reference initialization
Binds a reference to an object
Contents |
[edit] Syntax
T & ref = object ;
T T T |
(1) | ||||||||
T && ref = object ;
T T T |
(2) | (since C++11) | |||||||
given R fn ( T & arg ); or R fn ( T && arg );
fn fn |
(3) | ||||||||
given T & fn () { or T && fn () {
|
(4) | ||||||||
Class:: Class(...) : refmember( expr) {...}
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(5) | ||||||||
[edit] Explanation
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:
The effects of reference initialization are:
- If the initializer is a braced-init-list
{
arg1, arg2, ...}
, rules of list initialization are followed.
- Otherwise, if the reference is an lvalue reference:
-
- If object is an lvalue expression, and its type is
T
or derived fromT
, 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.
- If object is an lvalue expression, and its type is
-
- If object is an lvalue expression, and its type is implicitly convertible to a type that is either
T
or derived fromT
, equally or less cv-qualified, then the non-explicit conversion functions of the source type and its base classes that return lvalue references are considered and the best one is selected by overload resolution. The reference is then bound to the object identified by the lvalue returned by the conversion function (or to its base class subobject)
- If object is an lvalue expression, and its type is implicitly convertible to a type that is either
- Otherwise, if the reference is either rvalue reference or lvalue reference to const:
-
- If object is an xvalue, a class prvalue, an array prvalue, or a function lvalue type that is either
T
or derived fromT
, equally or less cv-qualified, then the reference is bound to the value of the initializer expression or to its base subobject.
- If object is an xvalue, a class prvalue, an array prvalue, or a function lvalue type that is either
-
- If object is a class type expression that is not reference-related to
T
and (since C++17) that can be implicitly converted to an xvalue, a class prvalue, or a function value of type that is eitherT
or derived fromT
, equally or less cv-qualified, then the reference is bound to the result of the conversion or to its base subobject. User-defined conversions are not considered (since C++17)
- If object is a class type expression that is not reference-related to
-
- Otherwise, a temporary of type
T
is constructed and copy-initialized from object. The reference is then bound to this temporary. Copy-initialization rules apply (explicit constructors are not considered).
- Otherwise, a temporary of type
[edit] Lifetime of a temporary
Whenever a reference is bound to a temporary or to a base subobject of a temporary, the lifetime of the temporary is extended to match the lifetime of the reference, with the following exceptions:
- a temporary bound to a return value of a function in a return statement is not extended: it is destroyed immediately at the end of the return expression. Such function always returns a dangling reference.
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(until C++14) |
- a temporary bound to a reference parameter in a function call exists until the end of the full expression containing that function call: if the function returns a reference, which outlives the full expression, it becomes a dangling reference.
- a temporary bound to a reference in the initializer used in a new-expression exists until the end of the full expression containing that new-expression, not as long as the initialized object. If the initialized object outlives the full expression, its reference member becomes a dangling reference.
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.
[edit] Notes
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.
[edit] Example
#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: buf_ref is now 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 // (Note: does not compile in C++17) 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; }