Initialize static variables in C++ class?

Some answers including even the accepted answer seem to be a little misleading.

You don't have to

• Always assign a value to static objects when initializing because that's optional.
• Create another .cpp file for initializing since it can be done in the same header file.

You can even initialize a static object in the same class scope just like a normal variable using the inline keyword.

Initialize with no values in the same file

#include <string>
class A
{
    static std::string str;
    static int x;
};
std::string A::str;
int A::x;

Initialize with values in the same file

#include <string>
class A
{
    static std::string str;
    static int x;
};
std::string A::str = "SO!";
int A::x = 900;

Initialize in the same class scope using the inline keyword

#include <string>
class A
{
    static inline std::string str = "SO!";
    static inline int x = 900;
};

Mike Seymour has given you the right answer, but to add...
C++ lets you declare and define in your class body only static const integral types, as the compiler tells. So you can actually do:

class Foo
{
    static const int someInt = 1;
    static const short someShort = 2;
    // etc.
};

And you can't do that with any other type, in that cases you should define them in your .cpp file.

Since C++11 it can be done inside a class with constexpr.

class stat {
    public:
        // init inside class
        static constexpr double inlineStaticVar = 22;
};

The variable can now be accessed with:

stat::inlineStaticVar

Static member variables must be declared in the class and then defined outside of it!

There's no workaround, just put their actual definition in a source file.

From your description it smells like you're not using static variables the right way. If they never change you should use constant variable instead, but your description is too generic to say something more.

Static member variables always hold the same value for any instance of your class: if you change a static variable of one object, it will change also for all the other objects (and in fact you can also access them without an instance of the class - ie: an object).

Just to add on top of the other answers. In order to initialize a complex static member, you can do it as follows:

Declare your static member as usual.

// myClass.h
class myClass
{
static complexClass s_complex;
//...
};

Make a small function to initialize your class if it's not trivial to do so. This will be called just the one time the static member is initialized. (Note that the copy constructor of complexClass will be used, so it should be well defined).

//class.cpp    
#include myClass.h
complexClass initFunction()
{
    complexClass c;
    c.add(...);
    c.compute(...);
    c.sort(...);
    // Etc.
    return c;
}

complexClass myClass::s_complex = initFunction();

I feel it is worth adding that a static variable is not the same as a constant variable.

using a constant variable in a class

struct Foo{
    const int a;
    Foo(int b) : a(b){}
}

and we would declare it like like so

fooA = new Foo(5);
fooB = new Foo(10);
// fooA.a = 5;
// fooB.a = 10;

For a static variable

struct Bar{
    static int a;
    Foo(int b){
        a = b;
    }
}
Bar::a = 0; // set value for a

which is used like so

barA = new Bar(5);
barB = new Bar(10);
// barA.a = 10;
// barB.a = 10;
// Bar::a = 10;

You see what happens here. The constant variable, which is instanced along with each instance of Foo, as Foo is instanced has a separate value for each instance of Foo, and it can't be changed by Foo at all.

Where as with Bar, their is only one value for Bar::a no matter how many instances of Bar are made. They all share this value, you can also access it with their being any instances of Bar. The static variable also abides rules for public/private, so you could make it that only instances of Bar can read the value of Bar::a;

If your goal is to initialize the static variable in your header file (instead of a *.cpp file, which you may want if you are sticking to a "header only" idiom), then you can work around the initialization problem by using a template. Templated static variables can be initialized in a header, without causing multiple symbols to be defined.

See here for an example:

Static member initialization in a class template

Optionally, move all your constants to .cpp file without declaration in .h file. Use anonymous namespace to make them invisible beyond the cpp module.

// MyClass.cpp

#include "MyClass.h"

// anonymous namespace
namespace
{
    string RE_ANY = "([^\\n]*)";
    string RE_ANY_RELUCTANT = "([^\\n]*?)";
}

// member function (static or not)
bool MyClass::foo()
{
    // logic that uses constants
    return RE_ANY_RELUCTANT.size() > 0;
}