Is there a way to pass an rvalue by reference in C++?












2















I'm trying to create "rollback" functionality for a small database assignment.
I have a stack of binary search trees that I am using to store backups of the database called:



GenStack<GenBST<Student>> masterStudentStack;


Both the stack and BST are my own implementations (per the instructions for my assignment).



I have no problem pushing a copy of the BST onto the stack,



masterStudentStack.push(*masterStudent);


however when I try to retrieve this BST and return it to my primary BST pointer
using:



void rollBack() {
masterStudent = new GenBST<Student>(masterStudentStack.pop());
}


I receive an error.



Menu.cpp:419:63: error: invalid initialization of non-const reference of 
type ‘GenBST<Student>&’ from an rvalue of type ‘GenBST<Student>’
masterStudent = new GenBST<Student>(masterStudentStack.pop());
~~~~~~~~~~~~~~~~~~~~~~^~
In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h:49:1: note: initializing argument 1 of
‘GenBST<T>::GenBST(GenBST<T>&) [with T = Student]’
GenBST<T>::GenBST(GenBST<T>& other) {
^~~~~~~~~


The copy constructor for the BST is functional when an lvalue is passed in by reference, (this is the declaration for the constructor)



GenBST(GenBST<T>& other);


but I don't know how to pop something off the stack in a way in which the copy constructor will accept it. So, my question is: Can I something create a new BST using the rvalue "stack.pop()"?



Thanks,
Matthew



Edit:



After adding "const" to my BST copy constructor, I got this error



In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T =
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:50:22: error: passing ‘const GenBST<Student>’ as ‘this’ argument
discards qualifiers [-fpermissive]
if(other.getRoot() == NULL) {

GenBST.h:77:17: note: in call to ‘GenTreeNode<T>* GenBST<T>::getRoot()
[with T = Student]’
GenTreeNode<T>* GenBST<T>::getRoot()
^~~~~~~~~
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) [with T = Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {


Here is my constructor and the method it calls:



template <class T>
GenBST<T>::GenBST(const GenBST<T>& other) {
if(other.getRoot() == NULL) {
root = NULL;
}
else {
copyTree(this->root, other.root);
}
}

template <class T>
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {
if(otherNode == NULL) {
thisNode = NULL;
}
else {
thisNode = new GenTreeNode<T>(otherNode->key);
copyTree(thisNode->left, otherNode->left);
copyTree(thisNode->right, otherNode->right);
}
}


Any ideas?



Edit 2:



Thank so much for the help everybody. I added const to both my getRoot() and copyTree() methods and am now down to one error.



GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) const [with T =
Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) const {









share|improve this question

























  • you also need to make getRoot() and copyTree be const methods

    – M.M
    Nov 26 '18 at 3:39













  • Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

    – M.M
    Nov 26 '18 at 3:40
















2















I'm trying to create "rollback" functionality for a small database assignment.
I have a stack of binary search trees that I am using to store backups of the database called:



GenStack<GenBST<Student>> masterStudentStack;


Both the stack and BST are my own implementations (per the instructions for my assignment).



I have no problem pushing a copy of the BST onto the stack,



masterStudentStack.push(*masterStudent);


however when I try to retrieve this BST and return it to my primary BST pointer
using:



void rollBack() {
masterStudent = new GenBST<Student>(masterStudentStack.pop());
}


I receive an error.



Menu.cpp:419:63: error: invalid initialization of non-const reference of 
type ‘GenBST<Student>&’ from an rvalue of type ‘GenBST<Student>’
masterStudent = new GenBST<Student>(masterStudentStack.pop());
~~~~~~~~~~~~~~~~~~~~~~^~
In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h:49:1: note: initializing argument 1 of
‘GenBST<T>::GenBST(GenBST<T>&) [with T = Student]’
GenBST<T>::GenBST(GenBST<T>& other) {
^~~~~~~~~


The copy constructor for the BST is functional when an lvalue is passed in by reference, (this is the declaration for the constructor)



GenBST(GenBST<T>& other);


but I don't know how to pop something off the stack in a way in which the copy constructor will accept it. So, my question is: Can I something create a new BST using the rvalue "stack.pop()"?



