Is it safe to use c_str() on a temporary string?
#include <iostream>
std::string get_data()
{
return "Hello";
}
int main()
{
const char* data = get_data().c_str();
std::cout << data << "n";
return 0;
}
"Hello" is printing on my machine; however, I am led to believe that this behavior is unspecified i.e. implementation-specific. Am I correct or will it always print "Hello", judging that the returned string is immutable and as such qualified as something that is constant?
c++ string object-lifetime
edited 1 hour ago
Hong Ooi
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
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|
show 5 more comments
#include <iostream>
std::string get_data()
{
return "Hello";
}
int main()
{
const char* data = get_data().c_str();
std::cout << data << "n";
return 0;
}
"Hello" is printing on my machine; however, I am led to believe that this behavior is unspecified i.e. implementation-specific. Am I correct or will it always print "Hello", judging that the returned string is immutable and as such qualified as something that is constant?
c++ string object-lifetime
edited 1 hour ago
Hong Ooi
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
Where does that string that gets returned go afterc_str()is called and returns a pointer to some data?
– tadman
5 hours ago
2
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
2
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing#include <string>so technically it would be a compiler error ;)
– Tas
5 hours ago
1
I'm a bit surprised that the documentation forstd::string::c_strdoesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly
– alter igel
5 hours ago
1
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago
|
show 5 more comments
#include <iostream>
std::string get_data()
{
return "Hello";
}
int main()
{
const char* data = get_data().c_str();
std::cout << data << "n";
return 0;
}
"Hello" is printing on my machine; however, I am led to believe that this behavior is unspecified i.e. implementation-specific. Am I correct or will it always print "Hello", judging that the returned string is immutable and as such qualified as something that is constant?
c++ string object-lifetime
edited 1 hour ago
Hong Ooi
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
#include <iostream>
std::string get_data()
{
return "Hello";
}
int main()
{
const char* data = get_data().c_str();
std::cout << data << "n";
return 0;
}
"Hello" is printing on my machine; however, I am led to believe that this behavior is unspecified i.e. implementation-specific. Am I correct or will it always print "Hello", judging that the returned string is immutable and as such qualified as something that is constant?
c++ string object-lifetime
c++ string object-lifetime
edited 1 hour ago
Hong Ooi
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 1 hour ago
Hong Ooi
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 1 hour ago
Hong Ooi
43.1k1097139
edited 1 hour ago
Hong Ooi
43.1k1097139
edited 1 hour ago
Hong Ooi
43.1k1097139
43.1k1097139
asked 5 hours ago
Aknin AbdoAknin Abdo
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 5 hours ago
Aknin AbdoAknin Abdo
491
asked 5 hours ago
Aknin AbdoAknin Abdo
491
491
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Aknin Abdo is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
1
Where does that string that gets returned go afterc_str()is called and returns a pointer to some data?
– tadman
5 hours ago
2
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
2
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing#include <string>so technically it would be a compiler error ;)
– Tas
5 hours ago
1
I'm a bit surprised that the documentation forstd::string::c_strdoesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly
– alter igel
5 hours ago
1
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago
|
show 5 more comments
1
Where does that string that gets returned go afterc_str()is called and returns a pointer to some data?
– tadman
5 hours ago
2
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
2
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing#include <string>so technically it would be a compiler error ;)
– Tas
5 hours ago
1
I'm a bit surprised that the documentation forstd::string::c_strdoesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly
– alter igel
5 hours ago
1
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago
1
1
Where does that string that gets returned go after
c_str() is called and returns a pointer to some data?– tadman
5 hours ago
Where does that string that gets returned go after
c_str() is called and returns a pointer to some data?– tadman
5 hours ago
2
2
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
2
2
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing
#include <string> so technically it would be a compiler error ;)– Tas
5 hours ago
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing
#include <string> so technically it would be a compiler error ;)– Tas
5 hours ago
1
1
I'm a bit surprised that the documentation for
std::string::c_str doesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly– alter igel
5 hours ago
I'm a bit surprised that the documentation for
std::string::c_str doesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly– alter igel
5 hours ago
1
1
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago
|
show 5 more comments
2 Answers
2
active
oldest
votes
The code exhibits undefined behavior.
get_data() returns a temporary which expires at the end of the full expression (*):
const char* data = get_data().c_str() ;
// ^~~~~~~~~~ ^
// this evaluates |
// to a prvalue |
// temporary expires here
data points to an internal of that object, so after the temporary ends you are left with a dangling pointer. Accessing it leads to Undefined Behavior. So the next line std::cout << data << "n"; makes the whole program exhibit Undefined Behavior.
*) There is an exception to this rule which doesn't apply here. If a prvalue is directly bound to a reference, the lifetime of the prvalue is extended to the lifetime of the reference.
