reference_wrapper Class
The latest version of this topic can be found at reference_wrapper Class.
Wraps a reference.
Syntax
template <class Ty>
class reference_wrapper
: public unary_function<T1, Ret> // see below
: public binary_function<T1, T2, Ret> // see below
{
public:
typedef Ty type;
typedef T0 result_type; // see below
reference_wrapper(Ty&);
Ty& get() const;
operator Ty&() const;
template <class T1, class T2, ..., class TN>
typename result_of<T(T1, T2, ..., TN)>::type operator()(T1&, T2&, ..., TN&);
private:
Ty *ptr; // exposition only
};
Remarks
A reference_wrapper<Ty> is copy constructible and assignable, and holds a pointer that points to an object of type Ty.
A specialization reference_wrapper<Ty> is derived from std::unary_function<T1, Ret> (hence defining the nested type result_type as a synonym for Ret and the nested type argument_type as a synonym for T1) only if the type Ty is:
a function type or pointer to function type taking one argument of type T1 and returning Ret; or
a pointer to a member function Ret T::f() cv, where cv represents the member function's cv-qualifiers; the type T1 is cv``T*; or
a class type that is derived from unary_function<T1, Ret>.
A specialization reference_wrapper<Ty> is derived from std::binary_function<T1, T2, Ret> (hence defining the nested type result_type as a synonym for Ret, the nested type first_argument_type as a synonym for T1, and the nested type second_argument_type as a synonym for T2) only if the type Ty is:
a function type or pointer to function type taking two arguments of types T1 and T2 and returning Ret; or
a pointer to a member function Ret T::f(T2) cv, where cv represents the member function's cv-qualifiers; the type T1 is cv``T*; or
a class type that is derived from binary_function<T1, T2, Ret>.
Constructors
| reference_wrapper::reference_wrapper | Constructs a reference_wrapper. |
Typedefs
| reference_wrapper::result_type | The weak result type of the wrapped reference. |
| reference_wrapper::type | The type of the wrapped reference. |
Member Functions
| reference_wrapper::get | Obtains the wrapped reference. |
Operators
| reference_wrapper::operator Ty& | Gets a pointer to the wrapped reference. |
| reference_wrapper::operator() | Calls the wrapped reference. |
Requirements
Header: <functional>
Namespace: std
reference_wrapper::get
Obtains the wrapped reference.
Ty& get() const;
Remarks
The member function returns INVOKE(get(), t1, t2, ..., tN).
Example
// std_tr1__functional__reference_wrapper_get.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int main()
{
int i = 1;
std::reference_wrapper<int> rwi(i);
std::cout << "i = " << i << std::endl;
std::cout << "rwi = " << rwi << std::endl;
rwi.get() = -1;
std::cout << "i = " << i << std::endl;
return (0);
}
i = 1
rwi = 1
i = -1
reference_wrapper::operator Ty&
Gets a pointer to the wrapped reference.
operator Ty&() const;
Parameters
Remarks
The member operator returns *ptr.
Example
// std_tr1__functional__reference_wrapper_operator_cast.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int main()
{
int i = 1;
std::reference_wrapper<int> rwi(i);
std::cout << "i = " << i << std::endl;
std::cout << "(int)rwi = " << (int)rwi << std::endl;
return (0);
}
i = 1
(int)rwi = 1
reference_wrapper::operator()
Calls the wrapped reference.
template <class T1, class T2, ..., class TN>
typename result_of<T(T1, T2, ..., TN)>::type operator()(T1& t1, T2& t2, ..., TN& tN);
Parameters
TN
The type of the Nth call argument.
tN
The Nth call argument.
Remarks
The template member operator returns INVOKE(get(), t1, t2, ..., tN).
Example
// std_tr1__functional__reference_wrapper_operator_call.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int neg(int val)
{
return (-val);
}
int main()
{
std::reference_wrapper<int (int)> rwi(neg);
std::cout << "rwi(3) = " << rwi(3) << std::endl;
return (0);
}
rwi
(3) = -3
reference_wrapper::reference_wrapper
Constructs a reference_wrapper.
explicit reference_wrapper(Ty& val);
Parameters
Ty
The type to wrap.
val
The value to wrap.
Remarks
The constructor sets the stored value ptr to &val.
Example
// std_tr1__functional__reference_wrapper_reference_wrapper.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int neg(int val)
{
return (-val);
}
int main()
{
int i = 1;
std::reference_wrapper<int> rwi(i);
std::cout << "i = " << i << std::endl;
std::cout << "rwi = " << rwi << std::endl;
rwi.get() = -1;
std::cout << "i = " << i << std::endl;
return (0);
}
i = 1
rwi = 1
i = -1
reference_wrapper::result_type
The weak result type of the wrapped reference.
typedef T0 result_type;
Remarks
The typedef is a synonym for the weak result type of a wrapped function.
Example
// std_tr1__functional__reference_wrapper_result_type.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int neg(int val)
{
return (-val);
}
int main()
{
typedef std::reference_wrapper<int (int)> Mywrapper;
Mywrapper rwi(neg);
Mywrapper::result_type val = rwi(3);
std::cout << "val = " << val << std::endl;
return (0);
}
val = -3
reference_wrapper::type
The type of the wrapped reference.
typedef Ty type;
Remarks
The typedef is a synonym for the template argument Ty.
Example
// std_tr1__functional__reference_wrapper_type.cpp
// compile with: /EHsc
#include <functional>
#include <iostream>
int neg(int val)
{
return (-val);
}
int main()
{
int i = 1;
typedef std::reference_wrapper<int> Mywrapper;
Mywrapper rwi(i);
Mywrapper::type val = rwi.get();
std::cout << "i = " << i << std::endl;
std::cout << "rwi = " << val << std::endl;
return (0);
}
i = 1
rwi = 1