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c++staticinitializationconstantsstatic-members
Thomas
Thomas

Reputation: 2959

Where should a static const data member be defined?

I have a class

class foo {
public:
   foo();
   foo( int );
private:
   static const string s;
};

Where is the best place to initialize the string s in the source file?

Upvotes: 146

Views: 196026

Answers (5)

plexando
plexando

Reputation: 1261

Since C++17 the inline specifier also applies to variables. You can now define static member variables in the class definition: 

#include <string>

class foo {
public:
   foo();
   foo( int );
private:
   inline static const std::string s { "foo" };
};

Upvotes: 26

Behnam Dezfouli
Behnam Dezfouli

Reputation: 177

Only integral values (e.g., static const int ARRAYSIZE) are initialized in header file because they are usually used in class header to define something such as the size of an array. Non-integral values are initialized in implementation file.

Upvotes: 1

user42690
user42690

Reputation:

Anywhere in one compilation unit (usually a .cpp file) would do:

foo.h

class foo {
    static const string s; // Can never be initialized here.
    static const char* cs; // Same with C strings.

    static const int i = 3; // Integral types can be initialized here (*)...
    static const int j; //     ... OR in cpp.
};

foo.cpp

#include "foo.h"
const string foo::s = "foo string";
const char* foo::cs = "foo C string";
// No definition for i. (*)
const int foo::j = 4;

(*) According to the standards you must define i outside of the class definition (like j is) if it is used in code other than just integral constant expressions. See David's comment below for details.

Upvotes: 203

Michael Burr
Michael Burr

Reputation: 340476

Static members need to be initialized in a .cpp translation unit at file scope or in the appropriate namespace:

const string foo::s( "my foo");

Upvotes: 13

GManNickG
GManNickG

Reputation: 504303

In a translation unit within the same namespace, usually at the top:

// foo.h
struct foo
{
    static const std::string s;
};

// foo.cpp
const std::string foo::s = "thingadongdong"; // this is where it lives

// bar.h
namespace baz
{
    struct bar
    {
        static const float f;
    };
}

// bar.cpp
namespace baz
{
    const float bar::f = 3.1415926535;
}

Upvotes: 13

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