For instance, this would have been fine:
int main()
{
const std::string& ref = get_data();
const char* data = ref.c_str();
std::cout << data << "n";
return 0;
}
answered 5 hours ago
bolovbolov
33.1k876141
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returnedstd::stringare separate objects.
– user4581301
4 hours ago
|
show 2 more comments
Yes it is, but not the way you're doing it.
If you did this:
std::cout << get_data().c_str() << 'n';
you'd be just fine.
That's because a temporary is guaranteed to live for the lifetime of the full expression it was created in. It may live longer in certain, very specific circumstances.
If you bind a const reference to a temporary, it's lifetime will be extended to be the lifetime of the name it was bound to. So, code like this:
std::string const &x = get_data();
std::cout << x.c_str() << 'n';
would also work because the temporary returned by get_data would be bound to the reference named x, and so as long as x remained a valid name to use, the temporary would still exist.
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
1
A const reference, I think you mean.
– Nemo
1 hour ago
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
The code exhibits undefined behavior.
get_data() returns a temporary which expires at the end of the full expression (*):
const char* data = get_data().c_str() ;
// ^~~~~~~~~~ ^
// this evaluates |
// to a prvalue |
// temporary expires here
data points to an internal of that object, so after the temporary ends you are left with a dangling pointer. Accessing it leads to Undefined Behavior. So the next line std::cout << data << "n"; makes the whole program exhibit Undefined Behavior.
*) There is an exception to this rule which doesn't apply here. If a prvalue is directly bound to a reference, the lifetime of the prvalue is extended to the lifetime of the reference.
For instance, this would have been fine:
int main()
{
const std::string& ref = get_data();
const char* data = ref.c_str();
std::cout << data << "n";
return 0;
}
answered 5 hours ago
bolovbolov
33.1k876141
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returnedstd::stringare separate objects.
– user4581301
4 hours ago
|
show 2 more comments
The code exhibits undefined behavior.
get_data() returns a temporary which expires at the end of the full expression (*):
const char* data = get_data().c_str() ;
// ^~~~~~~~~~ ^
// this evaluates |
// to a prvalue |
// temporary expires here
data points to an internal of that object, so after the temporary ends you are left with a dangling pointer. Accessing it leads to Undefined Behavior. So the next line std::cout << data << "n"; makes the whole program exhibit Undefined Behavior.
*) There is an exception to this rule which doesn't apply here. If a prvalue is directly bound to a reference, the lifetime of the prvalue is extended to the lifetime of the reference.
For instance, this would have been fine:
int main()
{
const std::string& ref = get_data();
const char* data = ref.c_str();
std::cout << data << "n";
return 0;
}
answered 5 hours ago
bolovbolov
33.1k876141
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returnedstd::stringare separate objects.
– user4581301
4 hours ago
|
show 2 more comments
The code exhibits undefined behavior.
get_data() returns a temporary which expires at the end of the full expression (*):
const char* data = get_data().c_str() ;
// ^~~~~~~~~~ ^
// this evaluates |
// to a prvalue |
// temporary expires here
data points to an internal of that object, so after the temporary ends you are left with a dangling pointer. Accessing it leads to Undefined Behavior. So the next line std::cout << data << "n"; makes the whole program exhibit Undefined Behavior.
*) There is an exception to this rule which doesn't apply here. If a prvalue is directly bound to a reference, the lifetime of the prvalue is extended to the lifetime of the reference.
For instance, this would have been fine:
int main()
{
const std::string& ref = get_data();
const char* data = ref.c_str();
std::cout << data << "n";
return 0;
}
answered 5 hours ago
bolovbolov
33.1k876141
The code exhibits undefined behavior.
get_data() returns a temporary which expires at the end of the full expression (*):
const char* data = get_data().c_str() ;
// ^~~~~~~~~~ ^
// this evaluates |
// to a prvalue |
// temporary expires here
data points to an internal of that object, so after the temporary ends you are left with a dangling pointer. Accessing it leads to Undefined Behavior. So the next line std::cout << data << "n"; makes the whole program exhibit Undefined Behavior.
*) There is an exception to this rule which doesn't apply here. If a prvalue is directly bound to a reference, the lifetime of the prvalue is extended to the lifetime of the reference.
For instance, this would have been fine:
int main()
{
const std::string& ref = get_data();
const char* data = ref.c_str();
std::cout << data << "n";
return 0;
}
answered 5 hours ago
bolovbolov
33.1k876141
edited 5 hours ago
answered 5 hours ago
bolovbolov
33.1k876141
answered 5 hours ago
bolovbolov
33.1k876141
answered 5 hours ago
bolovbolov
33.1k876141
33.1k876141
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returnedstd::stringare separate objects.
– user4581301
4 hours ago
|
show 2 more comments
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returnedstd::stringare separate objects.
– user4581301
4 hours ago
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
Your answer should include something with the words sequence point to get my upvote, because people still search for that - even though it doesn't appear in the standard.
