string erase appends elements with backward iterators

Question

I am trying to remove every '0' in a std::string, starting from the back. According to this, there are multiple ways to .erase backward iterators.

The weird thing is that they all append numbers to the std::string, instead of erasing them!

Here is a little sample

//'str' will be "5.090000"
std::string str = std::to_string(5.09);

auto it = std::remove_if(str.rbegin(), str.rend(), [](char value) {
    return value == '0';
});

1) Pre-C++11 way:

str.erase(--(it.base()));

2) C++11 way (1)

str.erase(std::next(it).base());

3) C++11 way (2)

std::advance(it, 1);
str.erase(it.base());

In all cases, str == "5.095.9". Why? Because as I see it, str should be 5.9, but it isn't...

My best guess is that I am doing something wrong with the backwards iterators, because if you split the string: 5.09 | 5.9, the first value has still the in-between '0', but not the last ones. The second value is actually the std::string I expected.

What I am doing wrong?

Look at the documentation of `std::remove_if` very carefully, and think about the range you are manipulating. — Kerrek SB, May 12 '16 at 17:20
i never used backward iterators, but my naive guess would be that you have to use them like this : `std::remove_if(str.rend(), str.rbegin(), [](char value)` — 463035818_is_not_an_ai, May 12 '16 at 17:20
@tobi303 http://en.cppreference.com/w/cpp/iterator/reverse_iterator - here's RTFM for you :D — LogicStuff, May 12 '16 at 17:21
What about using `str.find_last_not_of('0')` to find the last non-zero digit. — Bo Persson, May 12 '16 at 17:27
Why start from the back if you're removing all the `0`s, anyway? — LogicStuff, May 12 '16 at 17:29
@KerrekSB ooh, I see my error ;) Thanks a lot, although my error was in the `erase` logic :) — Rakete1111, May 12 '16 at 17:33
@LogicStuff my example gives a good reason: removing every unimportant `0` from a `double` converted into a `std::string`, but only the last ones (i.e. `5.09000000` -> `5.09`) — Rakete1111, May 12 '16 at 17:35
@Rakete1111 but your question says "...trying to remove every '0'..." — 463035818_is_not_an_ai, May 12 '16 at 17:39
@tobi303 yes, I know, I don't want to overcomplicate things ;) — Rakete1111, May 12 '16 at 17:40

score 1 · Accepted Answer · answered May 12 '16 at 17:58

I made 2 errors in my approach:

erase called with only 1 iterator removes the element the iterator is pointing at, not from the iterator to the end (as I falsely assumed)

So str.erase(std::next(it).base(), str.end()); - this is still wrong, continue reading ;)

As @KerrekSB pointed out, I didn't read the docs carefully enough: Because I am using std::reverse_iterator, the elements get pushed back to the front! So, as it points to the new end iterator (which is BTW before the not-removed elements), I have to delete the range from the beginning (str.begin()) to it.base().

TL;DR

The new working version is: str.erase(str.begin(), it.base());

string erase appends elements with backward iterators

1 Answers1