Reference declaration

Declares a named variable as a reference, that is, an alias to an already-existing object or function.

References are not objects; they do not necessarily occupy storage, although the compiler may allocate storage if it is necessary to implement the desired semantics (e.g. a non-static data member of reference type usually increases the size of the class by the amount necessary to store a memory address)

Because references are not objects, there are no arrays of references, no pointers to references, and no references to references:

int& a[3]; // error
int&* p; // error
int& &r; // error

It is permitted to form references to references through type manipulations in templates or typedefs, in which case the reference collapsing rules apply: rvalue reference to rvalue reference collapses to rvalue reference, all other combinations form lvalue reference:

typedef int&  lref;
typedef int&& rref;
int n;
lref&  r1 = n; // type of r1 is int&
lref&& r2 = n; // type of r2 is int&
rref&  r3 = n; // type of r3 is int&
rref&& r4 = 1; // type of r4 is int&&

(this, along with special rules for template argument deduction when T&& is used in a function template, forms the rules that make std::forward possible)

(since C++11)

[edit] Lvalue references

Lvalue references can be used to alias an existing object (optionally with different cv-qualification)

Run this code

#include <iostream>
#include <string>
int main()
{
    std::string s = "Ex";
    std::string& r1 = s;
    const std::string& r2 = s;
 
    r1 += "ample"; // modifies s
//  r2 += "!"; // error: cannot modify through reference to const
    std::cout << r2 << '\n'; // prints s, which now holds "Example"
}

They can also be used to implement pass-by-reference semantics in function calls:

Run this code

#include <iostream>
#include <string>
void double_string(std::string& s)
{
    s += s; // 's' is the same object as main()'s 'str'
}
int main()
{
    std::string str = "Test";
    double_string(str);
    std::cout << str << '\n';
}

When a function's return type is lvalue reference, the function call expression becomes an lvalue expression:

Run this code

#include <iostream>
#include <string>
char& char_number(std::string& s, std::size_t n)
{
    return s.at(n); // string::at() returns a reference to char
}
int main()
{
    std::string str = "Test";
    char_number(str, 1) = 'a'; // the function call is lvalue, can be assigned to
    std::cout << str << '\n';
}

Rvalue references

Rvalue references can be used to extend the lifetime of a modifiable temporary (note, lvalue references to const can extend lifetimes too, but they are not modifiable)

Run this code

#include <iostream>
#include <string>
int main()
{
    std::string s1 = "Test";
//    std::string&& r1 = s1; // error: can't bind to lvalue
 
    const std::string& r2 = s1 + s1; // OK, lvalue ref to const extends lifetime
//    r2 += "Test"; // error: can't modify through reference to const
 
    std::string&& r3 = s1 + s1; // OK, rvalue ref extends lifetime
    r3 += "Test"; // this rvalue can be modified
    std::cout << r3 << '\n';
}

More importantly, when a function has both rvalue reference and lvalue reference overloads, the rvalue reference overload binds to rvalues, while the lvalue reference overload binds to lvalues.

Run this code

#include <iostream>
void f(int& x)
{
     std::cout << "lvalue reference overload f(" << x << ")\n";
}
void f(const int& x)
{
     std::cout << "lvalue reference to const overload f(" << x << ")\n";
}
void f(int&& x)
{
     std::cout << "rvalue reference overload f(" << x << ")\n";
}
int main()
{
    int n = 1;
    const int x = 2;
    f(n); // calls f(int&)
    f(x); // calls f(const int&)
    f(3); // calls f(int&&)
          // would call f(const int&) if f(int&&) overload wasn't provided
}

This allows move constructors, move assignment operators, and other move-aware functions (e.g. vector::push_back() to be automatically selected when suitable.

(since C++11)

[edit] Dangling references

Although references, once initialized, always refer to valid objects or functions, it is possible to create a program where the life time of the referred-to object ends, but the reference remains accessible (dangling). Accessing such reference is undefined behavior. A common example a function returning a reference to an automatic variable:

std::string& f() {
    std::string str = "Example";
    return str; // exits the scope of str: str's destructor is called
}               // stack storage deallocated
...
std::string& r = f(); // r is a dangling reference
std::cout << r;      // Undefined: reads from a dangling reference
std::string s = f(); // Undefined: uses a dangling reference to copy-initialize s

Note that rvalue references and lvalue references to const extend the lifetimes of temporary objects, see reference initialization for rules and exceptions.

If the referred-to object was destroyed (e.g. by explicit destructor call), but the storage was not deallocated, a reference to the out-of-lifetime object may be used in limited ways, and may become valid if the object is recreated in the same storage, see Access outside of lifetime for details.

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)


`&` attr(optional) declarator	(1)

`&&` attr(optional) declarator	(2)	(since C++11)

declarator	-	any declarator except another reference declarator (there are no references to references)
attr(C++11)	-	optional list of attributes

Reference declaration

Contents

[edit] Syntax

[edit] Lvalue references

Rvalue references

[edit] Dangling references