Thanks,
Matthew



Edit:



After adding "const" to my BST copy constructor, I got this error



In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T =
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:50:22: error: passing ‘const GenBST<Student>’ as ‘this’ argument
discards qualifiers [-fpermissive]
if(other.getRoot() == NULL) {

GenBST.h:77:17: note: in call to ‘GenTreeNode<T>* GenBST<T>::getRoot()
[with T = Student]’
GenTreeNode<T>* GenBST<T>::getRoot()
^~~~~~~~~
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) [with T = Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {


Here is my constructor and the method it calls:



template <class T>
GenBST<T>::GenBST(const GenBST<T>& other) {
if(other.getRoot() == NULL) {
root = NULL;
}
else {
copyTree(this->root, other.root);
}
}

template <class T>
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {
if(otherNode == NULL) {
thisNode = NULL;
}
else {
thisNode = new GenTreeNode<T>(otherNode->key);
copyTree(thisNode->left, otherNode->left);
copyTree(thisNode->right, otherNode->right);
}
}


Any ideas?



Edit 2:



Thank so much for the help everybody. I added const to both my getRoot() and copyTree() methods and am now down to one error.



GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) const [with T =
Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) const {









share|improve this question

























  • you also need to make getRoot() and copyTree be const methods

    – M.M
    Nov 26 '18 at 3:39













  • Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

    – M.M
    Nov 26 '18 at 3:40














2












2








2








I'm trying to create "rollback" functionality for a small database assignment.
I have a stack of binary search trees that I am using to store backups of the database called:



GenStack<GenBST<Student>> masterStudentStack;


Both the stack and BST are my own implementations (per the instructions for my assignment).



I have no problem pushing a copy of the BST onto the stack,



masterStudentStack.push(*masterStudent);


however when I try to retrieve this BST and return it to my primary BST pointer
using:



void rollBack() {
masterStudent = new GenBST<Student>(masterStudentStack.pop());
}


I receive an error.



Menu.cpp:419:63: error: invalid initialization of non-const reference of 
type ‘GenBST<Student>&’ from an rvalue of type ‘GenBST<Student>’
masterStudent = new GenBST<Student>(masterStudentStack.pop());
~~~~~~~~~~~~~~~~~~~~~~^~
In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h:49:1: note: initializing argument 1 of
‘GenBST<T>::GenBST(GenBST<T>&) [with T = Student]’
GenBST<T>::GenBST(GenBST<T>& other) {
^~~~~~~~~


The copy constructor for the BST is functional when an lvalue is passed in by reference, (this is the declaration for the constructor)



GenBST(GenBST<T>& other);


but I don't know how to pop something off the stack in a way in which the copy constructor will accept it. So, my question is: Can I something create a new BST using the rvalue "stack.pop()"?



Thanks,
Matthew



Edit:



After adding "const" to my BST copy constructor, I got this error



In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T =
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:50:22: error: passing ‘const GenBST<Student>’ as ‘this’ argument
discards qualifiers [-fpermissive]
if(other.getRoot() == NULL) {

GenBST.h:77:17: note: in call to ‘GenTreeNode<T>* GenBST<T>::getRoot()
[with T = Student]’
GenTreeNode<T>* GenBST<T>::getRoot()
^~~~~~~~~
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) [with T = Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {


Here is my constructor and the method it calls:



template <class T>
GenBST<T>::GenBST(const GenBST<T>& other) {
if(other.getRoot() == NULL) {
root = NULL;
}
else {
copyTree(this->root, other.root);
}
}

template <class T>
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {
if(otherNode == NULL) {
thisNode = NULL;
}
else {
thisNode = new GenTreeNode<T>(otherNode->key);
copyTree(thisNode->left, otherNode->left);
copyTree(thisNode->right, otherNode->right);
}
}


Any ideas?



Edit 2:



Thank so much for the help everybody. I added const to both my getRoot() and copyTree() methods and am now down to one error.



GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) const [with T =
Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) const {









share|improve this question
















I'm trying to create "rollback" functionality for a small database assignment.
I have a stack of binary search trees that I am using to store backups of the database called:



GenStack<GenBST<Student>> masterStudentStack;


Both the stack and BST are my own implementations (per the instructions for my assignment).



I have no problem pushing a copy of the BST onto the stack,



masterStudentStack.push(*masterStudent);


however when I try to retrieve this BST and return it to my primary BST pointer
using:



void rollBack() {
masterStudent = new GenBST<Student>(masterStudentStack.pop());
}


I receive an error.



Menu.cpp:419:63: error: invalid initialization of non-const reference of 
type ‘GenBST<Student>&’ from an rvalue of type ‘GenBST<Student>’
masterStudent = new GenBST<Student>(masterStudentStack.pop());
~~~~~~~~~~~~~~~~~~~~~~^~
In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h:49:1: note: initializing argument 1 of
‘GenBST<T>::GenBST(GenBST<T>&) [with T = Student]’
GenBST<T>::GenBST(GenBST<T>& other) {
^~~~~~~~~


