Removing elements from vector using remove_if

Question

I am trying to remove vector elements using remove_if. But unsuccessfully. What am I doing wrong?

Here's my code:

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

void printme(std::vector<int>& a){
    for(const auto& item: a)
    std::cout << item << std::endl;
}

int main()
{
    std::vector<int> a {1, 2, 3, 4, 5, 6};
    printme(a);  
    a.erase( (std::remove_if(a.begin(), a.end(), [](const int& x){
        return x == 2;
        }), a.end()));
    printme(a);
}

My output is just:

1 2 3 4 5 6

Expected output:

1 2 3 4 5 6 1 3 4 5 6

Answer 1

You are using an overload of the std::vector::erase() member function that takes a single iterator as parameter. As the argument to erase() you are providing the iterator a.end(), since the following expression:

(std::remove_if(a.begin(), a.end(), [](const int& x){ return x == 2; }), a.end()))

evaluates to a.end() (i.e., because of the comma operator).

The iterator passed to the overload of erase() that takes a single iterator must be dereferenceable. However, the iterator a.end() is not dereferenceable, therefore, the call to erase() results in undefined behavior.

---

To use the overload that takes two iterators, remove the parenthesis surrounding the call to std::remove_if:

a.erase(std::remove_if(a.begin(), a.end(), [](const int& x){
        return x == 2;
        }), a.end());

Answer 2

You're adding superfluous parentheses, change it to

a.erase( std::remove_if(a.begin(), a.end(), [](const int& x){
    return x == 2;
    }), a.end());

Note that the comma operator just returns the last operand, that means you're passing a.end() to erase, which leads to UB.

Answer 3

The other answers have pointed out what the problem was. I want to say, it will be easier to notice these kinds of problems by simplifying your code.

I suggest using:

int main()
{
   std::vector<int> a {1, 2, 3, 4, 5, 6};
   printme(a);  

   auto it = std::remove_if(a.begin(), a.end(), [](const int& x){ return x == 2; });
   a.erase(it, a.end());

   printme(a);
}

Answer 4

You just had too many parentheses in the function call.

a.erase(std::remove_if(a.begin(), a.end(), [](const int& x) {return x == 2;}), a.end());

Just remove one parentheses before the std::remove_if and at the end of the call.