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# Reference declaration

From cppreference.com
< cpp‎ | language

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

### [edit] Syntax

A reference variable declaration is any simple declaration whose declarator has the form

`&` attr(optional) declarator | (1) |
`&&` attr(optional) declarator | (2) | (since C++11)

1) Lvalue reference declarator: the declaration S & D; declares `D` as an lvalue reference to the type determined by decl-specifier-seq `S`.
2) Rvalue reference declarator: the declaration S && D; declares `D` as an rvalue reference to type determined by decl-specifier-seq `S`.

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

A declaration of a reference (except for class members and function parameters) is required to contain an initializer: see reference initialization which must identify a valid object or function.

There are no references to void and no references to references.

Reference types cannot be cv-qualified at the top level; there is no syntax for that in declaration, and if a qualification is introduced through a typedef, decltype, or template type argument, it is ignored.

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.

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