I'm trying to cloning the vector itself, for example if the vector is [2,3] it will become [2,3,2,3].

This is my program:

#include <iostream>
#include <vector>
using namespace std;

int main() 
{
    vector<int> a;
    
    a.push_back(2);
    a.push_back(3);
    
    for(int i: a)
    {
        a.push_back(i);
    }
        
    for(int i: a)
        cout << i << " ";
        
    return 0;
}

So I'm just pushing the elements to the vector itself but the output is [2,3,2,0]. I don't understand why.

Help me with this problem.

CodePudding user response：

Range-for is syntactic sugar for an iterator-based for-loop, and std::vector::push_back() can invalidate iterators like the ones being used internally by the range-for:

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

You can use std::copy() after a resize():

vector<int> a;
a.push_back(2);
a.push_back(3);

auto size = a.size();
a.resize(size * 2);
std::copy(a.begin(), a.begin()   size, a.begin()   size);

CodePudding user response：

Do not use range base iterators while changing the container you are iterating. In this case, just store the size of the vector before inserting and then use a regular index based loop.

int sz = a.size();

for(int i =0 ; i < sz ;   i)
{
   a.push_back(a[i]);
}

CodePudding user response：

You are iterating over the vector which is being changed at the same time.

//This is problematic
for(int i: a)
{
  a.push_back(i);
}

You can instead do it this way:

#include <iostream>
#include <vector>
using namespace std;

int main() 
{
    vector<int> a;
    
    a.push_back(2);
    a.push_back(3);
    
    vector<int> b = a; //Store contents in another vector
    
    for(int i: b)
    {
        a.push_back(i);
    }
        
    for(int i: a)
        cout << i << " ";
        
    return 0;
}