Binds a reference to an object.
T T T T | (1) | |
T T T T | (2) | (since C++11) |
given R fn ( T & arg ); or R fn ( T && arg ); fn fn | (3) | |
inside T & fn () or T && fn ()
| (4) | |
given T & ref ; or T && ref ; inside the definition of Class Class | (5) |
A reference to T
can be initialized with an object of type T
, a function of type T
, or an object implicitly convertible to T
. Once initialized, a reference cannot be changed to refer to another object.
References are initialized in the following situations:
return
statement, when the function returns a reference typeThe effects of reference initialization are:
{
arg1, arg2, ... }
, rules of list initialization are followed. T
or derived from T
, and is equally or less cv-qualified, then the reference is bound to the object identified by the lvalue or to its base class subobject. double d = 2.0; double& rd = d; // rd refers to d const double& rcd = d; // rcd refers to d struct A {}; struct B : A {} b; A& ra = b; // ra refers to A subobject in b const A& rca = b; // rca refers to A subobject in b
T
, and object has conversion function to an lvalue whose type is either T
or derived from T
, equally or less cv-qualified, then the reference is bound to the object identified by the lvalue returned by the conversion function (or to its base class subobject). struct A {}; struct B : A { operator int&(); }; int& ir = B(); // ir refers to the result of B::operator int&
const
or rvalue reference (since C++11): T
or derived from T
, equally or less cv-qualified, then the reference is bound to the value of the initializer expression or to its base subobject (after materializing a temporary if necessary) (since C++17). struct A {}; struct B : A {}; extern B f(); const A& rca2 = f(); // bound to the A subobject of the B rvalue. A&& rra = f(); // same as above int i2 = 42; int&& rri = static_cast<int&&>(i2); // bound directly to i2
T
, and object has conversion function to a rvalue or a function lvalue whose type is either T
or derived from T
, equally or less cv-qualified, then the reference is bound to the result of the conversion function or to its base class subobject (after materializing a temporary if necessary) (since C++17). struct A {}; struct B : A {}; struct X { operator B(); } x; const A& r = x; // bound to the A subobject of the result of the conversion B&& rrb = x; // bound directly to the result of the conversion
T
. The reference is bound to the result of the conversion (after materializing a temporary) (since C++17). If the object (or, if the conversion is done by user-defined conversion, the result of the conversion function) is of type T
or derived from T
, it must be equally or less cv-qualified than T
, and, if the reference is an rvalue reference, must not be an lvalue (since C++11). const std::string& rs = "abc"; // rs refers to temporary copy-initialized from char array const double& rcd2 = 2; // rcd2 refers to temporary with value 2.0 int i3 = 2; double&& rrd3 = i3; // rrd3 refers to temporary with value 2.0
Whenever a reference is bound to a temporary or to a subobject thereof, the lifetime of the temporary is extended to match the lifetime of the reference, with the following exceptions:
return
statement is not extended: it is destroyed immediately at the end of the return expression. Such function always returns a dangling reference.
| (until C++14) |
struct A { int&& r; }; A a1{7}; // OK, lifetime is extended A a2(7); // well-formed, but dangling reference | (since C++20) |
In general, the lifetime of a temporary cannot be further extended by "passing it on": a second reference, initialized from the reference to which the temporary was bound, does not affect its lifetime.
References appear without initializers only in function parameter declaration, in function return type declaration, in the declaration of a class member, and with the extern
specifier.
#include <utility> #include <sstream> struct S { int mi; const std::pair<int, int>& mp; // reference member }; void foo(int) {} struct A {}; struct B : A { int n; operator int&() { return n; } }; B bar() { return B(); } //int& bad_r; // error: no initializer extern int& ext_r; // OK int main() { // Lvalues int n = 1; int& r1 = n; // lvalue reference to the object n const int& cr(n); // reference can be more cv-qualified volatile int& cv{n}; // any initializer syntax can be used int& r2 = r1; // another lvalue reference to the object n // int& bad = cr; // error: less cv-qualified int& r3 = const_cast<int&>(cr); // const_cast is needed void (&rf)(int) = foo; // lvalue reference to function int ar[3]; int (&ra)[3] = ar; // lvalue reference to array B b; A& base_ref = b; // reference to base subobject int& converted_ref = b; // reference to the result of a conversion // Rvalues // int& bad = 1; // error: cannot bind lvalue ref to rvalue const int& cref = 1; // bound to rvalue int&& rref = 1; // bound to rvalue const A& cref2 = bar(); // reference to A subobject of B temporary A&& rref2 = bar(); // same int&& xref = static_cast<int&&>(n); // bind directly to n // int&& copy_ref = n; // error: can't bind to an lvalue double&& copy_ref = n; // bind to an rvalue temporary with value 1.0 // Restrictions on temporary lifetimes std::ostream& buf_ref = std::ostringstream() << 'a'; // the ostringstream temporary // was bound to the left operand of operator<< // but its lifetime ended at the semicolon // so buf_ref is a dangling reference S a {1, {2, 3} }; // temporary pair {2, 3} bound to the reference member // a.mp and its lifetime is extended to match a S* p = new S{1, {2, 3} }; // temporary pair {2, 3} bound to the reference // member p->mp, but its lifetime ended at the semicolon // p->mp is a dangling reference delete p; }
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