– Wyck
5 hours ago
1
1
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
@Wyck I don't see how sequence points are relevant here. The only thing that matters is the lifetime of the temporary. And that lifetime is until the end of the full expression it appears on.
– bolov
5 hours ago
1
1
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
@Wyck newer standards don't use "sequence points" indeed. They use "sequenced after" and "sequenced before". I still don't see the connection to the problem at hand... Maybe I am missing something, could you please tell how sequencing relates here?
– bolov
5 hours ago
1
1
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
@Wyck a single statement can possibly have multiple sequencing considerations, but they would not affect when a temporary is destroyed
– kmdreko
5 hours ago
1
1
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returned
std::string are separate objects.– user4581301
4 hours ago
The only thing that this doesn't cover is the Asker's statement that judging that the returned string is immutable suggests that they might not know that the string literal and the returned
std::string are separate objects.– user4581301
4 hours ago
|
show 2 more comments
Yes it is, but not the way you're doing it.
If you did this:
std::cout << get_data().c_str() << 'n';
you'd be just fine.
That's because a temporary is guaranteed to live for the lifetime of the full expression it was created in. It may live longer in certain, very specific circumstances.
If you bind a const reference to a temporary, it's lifetime will be extended to be the lifetime of the name it was bound to. So, code like this:
std::string const &x = get_data();
std::cout << x.c_str() << 'n';
would also work because the temporary returned by get_data would be bound to the reference named x, and so as long as x remained a valid name to use, the temporary would still exist.
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
1
A const reference, I think you mean.
– Nemo
1 hour ago
add a comment |
Yes it is, but not the way you're doing it.
If you did this:
std::cout << get_data().c_str() << 'n';
you'd be just fine.
That's because a temporary is guaranteed to live for the lifetime of the full expression it was created in. It may live longer in certain, very specific circumstances.
If you bind a const reference to a temporary, it's lifetime will be extended to be the lifetime of the name it was bound to. So, code like this:
std::string const &x = get_data();
std::cout << x.c_str() << 'n';
would also work because the temporary returned by get_data would be bound to the reference named x, and so as long as x remained a valid name to use, the temporary would still exist.
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
1
A const reference, I think you mean.
– Nemo
1 hour ago
add a comment |
Yes it is, but not the way you're doing it.
If you did this:
std::cout << get_data().c_str() << 'n';
you'd be just fine.
That's because a temporary is guaranteed to live for the lifetime of the full expression it was created in. It may live longer in certain, very specific circumstances.
If you bind a const reference to a temporary, it's lifetime will be extended to be the lifetime of the name it was bound to. So, code like this:
std::string const &x = get_data();
std::cout << x.c_str() << 'n';
would also work because the temporary returned by get_data would be bound to the reference named x, and so as long as x remained a valid name to use, the temporary would still exist.
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
Yes it is, but not the way you're doing it.
If you did this:
std::cout << get_data().c_str() << 'n';
you'd be just fine.
That's because a temporary is guaranteed to live for the lifetime of the full expression it was created in. It may live longer in certain, very specific circumstances.
If you bind a const reference to a temporary, it's lifetime will be extended to be the lifetime of the name it was bound to. So, code like this:
std::string const &x = get_data();
std::cout << x.c_str() << 'n';
would also work because the temporary returned by get_data would be bound to the reference named x, and so as long as x remained a valid name to use, the temporary would still exist.
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
edited 1 hour ago
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
answered 4 hours ago
OmnifariousOmnifarious
41k11101162
41k11101162
1
A const reference, I think you mean.
– Nemo
1 hour ago
add a comment |
1
A const reference, I think you mean.
– Nemo
1 hour ago
1
1
A const reference, I think you mean.
– Nemo
1 hour ago
A const reference, I think you mean.
– Nemo
1 hour ago
add a comment |
Aknin Abdo is a new contributor. Be nice, and check out our Code of Conduct.
Aknin Abdo is a new contributor. Be nice, and check out our Code of Conduct.
Aknin Abdo is a new contributor. Be nice, and check out our Code of Conduct.
Aknin Abdo is a new contributor. Be nice, and check out our Code of Conduct.
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1
Where does that string that gets returned go after
c_str()is called and returns a pointer to some data?– tadman
5 hours ago
2
stackoverflow.com/questions/23464504/…
– Wyck
5 hours ago
2
Probably not a duplicate but helpful: stackoverflow.com/questions/349025/…. Also your interview question is missing
#include <string>so technically it would be a compiler error ;)– Tas
5 hours ago
1
I'm a bit surprised that the documentation for
std::string::c_strdoesn't mention destruction of the string as grounds for the returned pointer being invalidated (unless you consider the destructor to be a non-const member function). I think many people coming from a C background would benefit from having this written explicitly– alter igel
5 hours ago
1
@Tas: io-streams implement the shift-operators including overloads on basic_string ,so it needs its definition which requires it to include <string>. So it can't be a compiler error.
– engf-010
5 hours ago