Error Passing Const as this

Question

im working on a project for school and have this error but i cannot figure out out to fix it:

error: passing ‘const xArray’ as ‘this’ argument of ‘size_t xArray::PushBack(int)’ discards qualifiers [-fpermissive] a.PushBack(temp);

Here is the function that the error comes from:

    istream& operator>>(istream& in, const xArray& a)
    {
          int temp;
          in >> temp;
          a.PushBack(temp);

          return in;
    }

Here is my code for PushBack:

    size_t xArray::PushBack(int c)
    {

            if(len == arraySize)
            {
                     int* temp = new int[arraySize* 2];
                     size_t i;
                     for(i = 0; i < arraySize; i++)
                     {
                            temp[i] = data[i];
                     }
                     delete [] data;
                     data = temp;
                     data[len+1] = c;
                     len = len + 1;
            }
            else
            {
                  data[len + 1] = c;
                  len = len + 1;
            }
    }

Any help on how to fix or an explanation of this error would be appreciated Thanks in advance

It's const, therefore you can't modify it in any way, including by pushing stuff onto it. — user253751
It's exactly what the compiler says. You call a non const member function on a const object. In generally the istream >> operator changes the second element and should be passed by reference but not as a const reference. — Timothy Murphy

songyuanyao songyuanyao · Accepted Answer · 2016-03-03T02:10:53

For istream& operator>>(istream& in, const xArray& a), a is declared as const, and calling PushBack() on it will fail because xArray::PushBack() is a non-const member function.

You might change the parameter type of a to non-const reference, such as

istream& operator>>(istream& in, xArray& a)
{
    ...
}

Error Passing Const as this

1 Answers