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Why does this work (stack vs heap allocation)?
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Erdoeban Zsukloff zu Brecher Zhuang
Jul 31, 2016, 9:40:58 PM7/31/16
to
using namespace std;
vector<int> build_vector(void)
{
vector<int> result;
result.push_back(1);
return result;
}
int main(void)
{
auto V = build_vector();
// V should be broken but isn't !
for (auto vv : V) cout << vv ;
}
Isn't result allocated on stack and thus should be destroyed right upon
scope's "}" ? I distinctly recall getting compiler warnings and nasty memory
errors, so I got into habit of always using heap when returning anything
non-trivial, yet I lately see stuff like the snippet above in legitimate
authors ...
Thank you in advance !
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vector<int> build_vector(void)
{
vector<int> result;
result.push_back(1);
return result;
}
int main(void)
{
auto V = build_vector();
// V should be broken but isn't !
for (auto vv : V) cout << vv ;
}
Isn't result allocated on stack and thus should be destroyed right upon
scope's "}" ? I distinctly recall getting compiler warnings and nasty memory
errors, so I got into habit of always using heap when returning anything
non-trivial, yet I lately see stuff like the snippet above in legitimate
authors ...
Thank you in advance !
---
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https://www.avast.com/antivirus
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Richard Damon
Jul 31, 2016, 10:37:38 PM7/31/16
to
On 7/31/16 9:40 PM, Erdoeban Zsukloff zu Brecher Zhuang wrote:
> using namespace std;
> vector<int> build_vector(void)
> {
> vector<int> result;
> result.push_back(1);
> return result;
> }
> int main(void)
> {
> auto V = build_vector();
> // V should be broken but isn't !
> for (auto vv : V) cout << vv ;
> }
>
> Isn't result allocated on stack and thus should be destroyed right upon
> scope's "}" ? I distinctly recall getting compiler warnings and nasty
> memory errors, so I got into habit of always using heap when returning
> anything non-trivial, yet I lately see stuff like the snippet above in
> legitimate authors ...
>
> Thank you in advance !
>
Because you are returning the vector by value, so it is copied. The
> using namespace std;
> vector<int> build_vector(void)
> {
> vector<int> result;
> result.push_back(1);
> return result;
> }
> int main(void)
> {
> auto V = build_vector();
> // V should be broken but isn't !
> for (auto vv : V) cout << vv ;
> }
>
> Isn't result allocated on stack and thus should be destroyed right upon
> scope's "}" ? I distinctly recall getting compiler warnings and nasty
> memory errors, so I got into habit of always using heap when returning
> anything non-trivial, yet I lately see stuff like the snippet above in
> legitimate authors ...
>
> Thank you in advance !
>
problem is when you return a pointer to an auto value.
woodb...@gmail.com
Aug 1, 2016, 12:08:30 AM8/1/16
to
local variable. If you go by the standard, the
meaning of the word auto changed a few years ago.
Brian
Ebenezer Enterprises - In G-d we trust.
http://webEbenezer.net
Ike Naar
Aug 1, 2016, 1:29:29 AM8/1/16
to
would be okay, though.
Most likely Richard meant 'object with automatic storage duration'
when he used 'auto value'.
leigh.v....@googlemail.com
Aug 1, 2016, 7:59:27 AM8/1/16
to
Most optimisers will elide the copy using NRVO or move semantics.
/Leigh
/Leigh
Erdoeban Zsukloff zu Brecher Zhuang
Aug 11, 2016, 12:43:44 PM8/11/16
to
"Richard Damon" wrote in message news:IBynz.64968$Cu5....@fx36.iad...
%g++ var.cc -o var
var.cc: In function ‘VS_or_pc* build_vec()’:
var.cc:14:11: warning: address of local variable ‘result’ returned
[-Wreturn-local-addr]
VS_or_pc result;
^~~~~~
%./var.exe
zsh: segmentation fault (core dumped) ./var.exe
Thank you.
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