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Can you pop_back a vector and still use the iterator to the last element?

How do I erase an element from std::vector<> by index?Test if a vector contains a given elementWhat is the most effective way to get the index of an iterator of an std::vector?What is the easiest way to initialize a std::vector with hardcoded elements?std::vector iterator invalidationVector iterator comparisonusing erase with remove_ifC++ vector content not changed while using iterator

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8

1

I wonder what happens if I have an iterator on the last element of the vector and do pop_back.

std::set<int> s;

s.insert(5);



std::vector<int> v = {1, 2, 3, 4, 5};



for (auto it = v.begin(); it != v.end();) {

    if (s.count(*it)) {

        std::swap(*it, v.back());

        v.pop_back();

    } else {

        ++it;

    }

}

Code above works properly (v is {1, 2, 3, 4} after that block) at least with clang, but is it correct to check if it == v.end() if it is invalidated?

c++ vector iterator

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8

1

I wonder what happens if I have an iterator on the last element of the vector and do pop_back.

std::set<int> s;

s.insert(5);



std::vector<int> v = {1, 2, 3, 4, 5};



for (auto it = v.begin(); it != v.end();) {

    if (s.count(*it)) {

        std::swap(*it, v.back());

        v.pop_back();

    } else {

        ++it;

    }

}

Code above works properly (v is {1, 2, 3, 4} after that block) at least with clang, but is it correct to check if it == v.end() if it is invalidated?

c++ vector iterator

share|improve this question

edited yesterday

Nicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

asked yesterday

user3365922user3365922

6355 bronze badges

New contributor

user3365922 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

I wonder what happens if I have an iterator on the last element of the vector and do pop_back.

std::set<int> s;

s.insert(5);



std::vector<int> v = {1, 2, 3, 4, 5};



for (auto it = v.begin(); it != v.end();) {

    if (s.count(*it)) {

        std::swap(*it, v.back());

        v.pop_back();

    } else {

        ++it;

    }

}

Code above works properly (v is {1, 2, 3, 4} after that block) at least with clang, but is it correct to check if it == v.end() if it is invalidated?

c++ vector iterator

share|improve this question

edited yesterday

Nicol Bolas

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asked yesterday

user3365922user3365922

6355 bronze badges

New contributor

user3365922 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

std::set<int> s;

s.insert(5);



std::vector<int> v = {1, 2, 3, 4, 5};



for (auto it = v.begin(); it != v.end();) {

    if (s.count(*it)) {

        std::swap(*it, v.back());

        v.pop_back();

    } else {

        ++it;

    }

}

share|improve this question

edited yesterday

Nicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

asked yesterday

user3365922user3365922

6355 bronze badges

New contributor

user3365922 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

edited yesterday

Nicol Bolas

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asked yesterday

user3365922user3365922

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user3365922 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

edited yesterday

Nicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

edited yesterday

Nicol Bolas

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asked yesterday

user3365922user3365922

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user3365922 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked yesterday

user3365922user3365922

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1 Answer
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8

Your instincts are good; vector::pop_back invalidates iterators and references to the last element. If it is such an iterator, then it will be invalidated, and thus cannot be compared with v.end().

It would be much better to fix this by using algorithms and the erase-remove idiom:

auto last_it = std::remove_if(v.begin(), v.end(), [&](const auto &val) {return s.count(val) != 0;});

v.erase(last_it, v.end());

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answered yesterday

Nicol BolasNicol Bolas

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8

Your instincts are good; vector::pop_back invalidates iterators and references to the last element. If it is such an iterator, then it will be invalidated, and thus cannot be compared with v.end().

It would be much better to fix this by using algorithms and the erase-remove idiom:

auto last_it = std::remove_if(v.begin(), v.end(), [&](const auto &val) {return s.count(val) != 0;});

v.erase(last_it, v.end());

share|improve this answer

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

add a comment | 

8

Your instincts are good; vector::pop_back invalidates iterators and references to the last element. If it is such an iterator, then it will be invalidated, and thus cannot be compared with v.end().

It would be much better to fix this by using algorithms and the erase-remove idiom:

auto last_it = std::remove_if(v.begin(), v.end(), [&](const auto &val) {return s.count(val) != 0;});

v.erase(last_it, v.end());

share|improve this answer

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

add a comment | 

Your instincts are good; vector::pop_back invalidates iterators and references to the last element. If it is such an iterator, then it will be invalidated, and thus cannot be compared with v.end().

It would be much better to fix this by using algorithms and the erase-remove idiom:

auto last_it = std::remove_if(v.begin(), v.end(), [&](const auto &val) {return s.count(val) != 0;});

v.erase(last_it, v.end());

share|improve this answer

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

auto last_it = std::remove_if(v.begin(), v.end(), [&](const auto &val) {return s.count(val) != 0;});

v.erase(last_it, v.end());

share|improve this answer

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges

answered yesterday

Nicol BolasNicol Bolas

306k3737 gold badges511511 silver badges691691 bronze badges