std::vector<std::string> insert empty string instead

Question

In visual studio 2013, I created a std::vector and has store some strings in it. Then I want to make a copy of some string in the vector and append them to the end (suppose to move them to the end, after insert will do erase), but using insert method, I saw only empty strings at the end, very strange. I reproduced it with some simple test code,

std::vector<std::string> v;
std::string s = "0";
for (int i = 0; i < 7; ++i)
{
    s[0] = '0' + i; 
    v.push_back(s);
}
v.insert(v.end(), v.begin(), v.begin() + 3);
for (std::string& s : v) 
    std::cout << "\"" << s.c_str() << "\" ";

What I get there is "0" "1" "2" "3" "4" "5" "6" "" "" ""

I debugged into insert method, inside _Insert(..) method of vector class, it did some reallocating of memory, memory move/move and so on.

The first _Umove call move all 7 strings to new allocated memory, I think the std::move is invoked, the old memory has some empty string left.

Then the _Ucopy method try copy 3 items, but from old memory, as a result 3 empty string is attached. There is another _Umove call, I am not sure what's that for. After all thes, the old memory is freed and new memory attached to the vector.

Using a scalar type like int does not make wrong output, because the memory is copied, no std::move is invoked.

Am I doing something wrong here, or is it a MS Visual Studio's STL bug?

Some programmer dude · Accepted Answer · 2016-11-21 14:19:04Z

1

From this std::vector::insert reference:

Causes reallocation if the new size() is greater than the old capacity(). If the new size() is greater than capacity(), all iterators and references are invalidated

_{[Emphasis mine]}

You are adding elements to the vector while iterating the vector using iterators. Since this can cause the vector to be reallocated your iterators will be invalidated. That will lead to undefined behavior.

answered Nov 21, 2016 at 14:19

Some programmer dude

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6 Comments

Tiger Hwang Over a year ago

As I understand, reallocation cause iterators and reference invalidated, that should happen after insert is returned. Any iterator and reference to the vector is invalidated, but this happen during the call, it's strange to me. I try reserve some space before insert, then it works. It's only a workaround. I will try with other compiler later.

Some programmer dude Over a year ago

@TigerHwang You are passing two iterators to the insert function. As soon as the insert function makes space for the new elements, those iterators you pass become invalid. Using another compiler will not solve your problem. Reserving space is the only solution.

Tiger Hwang Over a year ago

My object is to move some strings in the vector to the end, so I tried another method std::move(v.begin(), v.begin() + 3, v.end()). This cause assertion failure due to iterator + offset out of range. With std::move(v.begin(), v.begin() + 3, v.end() - 3), it does not fail, but the result is not what I want. Any suggestion?

Some programmer dude Over a year ago

@TigerHwang In that case I suggest using a normal for loop that iterates three times. In the loop get the value of the first element, remove the first element, and add the value at the back.

Tiger Hwang Over a year ago

Thanks, I just remember a rotate method in std should do the trick.

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