Why is my code printing rvalue 2 times instead of rvalue & lvalue?C++0x rvalue references - lvalues-rvalue bindingWhat are rvalues, lvalues, xvalues, glvalues, and prvalues?Classes, Rvalues and Rvalue ReferencesWould you ever mark a C++ RValue reference parameter as constProviding different implementations of a class depending on lvalue/rvalue when using expression templatesVariadic template class constructor with lvalues and rvaluesC++ Operator Overloading [ ] for lvalue and rvalueWhy rvalue reference binding to xvalue doesn't work in my code?When to prefer const lvalue reference over rvalue reference templatesUnderstanding perfect forwarding
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Why is my code printing rvalue 2 times instead of rvalue & lvalue?
C++0x rvalue references - lvalues-rvalue bindingWhat are rvalues, lvalues, xvalues, glvalues, and prvalues?Classes, Rvalues and Rvalue ReferencesWould you ever mark a C++ RValue reference parameter as constProviding different implementations of a class depending on lvalue/rvalue when using expression templatesVariadic template class constructor with lvalues and rvaluesC++ Operator Overloading [ ] for lvalue and rvalueWhy rvalue reference binding to xvalue doesn't work in my code?When to prefer const lvalue reference over rvalue reference templatesUnderstanding perfect forwarding
So I wanted to practice the usage of std::forward and created a Test class with 2 constructors. 1 with T& and the other with T&& as overload. T& prints lvalue, and T&& prints rvalue so I know which one of the constructors is being used. I create 2 instances of class on stack and to my surprise both of which are using the T&& overload.
#include <iostream>
#include <type_traits>
#include <utility>
template <class T> auto forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
return std::move(t);
template <class T> class Test
public:
Test(T &) std::cout << "lvalue" << std::endl; ;
Test(T &&) std::cout << "rvalue" << std::endl; ;
;
int main()
int x = 5;
Test<int> a(forward(3));
Test<int> b(forward(x));
return 0;
I tried using the original std::forward function and implementing it but both times it printed rvalue x2. What am I doing wrong?
c++ forward rvalue lvalue stdmove
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
add a comment |
So I wanted to practice the usage of std::forward and created a Test class with 2 constructors. 1 with T& and the other with T&& as overload. T& prints lvalue, and T&& prints rvalue so I know which one of the constructors is being used. I create 2 instances of class on stack and to my surprise both of which are using the T&& overload.
#include <iostream>
#include <type_traits>
#include <utility>
template <class T> auto forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
return std::move(t);
template <class T> class Test
public:
Test(T &) std::cout << "lvalue" << std::endl; ;
Test(T &&) std::cout << "rvalue" << std::endl; ;
;
int main()
int x = 5;
Test<int> a(forward(3));
Test<int> b(forward(x));
return 0;
I tried using the original std::forward function and implementing it but both times it printed rvalue x2. What am I doing wrong?
c++ forward rvalue lvalue stdmove
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
2
autois never a reference.
– Evg
Jun 21 at 15:42
add a comment |
So I wanted to practice the usage of std::forward and created a Test class with 2 constructors. 1 with T& and the other with T&& as overload. T& prints lvalue, and T&& prints rvalue so I know which one of the constructors is being used. I create 2 instances of class on stack and to my surprise both of which are using the T&& overload.
#include <iostream>
#include <type_traits>
#include <utility>
template <class T> auto forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
return std::move(t);
template <class T> class Test
public:
Test(T &) std::cout << "lvalue" << std::endl; ;
Test(T &&) std::cout << "rvalue" << std::endl; ;
;
int main()
int x = 5;
Test<int> a(forward(3));
Test<int> b(forward(x));
return 0;
I tried using the original std::forward function and implementing it but both times it printed rvalue x2. What am I doing wrong?
c++ forward rvalue lvalue stdmove
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
So I wanted to practice the usage of std::forward and created a Test class with 2 constructors. 1 with T& and the other with T&& as overload. T& prints lvalue, and T&& prints rvalue so I know which one of the constructors is being used. I create 2 instances of class on stack and to my surprise both of which are using the T&& overload.
