Why is the copy constructor called twice in this code snippet?
I'm playing around with a few things to understand how copy constructors work. But I can't make sense of why the copy constructor is called twice for the creation of x2. I would have assumed it would be called once when the return value of createX() is copied into x2.
I also looked at a few related questions on SO, but as far as I can tell I couldn't find the same simple scenario as I am asking here.
By the way, I'm compiling with -fno-elide-constructors in order to see what's going on without optimizations.
#include <iostream>
struct X {
int i{2};
X() {
std::cout << "default constructor called" << std::endl;
}
X(const X& other) {
std::cout << "copy constructor called" << std::endl;
}
};
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x;
}
int main() {
X x1;
std::cout << "created x1" << std::endl;
std::cout << "x1: " << x1.i << std::endl << std::endl;
X x2 = createX();
std::cout << "created x2" << std::endl;
std::cout << "x2: " << x2.i << std::endl;
return 0;
}
This is the output:
default constructor called
created x1
x1: 2
default constructor called
created x on the stack
copy constructor called
copy constructor called
created x2
x2: 2
Can someone help me what I'm missing or overlooking here?
c++ c++14 copy-constructor
edited 12 hours ago
NathanOliver
93.8k16130199
asked 12 hours ago
c_studentc_student
635
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c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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add a comment |
I'm playing around with a few things to understand how copy constructors work. But I can't make sense of why the copy constructor is called twice for the creation of x2. I would have assumed it would be called once when the return value of createX() is copied into x2.
I also looked at a few related questions on SO, but as far as I can tell I couldn't find the same simple scenario as I am asking here.
By the way, I'm compiling with -fno-elide-constructors in order to see what's going on without optimizations.
#include <iostream>
struct X {
int i{2};
X() {
std::cout << "default constructor called" << std::endl;
}
X(const X& other) {
std::cout << "copy constructor called" << std::endl;
}
};
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x;
}
int main() {
X x1;
std::cout << "created x1" << std::endl;
std::cout << "x1: " << x1.i << std::endl << std::endl;
X x2 = createX();
std::cout << "created x2" << std::endl;
std::cout << "x2: " << x2.i << std::endl;
return 0;
}
This is the output:
default constructor called
created x1
x1: 2
default constructor called
created x on the stack
copy constructor called
copy constructor called
created x2
x2: 2
Can someone help me what I'm missing or overlooking here?
c++ c++14 copy-constructor
edited 12 hours ago
NathanOliver
93.8k16130199
asked 12 hours ago
c_studentc_student
635
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
I'm playing around with a few things to understand how copy constructors work. But I can't make sense of why the copy constructor is called twice for the creation of x2. I would have assumed it would be called once when the return value of createX() is copied into x2.
I also looked at a few related questions on SO, but as far as I can tell I couldn't find the same simple scenario as I am asking here.
By the way, I'm compiling with -fno-elide-constructors in order to see what's going on without optimizations.
#include <iostream>
struct X {
int i{2};
X() {
std::cout << "default constructor called" << std::endl;
}
X(const X& other) {
std::cout << "copy constructor called" << std::endl;
}
};
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x;
}
int main() {
X x1;
std::cout << "created x1" << std::endl;
std::cout << "x1: " << x1.i << std::endl << std::endl;
X x2 = createX();
std::cout << "created x2" << std::endl;
std::cout << "x2: " << x2.i << std::endl;
return 0;
}
This is the output:
default constructor called
created x1
x1: 2
default constructor called
created x on the stack
copy constructor called
copy constructor called
created x2
x2: 2
Can someone help me what I'm missing or overlooking here?
c++ c++14 copy-constructor
edited 12 hours ago
NathanOliver
93.8k16130199
asked 12 hours ago
c_studentc_student
635
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
I'm playing around with a few things to understand how copy constructors work. But I can't make sense of why the copy constructor is called twice for the creation of x2. I would have assumed it would be called once when the return value of createX() is copied into x2.
I also looked at a few related questions on SO, but as far as I can tell I couldn't find the same simple scenario as I am asking here.
By the way, I'm compiling with -fno-elide-constructors in order to see what's going on without optimizations.
#include <iostream>
struct X {
int i{2};
X() {
std::cout << "default constructor called" << std::endl;
}
X(const X& other) {
std::cout << "copy constructor called" << std::endl;
}
};
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x;
}
int main() {
X x1;
std::cout << "created x1" << std::endl;
std::cout << "x1: " << x1.i << std::endl << std::endl;
X x2 = createX();
std::cout << "created x2" << std::endl;
std::cout << "x2: " << x2.i << std::endl;
return 0;
}
This is the output:
default constructor called
created x1
x1: 2
default constructor called
created x on the stack
copy constructor called
copy constructor called
created x2
x2: 2
Can someone help me what I'm missing or overlooking here?
c++ c++14 copy-constructor
c++ c++14 copy-constructor
edited 12 hours ago
NathanOliver
93.8k16130199
asked 12 hours ago
c_studentc_student
635
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 12 hours ago
NathanOliver
93.8k16130199
asked 12 hours ago
c_studentc_student
635
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c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 12 hours ago
NathanOliver
93.8k16130199
edited 12 hours ago
NathanOliver
93.8k16130199
edited 12 hours ago
NathanOliver
93.8k16130199
93.8k16130199
asked 12 hours ago
c_studentc_student
635
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 12 hours ago
c_studentc_student
635
asked 12 hours ago
c_studentc_student
635
635
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
c_student is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
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add a comment |
add a comment |
2 Answers
2
active
oldest
votes
What you have to remember here is that the return value of a function is a distinct object. When you do
return x;
you copy initialize the return value object with x. This is the first copy constructor call you see. Then
X x2 = createX();
uses the returned object to copy initialize x2 so that is the second copy you see.
