ofi's blog

Bug In Vectors
By ofi, history, 7 years ago, In English 
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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7 years ago, # |
  Vote: I like it +51 Vote: I do not like it

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.

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    7 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    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...

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      7 years ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      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.

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        7 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        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)

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      7 years ago, # ^ |
      ← Rev. 2   Vote: I like it +8 Vote: I do not like it

      Personally, I see no harm if it is written in Word, as long as the idea is clearly expressed :)

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7 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

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

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7 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Thanks, it really was usefull.

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