Editing Reference (C++) (section)

== Syntax and terminology ==
=== Lvalue reference ===
The declaration of the form:

 <Type>& <Name>

where <code><Type></code> is a [[Data type|type]] and <code><Name></code> is an [[identifier]] is said to define an identifier whose type is '''[[Value (computer science)#lrvalue|lvalue]] reference to <code><Type></code>'''.<ref>[[#cpp-std|ISO/IEC 14822]], clause 9.3.3.2, paragraph 1.</ref>

Examples:
<syntaxhighlight lang="cpp">
int a = 5;
int& r_a = a;

extern int& r_b;
</syntaxhighlight>

Here, <code>r_a</code> and <code>r_b</code> are of type "lvalue reference to <code>int</code>"

<syntaxhighlight lang="cpp">int& Foo();</syntaxhighlight>

<code>Foo</code> is a function that returns an "lvalue reference to <code>int</code>"

<syntaxhighlight lang="cpp">void Bar(int& r_p);</syntaxhighlight>

<code>Bar</code> is a function with a reference parameter, which is an "lvalue reference to <code>int</code>"

<syntaxhighlight lang="cpp">class MyClass { int& m_b; /* ... */ };</syntaxhighlight>

<code>MyClass</code> is a <code>class</code> with a member which is lvalue reference to <code>int</code>

<syntaxhighlight lang="cpp">
int FuncX() { return 42 ; };
int (&f_func)() = FuncX;
int (&&f_func2)() = FuncX; // essentially equivalent to the above
</syntaxhighlight>

<code>FuncX</code> is a function that returns a (non-reference type) <code>int</code> and <code>f_func</code> is an ''[[aliasing (computing)|alias]]'' for <code>FuncX</code>

<syntaxhighlight lang="cpp">const int& ref = 65;</syntaxhighlight>

<code>[[const (computer programming)|const]] int& ref</code> is an lvalue reference to <code>const int</code> pointing to a piece of storage having value 65.

<syntaxhighlight lang="cpp">
int arr[3];
int (&arr_lvr)[3] = arr;
int (&&arr_rvr)[3] = std::move(arr);
typedef int arr_t[3];
int (&&arr_prvl)[3] = arr_t{}; // arr_t{} is an array prvalue
int *const & ptr_clv = arr; // same as int *const & ptr_clv = &arr[0];
int *&& ptr_rv = arr;
// int *&arr_lv = arr; // Error: Initializing an lvalue reference to non-const type with an rvalue
</syntaxhighlight>

<code>arr_lvr</code> is a reference to an array. When initializing a reference to array, array-to-pointer conversion does not take place, but it does take place when initializing a reference to pointer. Since array-to-pointer conversion returns a prvalue, only lvalue references to <code>const</code> and rvalue references can be initialized with its result. Similarly, when initializing a reference to function, function-to-pointer conversion does not take place (see <code>f_func</code> above), but it does take place when initializing a reference to function pointer:

<syntaxhighlight lang="cpp">
int FuncX() { return 42 ; };
int (*const &pf_func)() = FuncX; // same as int (*const &pf_func)() = &FuncX;
int (* &&pf_func2)() = FuncX;
</syntaxhighlight>

=== Rvalue reference ===
The declaration of the form:

 <Type>&& <Name>

where <code><Type></code> is a type and <code><Name></code> is an identifier is said to define an identifier whose type is '''[[rvalue reference]] to <code><Type></code>'''. 
Since the name of an [[Value (computer science)#lrvalue|rvalue]] reference is itself an lvalue, <code>std::move</code> must be used to pass an rvalue reference to a function overload accepting an rvalue reference parameter. Rvalue references to cv-unqualified type template parameters of that same function template or <code>auto&&</code> except when deduced from a brace-enclosed initializer list are called '''forwarding references''' (referred to as "universal references" in some older sources<ref>{{cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4164.pdf|title=Forwarding References|last1=Sutter|first1=Herb|last2=Stroustrup|first2=Bjarne|last3=Dos Reis|first3=Gabriel}}</ref>) and can act as lvalue or rvalue references depending on what is passed to them.<ref>[[#cpp-std|ISO/IEC 14822]], clause 13.10.2.1, paragraph 3.</ref>
When found in function parameters, they are sometimes used with <code>std::forward</code> to forward the function argument to another function while preserving the value category (lvalue or rvalue) it had when passed to the calling function.<ref>{{cite web|url=http://thbecker.net/articles/rvalue_references/section_01.html|title=C++ Rvalue References Explained|last=Becker|first=Thomas|accessdate=2022-11-25}}</ref>

Types which are of kind "reference to <code><Type></code>" 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.