iterator - no matching erase function in c++11

Question

I can't exmplain this behaviour:

for (vector<File>::const_iterator it = this->files.begin(); it != this->files.end(); ++it) {
    if (...) erase(it); // break after, no need of ++it in else branch
}

where File is my own class (std not included) and this->files is vector of Files

when I compile the code I get (see line 2)

Path.cpp: In member function ‘void Path::rmFile(File&)’:
Path.cpp:190:24: error: no matching function for call to ‘std::vector<File>::erase(std::vector<File>::const_iterator&)’
Path.cpp:190:24: note: candidates are:
In file included from /usr/include/c++/4.7/vector:70:0,
             from Path.h:5,
             from Path.cpp:1:
/usr/include/c++/4.7/bits/vector.tcc:135:5: note: std::vector<_Tp, _Alloc>::iterator     std::vector<_Tp, _Alloc>::erase(std::vector<_Tp, _Alloc>::iterator) [with _Tp = File; _Alloc =     std::allocator<File>; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<File*,     std::vector<File> >; typename std::_Vector_base<_Tp, _Alloc>::pointer = File*]
/usr/include/c++/4.7/bits/vector.tcc:135:5: note:   no known conversion for argument 1     from ‘std::vector<File>::const_iterator {aka __gnu_cxx::__normal_iterator<const File*,     std::vector<File> >}’ to ‘std::vector<File>::iterator {aka __gnu_cxx::__normal_iterator<File*,     std::vector<File> >}’
/usr/include/c++/4.7/bits/vector.tcc:147:5: note: std::vector<_Tp, _Alloc>::iterator     std::vector<_Tp, _Alloc>::erase(std::vector<_Tp, _Alloc>::iterator, std::vector<_Tp,     _Alloc>::iterator) [with _Tp = File; _Alloc = std::allocator<File>; std::vector<_Tp,     _Alloc>::iterator = __gnu_cxx::__normal_iterator<File*, std::vector<File> >; typename     std::_Vector_base<_Tp, _Alloc>::pointer = File*]
/usr/include/c++/4.7/bits/vector.tcc:147:5: note:   candidate expects 2 arguments, 1 provided
make: *** [Path.o] Error 1

even the doc says it's ok, but error no matching function for call to std::vector::erase(std::vector::const_iterator&) is really wierd.

I really need to be able to delete vector item by iterator. Can anybody help me please? Thaks in advance.

@chris `const_iterator` means no modification *through* this iterator. If you want to call `erase`, you already need a non-const way to access the container. The iterator merely provides the position where to modify, so the restriction has been lifted in C++11. — dyp, Nov 30 '13 at 22:55
I guess this question is a duplicate: http://stackoverflow.com/q/15987893/420683 You'd need a more recent version of libstdc++; AFAIK it's fixed now (SVN), but I don't know since which version. — dyp, Nov 30 '13 at 23:01
Not entirely sure about what your comment means. After `erase(it)`, you absolutely must not do `++it`. — Alan Stokes, Nov 30 '13 at 23:03

Quuxplusone · Accepted Answer · 2018-03-03T07:51:02.790

You have three bugs here.

for (vector<File>::const_iterator it = this->files.begin(); it != this->files.end(); ++it) {
    if (...) erase(it); // break after, no need of ++it in else branch
}

The first bug is that you incorrectly cut-and-pasted your code into StackOverflow. What you meant to paste was

for (vector<File>::const_iterator it = this->files.begin(); it != this->files.end(); ++it) {
    if (...) this->files.erase(it); // break after, no need of ++it in else branch
}

The second bug is what the compiler is warning you about: there's no way to modify a collection through a const_iterator. (EDIT: Okay, apparently C++11 added such a way, but libstdc++ didn't support it immediately.) That's what the const_ part means! If you want to modify the collection, use a plain old iterator:

for (vector<File>::iterator it = this->files.begin(); it != this->files.end(); ++it) {
    if (...) this->files.erase(it); // LOOK OUT, THERE'S STILL A BUG
}

The third bug is that once you call std::vector::erase on a collection, all iterators (and const_iterators) into that collection become unusable. The Standard terminology for this is that erase invalidates iterators. (The reason for this is that std::vector behaves basically like a big heap-allocated buffer, and calling resize on the vector is allowed to do the equivalent of realloc⁽¹⁾ on the buffer, and calling erase is allowed to call resize (because if you erase half the elements in the vector, you're probably expecting the heap-allocation to shrink accordingly).)

So, what you're trying to do won't work using that naive for-loop approach. What you need to do is use a standard algorithm, namely remove_if:

#include <algorithm>

auto predicate = [](const File& f) { return f.ShouldBeErasedOrWhatever(); }
auto newEndIterator = std::remove_if(this->files.begin(), this->files.end(), predicate);
this->files.erase(newEndIterator, this->files.end());  // erase everything after "newEndIterator"

Replace f.ShouldBeErasedOrWhatever() with whatever your "..." is in the original code. Now you have valid, idiomatic C++11 that does the right thing — with no bugs!

⁽¹⁾ – Note on "the equivalent of realloc": Of course it's not really realloc; it's really a type-safe process that calls move-constructors and destructors as needed. vector knows it's not generally safe to memcpy arbitrary objects in C++.

score 2 · Answer 2 · answered Nov 30 '13 at 22:23

2

Assuming your example code is incorrect and it really is files.erase(it), then const_iterator version was only added in C++11 which it looks like you don't have since you're not using auto.

answered Nov 30 '13 at 22:23

uk4321

1,028
8
18

iterator - no matching erase function in c++11

2 Answers2