The copy constructor for the BST is functional when an lvalue is passed in by reference, (this is the declaration for the constructor)



GenBST(GenBST<T>& other);


but I don't know how to pop something off the stack in a way in which the copy constructor will accept it. So, my question is: Can I something create a new BST using the rvalue "stack.pop()"?



Thanks,
Matthew



Edit:



After adding "const" to my BST copy constructor, I got this error



In file included from Menu.h:7:0,
from Menu.cpp:1:
GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T =
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:50:22: error: passing ‘const GenBST<Student>’ as ‘this’ argument
discards qualifiers [-fpermissive]
if(other.getRoot() == NULL) {

GenBST.h:77:17: note: in call to ‘GenTreeNode<T>* GenBST<T>::getRoot()
[with T = Student]’
GenTreeNode<T>* GenBST<T>::getRoot()
^~~~~~~~~
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) [with T = Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {


Here is my constructor and the method it calls:



template <class T>
GenBST<T>::GenBST(const GenBST<T>& other) {
if(other.getRoot() == NULL) {
root = NULL;
}
else {
copyTree(this->root, other.root);
}
}

template <class T>
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) {
if(otherNode == NULL) {
thisNode = NULL;
}
else {
thisNode = new GenTreeNode<T>(otherNode->key);
copyTree(thisNode->left, otherNode->left);
copyTree(thisNode->right, otherNode->right);
}
}


Any ideas?



Edit 2:



Thank so much for the help everybody. I added const to both my getRoot() and copyTree() methods and am now down to one error.



