C++ – What does it mean to have an undefined reference to a static member

cc++-faq

I just wrote a class with some static data members, but now I am getting errors about "undefined references". Why doesn't this work? What am I doing wrong?

_{_(Note: This is meant to be an entry to [Stack Overflow's C++ FAQ](https://stackoverflow.com/questions/tagged/c++-faq). If you want to critique the idea of providing an FAQ in this form, then [the posting on meta that started all this](https://meta.stackexchange.com/questions/68647/setting-up-a-faq-for-the-c-tag) would be the place to do that. Answers to that question are monitored in the [C++ chatroom](https://chat.stackoverflow.com/rooms/10/c-lounge), where the FAQ idea started out in the first place, so your answer is very likely to get read by those who came up with the idea.)_}

Best Answer

To understand this, you should have a good understanding of compiling and linking, and the differences between declarations and definitions.

Consider the following class:

//In header file
class Example {
    static bool exampleStaticMember;
};

Here, exampleStaticMember is declared but not defined. This means that if exampleStaticMember is used in a way that means that it must have an address then there must be a separate definition for it. In general, no declaration of a static data member in a class definition is a definition of that member.

The required declaration is usually put in the cpp file which contains the other definitions for the members of the class. It must be in the same namespace as the class definition. The definition typically looks like:

//In source file:
//This may optionally have an initialiser (eg "= true")
bool Example::exampleStaticMember;

The definition can be put in any cpp file, but it should not be put in the header with the class, because that would be likely to break the One Definition Rule.

As a special case, if the static member variable is an const integral or enumeration type then it can have an initialiser in the class definition:

//In header file
class Example {
    static const int initialised = 15;
};

In this case, the definition in the cpp file is still required, but it is not allowed to have an initialiser:

//In source file
//Note: no initialiser!
const int Example::initialised;

Static members that have been initialised like this can be used in constant expressions.

Templates

For a static data member of a template, things are slightly different. The static member should be defined in the header along with the rest of the class:

//In header file
template<typename T>
class Example {
    static int exampleInt;
    static T exampleT;
}
template<typename T> int Example<T>::exampleInt;
template<typename T> T Example<T>::exampleT;

This works because there is a specific exception to the One Definition Rule for static data members of class templates.

Other uses of static

When the static keyword is applied to functions and objects that are not in a class scope it can take on a very different meaning.

When applied to objects in a function scope, it declares an object that is initialised in the first execution of the function and that subsequently keeps its value between function calls.

When applied to objects or functions at namespace scope (outside of any class or function definition), it declares objects or functions with internal linkage. This usage is deprecated for objects, as the unnamed-namespace provides a better alternative.

Related Solutions

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

C++ – What does the explicit keyword mean

The compiler is allowed to make one implicit conversion to resolve the parameters to a function. What this means is that the compiler can use constructors callable with a single parameter to convert from one type to another in order to get the right type for a parameter.

Here's an example class with a constructor that can be used for implicit conversions:

class Foo
{
public:
  // single parameter constructor, can be used as an implicit conversion
  Foo (int foo) : m_foo (foo) 
  {
  }

  int GetFoo () { return m_foo; }

private:
  int m_foo;
};

Here's a simple function that takes a Foo object:

void DoBar (Foo foo)
{
  int i = foo.GetFoo ();
}

and here's where the DoBar function is called:

int main ()
{
  DoBar (42);
}

The argument is not a Foo object, but an int. However, there exists a constructor for Foo that takes an int so this constructor can be used to convert the parameter to the correct type.

The compiler is allowed to do this once for each parameter.

Prefixing the explicit keyword to the constructor prevents the compiler from using that constructor for implicit conversions. Adding it to the above class will create a compiler error at the function call DoBar (42). It is now necessary to call for conversion explicitly with DoBar (Foo (42))

The reason you might want to do this is to avoid accidental construction that can hide bugs.
Contrived example:

You have a MyString class with a constructor that constructs a string of the given size. You have a function print(const MyString&) (as well as an overload print (char *string)), and you call print(3) (when you actually intended to call print("3")). You expect it to print "3", but it prints an empty string of length 3 instead.

Best Answer

Related Solutions

C++ – What are the differences between a pointer variable and a reference variable in C++

C++ – What does the explicit keyword mean

Related Topic