remove_if erases everything from vector - c++

Question

I'm writing a program for an assignment - it's supposed to be a database
for information about employees in a company. Basically, a vector containing
structures (individual employees).
The trouble I'm having is that remove_if erases everything from the vector - instead of an individual employee.
If I understood documentation/other topics correctly, that function should
do two things - rearrange elements of the vector, and return an
iterator to the first element outside the new range - but it doesn't do
it, it returns an iterator to the first element - and so when the
erase() function is called, all elements are deleted. At least
that's what I found when debugging it.
Here's a mcve of my code:
#include <iostream>
#include <vector>
#include <algorithm>
struct employee {
int number;
};
int main()
{
//creating the vector and adding some values to it
employee one{ 1 };
employee two{ 2 };
employee three{ 3 };
std::vector <employee> staff{ one, two, three };
int m = 2; //some parameter I want to pass to lambda function
auto it = std::remove_if(staff.begin(), staff.end(),
[m](employee a) {
if (a.number == 2)
return true; }
);
staff.erase(it, staff.end());
for (auto it = staff.begin(); it != staff.end(); it++)
std::cout << it->number << std::endl;
system("pause");
return 0;
}
I realise that I could've done the same thing in a loop - in fact, I did, but I just can't wrap my head around why doesn't this approach work. Also, a list would've probably been a better choice for this program (with it, the for loop would have taken fewer instructions to compute), but I've already finished the program, and right now I just really want to know why didn't remove_if work.
Thanks!
EDIT: As #drescherjm pointed out, that was due to the fact that the lambda function didn't return false when the when the if statement wasn't met.
So the question is answered.

Answer 1

The main problem is you are not returning a value when your condition in your lambda is not met. This is undefined behavior not to return a value.
auto it = std::remove_if(staff.begin(), staff.end(),
[m](employee a) {
if (a.number == 2)
return true; }
);
A simple solution is to remove the if and just return the conditional.
auto it = std::remove_if(staff.begin(), staff.end(),
[m](employee a) {
return (a.number == 2);
}
);
However as #killzonekid mentioned this is not correct because you are still not using the parameter.
auto it = std::remove_if(staff.begin(), staff.end(),
[m](employee a) {
return (a.number == m);
}
);
Replacing the fixed 2 with m should take care of that.