GenBST.h: In instantiation of ‘GenBST<T>::GenBST(const GenBST<T>&) [with T = 
Student]’:
Menu.cpp:419:65: required from here
GenBST.h:54:32: error: binding ‘GenTreeNode<Student>* const’ to reference of
type ‘GenTreeNode<Student>*&’ discards qualifiers
copyTree(this->root, other.root);
~~~~~~^~~~
GenBST.h:108:6: note: initializing argument 2 of ‘void
GenBST<T>::copyTree(GenTreeNode<T>*&, GenTreeNode<T>*&) const [with T =
Student]’
void GenBST<T>::copyTree(GenTreeNode<T> *& thisNode, GenTreeNode<T> *&
otherNode) const {






c++ stack binary-search-tree rvalue lvalue






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 26 '18 at 3:44







Matthew Parnham

















asked Nov 26 '18 at 3:20









Matthew ParnhamMatthew Parnham

224




224













  • you also need to make getRoot() and copyTree be const methods

    – M.M
    Nov 26 '18 at 3:39













  • Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

    – M.M
    Nov 26 '18 at 3:40



















  • you also need to make getRoot() and copyTree be const methods

    – M.M
    Nov 26 '18 at 3:39













  • Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

    – M.M
    Nov 26 '18 at 3:40

















you also need to make getRoot() and copyTree be const methods

– M.M
Nov 26 '18 at 3:39







you also need to make getRoot() and copyTree be const methods

– M.M
Nov 26 '18 at 3:39















Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

– M.M
Nov 26 '18 at 3:40





Also you probably shouldn't be using new and delete with masterStudent (it could be a GenBST instead of a pointer to one , since you seem to be using value semantics with it anyway)

– M.M
Nov 26 '18 at 3:40












1 Answer
1






active

oldest

votes


















2














The canonical form of a copy constructor takes a reference to a const object to copy. Conceptually, making a copy of something normally implies that the original object remains unchanged. There should hardly ever be a need to modify the object you're copying. An rvalue can be bound to a reference to const but not to a reference to non-const. Unless making a copy of a GenBST really does actually entail modifying the object you're copying (I assume and sincerely hope it does not), you can simply change the signature of your copy constructor to



GenBST(const GenBST& other);





share|improve this answer


























  • Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

    – Matthew Parnham
    Nov 26 '18 at 3:32













  • Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

    – Michael Kenzel
    Nov 26 '18 at 3:40













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1 Answer
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oldest

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1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









2














The canonical form of a copy constructor takes a reference to a const object to copy. Conceptually, making a copy of something normally implies that the original object remains unchanged. There should hardly ever be a need to modify the object you're copying. An rvalue can be bound to a reference to const but not to a reference to non-const. Unless making a copy of a GenBST really does actually entail modifying the object you're copying (I assume and sincerely hope it does not), you can simply change the signature of your copy constructor to



GenBST(const GenBST& other);





share|improve this answer


























  • Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

    – Matthew Parnham
    Nov 26 '18 at 3:32













  • Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

    – Michael Kenzel
    Nov 26 '18 at 3:40


















2














The canonical form of a copy constructor takes a reference to a const object to copy. Conceptually, making a copy of something normally implies that the original object remains unchanged. There should hardly ever be a need to modify the object you're copying. An rvalue can be bound to a reference to const but not to a reference to non-const. Unless making a copy of a GenBST really does actually entail modifying the object you're copying (I assume and sincerely hope it does not), you can simply change the signature of your copy constructor to



GenBST(const GenBST& other);





share|improve this answer


























  • Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

    – Matthew Parnham
    Nov 26 '18 at 3:32













  • Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

    – Michael Kenzel
    Nov 26 '18 at 3:40
















2












2








2







The canonical form of a copy constructor takes a reference to a const object to copy. Conceptually, making a copy of something normally implies that the original object remains unchanged. There should hardly ever be a need to modify the object you're copying. An rvalue can be bound to a reference to const but not to a reference to non-const. Unless making a copy of a GenBST really does actually entail modifying the object you're copying (I assume and sincerely hope it does not), you can simply change the signature of your copy constructor to



GenBST(const GenBST& other);





share|improve this answer















The canonical form of a copy constructor takes a reference to a const object to copy. Conceptually, making a copy of something normally implies that the original object remains unchanged. There should hardly ever be a need to modify the object you're copying. An rvalue can be bound to a reference to const but not to a reference to non-const. Unless making a copy of a GenBST really does actually entail modifying the object you're copying (I assume and sincerely hope it does not), you can simply change the signature of your copy constructor to



GenBST(const GenBST& other);






share|improve this answer














share|improve this answer



share|improve this answer








edited Nov 26 '18 at 3:34

























answered Nov 26 '18 at 3:27









Michael KenzelMichael Kenzel

4,2781819




4,2781819













  • Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

    – Matthew Parnham
    Nov 26 '18 at 3:32













  • Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

    – Michael Kenzel
    Nov 26 '18 at 3:40





















  • Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

    – Matthew Parnham
    Nov 26 '18 at 3:32













  • Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

    – Michael Kenzel
    Nov 26 '18 at 3:40



















Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

– Matthew Parnham
Nov 26 '18 at 3:32







Thank you for the quick reply! I tried this, but it resulted in a different error that I don't quite understand. I'll attach it to my original post.

– Matthew Parnham
Nov 26 '18 at 3:32















Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

– Michael Kenzel
Nov 26 '18 at 3:40







Well, the problem is that const is transitive. Your copyTree() method expects references to non-const objects but you give it references to objects that are now const. You'll have to add const there too. With const-correctness it's all or nothing…

– Michael Kenzel
Nov 26 '18 at 3:40




















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