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std::remove_ifは実際には要素の削除を行わない。

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std::remove_ifは条件を満たす(述語が真を返す)要素を実際削除するわけではなく、「削除する要素を詰めて、サイズが縮んだコンテナの新しい終端イテレータを返す」だけなので、その後の要素の削除は自分でやらないといけません。

#include <vector>
#include <algorithm>
#include <iostream>

int main()
{
std::vector<int> v = {1,2,3,4,5,6,7};

// "1 2 3 4 5 6 7"
for(auto i=v.begin();i!=v.end();++i)
  {std::cout << *i << " ";}
std::cout << std::endl;

auto itrNewEnd = std::remove_if(v.begin(),v.end()
                                ,[](int i)->bool
                                 { return 3<=i&&i<=5; });

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

// "1 2 6 7"
// 上記のv.erase()をコメントアウトすると "1 2 6 7 5 6 7"
for(auto i=v.begin();i!=v.end();++i)
  {std::cout << *i << " ";}
std::cout << std::endl;

return 0;
}
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