Question

I have a simple function that loops while there are still elements in a vector. Inside the loop, a single element is popped from the end of the vector using pop_back(). For some reason my code is removing 2 elements every time this is called.

vector<Vertex> vertices;

while ( vertices.size() != 0 ) {
    std::cerr << "We are in the loop, size: " << vertices.size() << std::endl;
    Vertex tmp = vertices.back();
    // do stuff with tmp, not shown here;  
    vertices.pop_back();
}

The output is as follows:

We are in the loop, size: 3
We are in the loop, size: 1

To clarify, this is the output of the exact code above.

EDIT:

vector<Vertex> vertices;

while ( vertices.size() != 0 ) {

    Vertex tmp = vertices.back();
    std::cerr << "We are in the loop, size: " << vertices.size() << std::endl;
    vertices.pop_back();
    std::cerr << "We are in the loop, size: " << vertices.size() << std::endl;
}

Output:

We are in the loop, size: 3
We are in the loop, size: 1
We are in the loop, size: 1
We are in the loop, size: 0

EDIT 2:

I changed my implementation from vector to deque. Using the exact same commands I’ve managed to achieve the desired output:

We are in the loop, size: 3 
We are in the loop, size: 2 
We are in the loop, size: 2 
We are in the loop, size: 1 
We are in the loop, size: 1 
We are in the loop, size: 0

Still can’t explain the behaviour from before; thanks for the help everyone.

Answer 1

As Kerrek SB mentioned error is not in the code given, I tried following code and it works fine.

#include <iostream>
#include <vector>

int main ( int argc, char **argv) {
    std::vector<int> v;
    for ( int i = 0; i < 10; i++) {
        v.push_back(i);
    }
    while ( !v.empty()) {
        std::cerr << "We are in the loop, size: " << v.size() << std::endl;
        int tmp = v.back();
        v.pop_back();
    }
}