Can you reuse a moved std::string?

Question

Given this example:

 std::vector<std::string> split(const std::string& str) {
    std::vector<std::string> result;
    std::string curr;
    for (auto c : str) {
        if (c == DELIMITER) {
            result.push_back(std::move(curr)); // ATTENTION HERE!
        } else {
            curr.push_back(c);
        }
    }
    result.push_back(std::move(curr));
    return result;
}

Can I reuse the curr std:string? This snippet seems working: after curr is moved inside the result vector, it becomes empty. I want to be sure this is not an undefined behavior in the standard and it isn't working only because of luck.

It's left in a valid but unspecified state. Not guaranteed to be empty. — T.C., Dec 09 '14 at 10:29
i think the difference here is subtle, e.g: small string optimization — Alessandro Pezzato, Dec 09 '14 at 11:03

score 14 · Accepted Answer · answered Dec 09 '14 at 10:34

With a few exceptions (smart pointers, for instance), moved-from objects are left in a valid but unspecified state.

In a std::string that uses the small string optimization, for instance, if the string is small, there is no dynamic allocation, and a move is a copy. In that case it is perfectly valid for the implementation to leave the source string untouched, and not incur the extra cost of emptying the string.

score 4 · Answer 2 · edited May 23 '17 at 11:50

4

May be, this link will be useful

In short, for reuse you need to call .clear method

edited May 23 '17 at 11:50

Community

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1

answered Dec 09 '14 at 10:41

RainLabs

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1

7

You don't _need_ to call `clear()`. You can just assign to it, or resize it, or any other operation with no preconditions. – Jonathan Wakely Dec 09 '14 at 11:16

score 1 · Answer 3 · answered Dec 09 '14 at 10:32

1

From http://en.cppreference.com/w/cpp/utility/move

...all standard library functions that accept rvalue reference parameters (such as std::vector::push_back) are guaranteed to leave the moved-from argument in valid but unspecified state.

answered Dec 09 '14 at 10:32

Neska

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Didn't answer the question of the op about whether it can be reused or not. – JiaHao Xu Oct 25 '18 at 03:28

Can you reuse a moved std::string?

3 Answers3

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