Error while concatenating a vector to itself in c-CodePudding

I am simply trying to concatenate a vector to itself but the following code is not working and I am not able to find the issue. If my input vector is {1,2,1}, the o/p I am getting is {1,2,1,1,16842944,1}. Please tell where I am wrong. The output I want is [1,2,1,1,2,1]

 vector<int> getConcatenation(vector<int>& nums) {
        
        int size=nums.size();
        auto itr=nums.begin();

        while(size--)
        {

            nums.push_back(*itr);
             itr  ;
        }
        
        return nums;      
    }

CodePudding user response：

In your original program push_back invalidates the iterators and using those invalidated iterators can lead to undefined behavior.

One way to solve this would be to use std::copy_n with std::vector::resize as shown below:

 vector<int> getConcatenation(vector<int>& nums) {
        
       std::vector<int>::size_type old_Size = nums.size();
       nums.resize(2 * old_Size);
       std::copy_n(nums.begin(), old_Size, nums.begin()   old_Size);
        
        return nums; //NO NEED for this return since the function took vector by reference and so the change is already reflected on passed vector     
    }

Also you would need to add #include <algorithm> for std::copy_n.

Note that since your function takes the vector be reference, there is no need to return nums because the changes you do on nums is already reflected on the original vector. So you can use void as the return type of the function and then remove the return statement.