One thing to note is that
return x;
will try to move x into the return object if it can. Had you made a move constructor you would have seen this called. The reason for this is that since local objects go out of scope at the end of the function, the compiler treats the object as an rvalue and only if that does not find a valid overload does it fall back to returning it as an lvalue.
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of-fno-elide-constructorsbut I'm assuming it prevents RVO.
– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
|
show 1 more comment
First copy is in return of createX
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x; // First copy
}
Second one is to create x2 from the temporary return by createX.
X x2 = createX(); // Second copy
Notice that in C++17, second copy is forced to be elided.
answered 12 hours ago
Jarod42Jarod42
117k12103186
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
What you have to remember here is that the return value of a function is a distinct object. When you do
return x;
you copy initialize the return value object with x. This is the first copy constructor call you see. Then
X x2 = createX();
uses the returned object to copy initialize x2 so that is the second copy you see.
One thing to note is that
return x;
will try to move x into the return object if it can. Had you made a move constructor you would have seen this called. The reason for this is that since local objects go out of scope at the end of the function, the compiler treats the object as an rvalue and only if that does not find a valid overload does it fall back to returning it as an lvalue.
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of-fno-elide-constructorsbut I'm assuming it prevents RVO.
– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
|
show 1 more comment
What you have to remember here is that the return value of a function is a distinct object. When you do
return x;
you copy initialize the return value object with x. This is the first copy constructor call you see. Then
X x2 = createX();
uses the returned object to copy initialize x2 so that is the second copy you see.
One thing to note is that
return x;
will try to move x into the return object if it can. Had you made a move constructor you would have seen this called. The reason for this is that since local objects go out of scope at the end of the function, the compiler treats the object as an rvalue and only if that does not find a valid overload does it fall back to returning it as an lvalue.
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of-fno-elide-constructorsbut I'm assuming it prevents RVO.
– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
|
show 1 more comment
What you have to remember here is that the return value of a function is a distinct object. When you do
return x;
you copy initialize the return value object with x. This is the first copy constructor call you see. Then
X x2 = createX();
uses the returned object to copy initialize x2 so that is the second copy you see.
One thing to note is that
return x;
will try to move x into the return object if it can. Had you made a move constructor you would have seen this called. The reason for this is that since local objects go out of scope at the end of the function, the compiler treats the object as an rvalue and only if that does not find a valid overload does it fall back to returning it as an lvalue.
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
What you have to remember here is that the return value of a function is a distinct object. When you do
return x;
you copy initialize the return value object with x. This is the first copy constructor call you see. Then
X x2 = createX();
uses the returned object to copy initialize x2 so that is the second copy you see.
One thing to note is that
return x;
will try to move x into the return object if it can. Had you made a move constructor you would have seen this called. The reason for this is that since local objects go out of scope at the end of the function, the compiler treats the object as an rvalue and only if that does not find a valid overload does it fall back to returning it as an lvalue.
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
edited 12 hours ago
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
answered 12 hours ago
NathanOliverNathanOliver
93.8k16130199
93.8k16130199
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of-fno-elide-constructorsbut I'm assuming it prevents RVO.
– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
|
show 1 more comment
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of-fno-elide-constructorsbut I'm assuming it prevents RVO.
– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
@FrançoisAndrieux Yeah. Updating the wording to make that more explicit that isn't something that is just allowed but has to be done.
– NathanOliver
12 hours ago
I'm not sure of the details of
-fno-elide-constructors but I'm assuming it prevents RVO.– François Andrieux
12 hours ago
I'm not sure of the details of
-fno-elide-constructors but I'm assuming it prevents RVO.– François Andrieux
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
@FrançoisAndrieux It does not prevent guaranteed RVO if using C++17 since that is mandated that it has to happen, there isn't actually a temporary object created. See here where it only uses a single copy as the second one is mandated not to happen.
– NathanOliver
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
So we have to assume OP is using a pre-C++17 standard?
– François Andrieux
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
Kind of. The compiler they are using could have a bug, or they are using pre C++17. I'm inclined to believe it is the latter.
– NathanOliver
12 hours ago
|
show 1 more comment
First copy is in return of createX
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x; // First copy
}
Second one is to create x2 from the temporary return by createX.
X x2 = createX(); // Second copy
Notice that in C++17, second copy is forced to be elided.
answered 12 hours ago
Jarod42Jarod42
117k12103186
add a comment |
First copy is in return of createX
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x; // First copy
}
Second one is to create x2 from the temporary return by createX.
X x2 = createX(); // Second copy
Notice that in C++17, second copy is forced to be elided.
answered 12 hours ago
Jarod42Jarod42
117k12103186
add a comment |
First copy is in return of createX
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x; // First copy
}
Second one is to create x2 from the temporary return by createX.
X x2 = createX(); // Second copy
Notice that in C++17, second copy is forced to be elided.
answered 12 hours ago
Jarod42Jarod42
117k12103186
First copy is in return of createX
X createX() {
X x;
std::cout << "created x on the stack" << std::endl;
return x; // First copy
}
Second one is to create x2 from the temporary return by createX.
X x2 = createX(); // Second copy
Notice that in C++17, second copy is forced to be elided.
answered 12 hours ago
Jarod42Jarod42
117k12103186
answered 12 hours ago
Jarod42Jarod42
117k12103186
answered 12 hours ago
Jarod42Jarod42
117k12103186
answered 12 hours ago
Jarod42Jarod42
117k12103186
117k12103186
add a comment |
add a comment |
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