reference initialization

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C++
 
C++ language
 
Initialization
 

Binds a reference to an object

Syntax

T & ref = object ;

T & ref = { arg1, arg2, ... };

T & ref ( object ) ;

T & ref { arg1, arg2, ... } ;

(1)
T && ref = object ;

T && ref = { arg1, arg2, ... };

T && ref ( object ) ;

T && ref { arg1, arg2, ... } ;

(2) (since C++11)
given R fn ( T & arg ); or R fn ( T && arg );

fn ( object )

fn ( { arg1, arg2, ... } )

(3)
given T & fn () { or T && fn () {

return object ;

(4)
Class::Class(...) : refmember( expr) {...} (5)

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:

1) When a named lvalue reference variable is declared with an initializer
2) When a named rvalue reference variable is declared with an initializer
3) In a function call expression, when the function parameter has reference type
4) In the return statement, when the function returns a reference type
5) When a non-static data member of reference type is initialized using a member initializer

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 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
  • If object is an lvalue expression, and its type is implicitly convertible to a type that is either T or derived from T, 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) 
struct A { };
struct B : A { operator int&(); } b;
int& ir = B();         // ir refers to the result of B::operator int&
  • Otherwise, if the reference is either rvalue reference or lvalue reference to const:
  • If object is a non-bit-field xvalue, a class prvalue, an array prvalue, (until C++17)rvalue (since C++17) or a function lvalue, and its type is either 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 { } b;
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
  • If object is a class type expression that is not same or derived from T and (since C++17) that can be implicitly converted to an xvalue, a class prvalue, (until C++17)rvalue (since C++17) or a function value of type that is either T or derived from T, equally or less cv-qualified, then the reference is bound to the result of the conversion or to its base subobject (after materializing a temporary if necessary). (since C++17) User-defined conversions are not considered (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
  • Otherwise, a temporary of type T is constructed and copy-initialized from object. Copy-initialization rules apply (explicit constructors are not considered). If T or the type of object is a class type, the reference is then direct-initialized with the result of the conversion, except user-defined conversions are ignored for that direct-initialization. For non-class types, the reference is bound directly to this temporary (until C++17)the result of the conversion (after materializing a temporary if necessary) (since C++17) 
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

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.
  • a temporary bound to a reference member in a constructor initializer list persists only until the constructor exits, not as long as the object exists. (note: such initialization is ill-formed as of DR 1696)
(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.

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.

Example

Run this code
#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;
}


See also

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