C++ - Removing element by value of custom data type vector

Question

I am trying to delete vector element by value for custom data type vector. It works fine If I use the simple data type like int etc instead of hello data type.

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

class hello
{

public:
    hello() {
        x = false;
    }
    bool x;
};

int main() {

   hello f1;
   hello f2;
   hello f3;

   std::vector <hello> vector_t;

   vector_t.push_back(f1);
   vector_t.push_back(f2);
   vector_t.push_back(f3);

   for (unsigned int i = 0; i < vector_t.size(); i++)
       {
       if (vector_t[i].x)
       {
            vector_t.erase(std::remove(vector_t.begin(), vector_t.end(), i), vector_t.end());
        }
    }

    return 0;
}

It shows an error:

binary '==': no operator found which takes a left-hand operand of type 'hello' (or there is no acceptable conversion) vector_test

Answer 1

Looks like you want to rather use remove_if where the .x member is true.

vector_t.erase(std::remove_if(vector_t.begin(), vector_t.end(), [](const hello &h) { return h.x; }), vector_t.end());

The for loop and if condition aren't necessary, they're not needed this way.

Answer 2

remove tries to find all elements that compare equal to whatever you passed to it. If you do not tell the compiler how to compare hello objects with the integer i value, it cannot do that.

What you probably wanted to do was to just remove the i-th element of the vector if it satisfies your criterion:

for (unsigned int i = 0; i < vector_t.size(); i++)
{
    if (vector_t[i].x)
    {
        vector_t.erase(vector_t.begin() + i);
        --i; // The next element is now at position i, don't forget it!
    }
}

The most idiomatic way would be using std::remove_if as shown in acgraig5075's answer.

Answer 3

It shows an error:

binary '==': no operator found which takes a left-hand operand of type 'hello' (or there is no acceptable conversion) vector_test

You can provide the obviously missing operator == for your class which will solve the issue:

bool operator==(hello const &h)
{
    return this->x == h.x;
} 

Your remove/erase should look like this though:

vector_t.erase(std::remove(vector_t.begin(), vector_t.end(), vector_t[i]), vector_t.end());

Demo: https://ideone.com/E3aV76