### ofi's blog

By ofi, history, 8 months ago, ,
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

•
• +49
•

 » 8 months ago, # |   +51 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.
•  » » 8 months ago, # ^ |   0 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++17 doing the expected thing.Btw, pretty funny that you are allowed to write C++ proposals in Word...
•  » » » 8 months ago, # ^ |   +3 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.
•  » » » » 8 months ago, # ^ |   0 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)
•  » » » 8 months ago, # ^ | ← Rev. 2 →   +8 Personally, I see no harm if it is written in Word, as long as the idea is clearly expressed :)
 » 8 months ago, # | ← Rev. 2 →   0 The following modification produces the expected output. int w = f(); v[0] = w; Global variables should be used carefully so as to avoid such unexpected side effects. Best wishes
 » 8 months ago, # |   +8 Thanks, it really was usefull.