#include <iostream>
#include <type_traits>
#include <utility>
template <class T> auto forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
return std::move(t);
template <class T> class Test
public:
Test(T &) std::cout << "lvalue" << std::endl; ;
Test(T &&) std::cout << "rvalue" << std::endl; ;
;
int main()
int x = 5;
Test<int> a(forward(3));
Test<int> b(forward(x));
return 0;
I tried using the original std::forward function and implementing it but both times it printed rvalue x2. What am I doing wrong?
c++ forward rvalue lvalue stdmove
c++ forward rvalue lvalue stdmove
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
edited Jun 21 at 15:46
Evg
6,0573 gold badges20 silver badges43 bronze badges
6,0573 gold badges20 silver badges43 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
asked Jun 21 at 15:34
3l4x3l4x
483 bronze badges
483 bronze badges
2
autois never a reference.
– Evg
Jun 21 at 15:42
add a comment |
2
autois never a reference.
– Evg
Jun 21 at 15:42
2
2
auto is never a reference.– Evg
Jun 21 at 15:42
auto is never a reference.– Evg
Jun 21 at 15:42
add a comment |
1 Answer
1
active
oldest
votes
Your problem stems from the return type of forward. You use auto as the return type which will not deduce a reference for you. That means when you do return, no matter which branch it returns from, you return by value which means you have a prvalue.
What you need is decltype(auto) so you return an rvalue or lvalue reference, depending on the return statement. Using
template <class T> decltype(auto) forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
else
return std::move(t);
gives you the output:
rvalue
lvalue
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
Isn't there such a thing as anauto&, or am I making up silliness?
– Chipster
Jun 21 at 15:46
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
add a comment |
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1 Answer
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active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Your problem stems from the return type of forward. You use auto as the return type which will not deduce a reference for you. That means when you do return, no matter which branch it returns from, you return by value which means you have a prvalue.
What you need is decltype(auto) so you return an rvalue or lvalue reference, depending on the return statement. Using
template <class T> decltype(auto) forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
else
return std::move(t);
gives you the output:
rvalue
lvalue
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
Isn't there such a thing as anauto&, or am I making up silliness?
– Chipster
Jun 21 at 15:46
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
add a comment |
Your problem stems from the return type of forward. You use auto as the return type which will not deduce a reference for you. That means when you do return, no matter which branch it returns from, you return by value which means you have a prvalue.
What you need is decltype(auto) so you return an rvalue or lvalue reference, depending on the return statement. Using
template <class T> decltype(auto) forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
else
return std::move(t);
gives you the output:
rvalue
lvalue
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
Isn't there such a thing as anauto&, or am I making up silliness?
– Chipster
Jun 21 at 15:46
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
add a comment |
Your problem stems from the return type of forward. You use auto as the return type which will not deduce a reference for you. That means when you do return, no matter which branch it returns from, you return by value which means you have a prvalue.
What you need is decltype(auto) so you return an rvalue or lvalue reference, depending on the return statement. Using
template <class T> decltype(auto) forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
else
return std::move(t);
gives you the output:
rvalue
lvalue
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
Your problem stems from the return type of forward. You use auto as the return type which will not deduce a reference for you. That means when you do return, no matter which branch it returns from, you return by value which means you have a prvalue.
What you need is decltype(auto) so you return an rvalue or lvalue reference, depending on the return statement. Using
template <class T> decltype(auto) forward(T &&t)
if constexpr (std::is_lvalue_reference<T>::value)
return t;
else
return std::move(t);
gives you the output:
rvalue
lvalue
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
edited Jun 21 at 15:47
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
answered Jun 21 at 15:43
NathanOliverNathanOliver
107k19 gold badges159 silver badges236 bronze badges
107k19 gold badges159 silver badges236 bronze badges
Isn't there such a thing as anauto&, or am I making up silliness?
– Chipster
Jun 21 at 15:46
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
add a comment |
Isn't there such a thing as anauto&, or am I making up silliness?
– Chipster
Jun 21 at 15:46
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
Isn't there such a thing as an
auto&, or am I making up silliness?– Chipster
Jun 21 at 15:46
Isn't there such a thing as an
auto&, or am I making up silliness?– Chipster
Jun 21 at 15:46
3
3
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
@Chipster There is, but that will always return an lvalue reference, which would not be what you want in a forwarding function.
– NathanOliver
Jun 21 at 15:47
thank you, problem solved
– 3l4x
Jun 21 at 15:49
thank you, problem solved
– 3l4x
Jun 21 at 15:49
2
2
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
@3l4x You'e welcome. Good first effort BTW. Generic code and forwarding can be pretty tough to get right.
– NathanOliver
Jun 21 at 15:51
1
1
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
@NathanOliver, even for compilers. ;)
– Evg
Jun 21 at 16:30
add a comment |
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2
autois never a reference.– Evg
Jun 21 at 15:42