Reference (C++)

The declaration of the form:

<Type>& <Name>

where <Type> is a type and <Name> is an identifier whose type is reference to <Type>.

Examples:

int a = 5;
int& r_a = a;

extern int& r_b;

Here, r_a and r_b are of type "reference to int"

int& Foo();

Foo is a function that returns a "reference to int"

void Bar(int& r_p);

Bar is a function with a reference parameter, which is a "reference to int"

class MyClass { int& m_b; /* ... */ };

MyClass is a class with a member which is reference to int

int FuncX() { return 42 ; };
int (&f_func)() = FuncX;

FuncX is a function that returns a (non-reference type) int and f_func is an alias for FuncX

const int& ref = 65;

const int& ref is a constant reference pointing to a piece of storage having value 65.

Types which are of kind "reference to <Type>" are sometimes called reference types. Identifiers which are of reference type are called reference variables. To call them variable, however, is in fact a misnomer, as we will see.

C++ references differ from pointers in several essential ways:

• It is not possible to refer directly to a reference object after it is defined; any occurrence of its name refers directly to the object it references.
• Once a reference is created, it cannot be later made to reference another object; it cannot be reseated. This is often done with pointers.
• References cannot be null, whereas pointers can; every reference refers to some object, although it may or may not be valid. Note that for this reason, containers of references are not allowed.
• References cannot be uninitialized. Because it is impossible to reinitialize a reference, they must be initialized as soon as they are created. In particular, local and global variables must be initialized where they are defined, and references which are data members of class instances must be initialized in the initializer list of the class's constructor. For example:

  int& k; // compiler will complain: error: `k' declared as reference but not initialized
  

There is a simple conversion between pointers and references: the address-of operator (&) will yield a pointer referring to the same object when applied to a reference, and a reference which is initialized from the dereference (*) of a pointer value will refer to the same object as that pointer, where this is possible without invoking undefined behavior. This equivalence is a reflection of the typical implementation, which effectively compiles references into pointers which are implicitly dereferenced at each use. Though that is usually the case, the C++ Standard does not force compilers to implement references using pointers.

A consequence of this is that in many implementations, operating on a variable with automatic or static lifetime through a reference, although syntactically similar to accessing it directly, can involve hidden dereference operations that are costly.

Also, because the operations on references are so limited, they are much easier to understand than pointers and are more resistant to errors. While pointers can be made invalid through a variety of mechanisms, ranging from carrying a null value to out-of-bounds arithmetic to illegal casts to producing them from arbitrary integers, a previously-valid reference only becomes invalid in two cases:

• If it refers to an object with automatic allocation which goes out of scope,
• If it refers to an object inside a block of dynamic memory which has been freed.

The first is easy to detect automatically if the reference has static scoping, but is still a problem if the reference is a member of a dynamically allocated object; the second is more difficult to detect. These are the only concerns with references, and are suitably addressed by a reasonable allocation policy.

• Other than just a helpful replacement for pointers, one convenient application of references is in function parameter lists, where they allow passing of parameters used for output with no explicit address-taking by the caller. For example:

void Square(int x, int& out_result) {
  out_result = x * x;
}

Then, the following call would place 9 in y:

int y;
Square(3, y);

However, the following call would give a compiler error, since reference parameters not qualified with const can only be bound to addressable values:

Square(3, 6);

• Returning a reference allows function calls to be assigned to:

  int& Preinc(int& x) {
    return ++x;  // "return x++;" would have been wrong
  }
  
  Preinc(y) = 5;  // same as ++y, y = 5
  

• In many implementations, normal parameter-passing mechanisms often imply an expensive copy operation for large parameters. References qualified with const are a useful way of passing large objects between functions that avoids this overhead:

  void FSlow(BigObject x) { /* ... */ }  
  void FFast(const BigObject& x) { /* ... */ }
  
  BigObject y;
  
  FSlow(y);  // Slow, copies y to parameter x.
  FFast(y);  // Fast, gives direct read-only access to y.
  

If FFast actually requires its own copy of x that it can modify, it must create a copy explicitly. While the same technique could be applied using pointers, this would involve modifying every call site of the function to add cumbersome address-of (&) operators to the argument, and would be equally difficult to undo, if the object became smaller later on.

Continuing the relationship between references and pointers (in C++ context), the former exhibit polymorphic capabilities, as one might expect:

#include <iostream>

class A {
 public:
  A() = default;
  virtual void Print() { std::cout << "This is class A\n"; }
};

class B : public A {
 public:
  B() = default;
  virtual void Print() { std::cout << "This is class B\n"; }
};

int main() {
  A a;
  A& ref_to_a = a;

  B b;
  A& ref_to_b = b;

  ref_to_a.Print();
  ref_to_b.Print();
}

The source above is valid C++ and generates the following output:
This is class A

This is class B

References are defined by the ISO C++ standard as follows (excluding the example section):

In a declaration T D where D has the form

& D1

and the type of the identifier in the declaration T D1 is "derived-declarator-type-list T," then the type of the identifier of D is "derived-declarator-type-list reference to T." Cv-qualified references are ill-formed except when the cv-qualifiers (const and volatile) are introduced through the use of a typedef (7.1.3) or of a template type argument (14.3), in which case the cv-qualifiers are ignored. [Example: in

typedef int& A;
const A aref = 3;  // ill-formed;
// non-const reference initialized with rvalue

the type of aref is "reference to int", not "const reference to int". ] [Note: a reference can be thought of as a name of an object. ] A declarator that specifies the type "reference to cv void" is ill-formed.

It is unspecified whether or not a reference requires storage (3.7).

There shall be no references to references, no arrays of references, and no pointers to references. The declaration of a reference shall contain an initializer (8.5.3) except when the declaration contains an explicit extern specifier (7.1.1), is a class member (9.2) declaration within a class declaration, or is the declaration of a parameter or a return type (8.3.5); see 3.1. A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior. As described in 9.6, a reference cannot be bound directly to a bitfield. ]

— ISO/IEC 14882:1998(E), the ISO C++ standard, in section 8.3.2 [dcl.ref]

• References in the C++ FAQ Lite