int size = 10;

mainfile.cpp

extern const int size;

void main()
{
  int mas[size];
}

int mas[size];

This line has an issue. Please any guess??

Solution

  • First of all constants have internal linkage. Thus these declarations

    file1.cpp
const int size = 10;

    and 

    mainfile.cpp
extern const int size;

    refer to different entities.

    The constant declared in file1.cpp is not visible outside its corresponding compilation unit.

    According to the C++ Standard (3.5 Program and linkage)

    3 A name having namespace scope (3.3.6) has internal linkage if it is the name of

    — a non-volatile variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage; or

    In mainfile the value of size is not specified so the compiler will issue an error for statement

    int mas[size];

    becuase the size of an array shall be a compile-time constant expression.

    The simplest solution is to place the constant definition

    const int size = 10;

    in some common headet file that will be included in each translation unit where there is a reference to the constant.