vector<int> v;
int f()
{
v.push_back(-1);
return 1;
}
int main()
{
v.push_back(0);
cout<<v[0]<<endl;
v[0]=f();
cout<<v[0]<<' '<<v[1]<<endl;
return 0;
}
expected output:
0
1 -1
output:
0
0 1
vector<int> v;
int f()
{
v.push_back(-1);
return 1;
}
int main()
{
v.push_back(0);
cout<<v[0]<<endl;
v[0]=f();
cout<<v[0]<<' '<<v[1]<<endl;
return 0;
}
expected output:
0
1 -1
output:
0
0 1
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When evaluating
v[0]=f();vector resizes from 1 to 2, reallocation happens, but the left-hand side address is evaluated before reallocation happens, so you perform an assignment to the memory region that is not a content of a vector any more.The order of evaluation of sides in an assignment operator is not defined, thus you have UB.
This paper "Refining Expression Evaluation Order for Idiomatic C++" was proposed for the C++17 standard. Pretty sure that it also did get accepted, so it shouldn't be UB anymore. Though I didn't find any proof other than
g++ -std=c++17doing the expected thing.Btw, pretty funny that you are allowed to write C++ proposals in Word...
To get proof open the latest working draft based on c++17 branch n4659 and check if the changes from proposal are there. [Cppreference]http://en.cppreference.com/w/cpp/language/eval_order) is also a good source.
Yeah, should have checked it yesterday, but I was too tired.
Just for reference, the C++17 draft says: "The right operand is sequenced before the left operand." (under 8.18 Assignment and compound assignment operators)
Personally, I see no harm if it is written in Word, as long as the idea is clearly expressed :)
The following modification produces the expected output.
Global variables should be used carefully so as to avoid such unexpected side effects.
Best wishes
Thanks, it really was usefull.