Problem with lvalue

[Delcho]

Can someone tell me why I get an error message using temporary as lvalue [-fpermissive] on k.back().koef+=front().koef;

  #include<iostream>
using namespace std;
struct polinom
{
    int koef;
    int stepen;
}c;
struct polinom1
{
    int koef;
    int stepen;
}o;
struct item
{
    polinom val;
    item* next;
};
class queue
{
    private:
    item* first;
    item* last;
    public:
    queue();
    ~queue();
    void push(polinom c);
    void pop();
    bool empty();
    polinom front();
    polinom back();
    void read(int &n);
    void izhod(int n);
    void sort(int n);
    void subirane(int n);
};

int main()
{
    int n=0;
    queue a;
    a.read(n);
    a.sort(n);
    cout<<" "<<endl;
    a.subirane(n);
    a.izhod(n);
}
void queue::push(polinom c)
{
    item* p= new item;
    p->val=c;
    p->next=NULL;
    if(first == NULL)
    {
        first = p;
        last=first;
    }
    else
    {
        last->next = p;
        last=p;
    }
}

queue::queue()
{
    first = NULL;
    last = NULL;
}
queue::~queue()
{
      while(first!=NULL)
      {
            item *temp=first;
            first=first->next;
            delete temp;
      }
}
void queue::pop()
{
      if(first!=NULL)
      {
            item *temp=first;
            first=first->next;
            delete temp;
            if(first==NULL)
            {
                last=NULL;
            }
      }
}
bool queue::empty()
{
    return last==NULL;
}
polinom queue::front()
{
    if(first != NULL)
        return first->val;
}
polinom queue::back()
{
    if(last != NULL)
        return last->val;
}
void queue::read(int &n)
{
    while(cin)
        {
            cout<<"Vavedete koeficient ";
            cin>>c.koef;
            if(!cin) break;
            cout<<"Vavete stepen ";
            cin>>c.stepen;
            push(c);
            n++;
        }
        cin.clear();
}
void queue::izhod(int n)
{
    int br=0;
    polinom d;
    while(!empty())
    {
        d=front();
        br++;
        if(empty()) break;
        if(br<n)
        {
         cout<<d.koef<<"*x^"<<d.stepen<<"+";
        }
        else
        {
         cout<<d.koef<<"*x^"<<d.stepen;
         break;
        }
        pop();
    }
}
void queue::sort(int n)
{
    polinom o[20],swap;
    polinom d;
    int k=0;
    for(int i=0;i<n;i++)
    {
       d=front();
       o[i]=d;
       pop();
    }
    for(int z=0;z<n;z++)
    {
        cout<<"*"<<o[z].stepen<<"*";
    }
    for (int i=0;i<n-1;i++)
     for (int j=0;j<n-i-1;j++)
        if (o[j].stepen<o[j+1].stepen)
        {
           swap=o[j];
           o[j]=o[j+1];
           o[j+1]=swap;
        }
    for(int p=0;p<n;p++)
    {
        push(o[p]);
    }
}
void queue::subirane(int n)
{
    queue k;
    k.push(front());
    pop();
    while(!empty())
    {
        if(k.back().stepen==front().stepen)
        {
            k.back().koef+=front().koef;
        }
        else
        {
            k.push(front());
        }
        pop();
    }
}

[jonnin]

polinom front();
^^ This is why. Front() should return a reference, not a copy.

try
polinom& front();

[nuderobmonkey]

Same with back()

[Ganado]

Here's a minimal program that also makes the same error message, in case it's still confusing:

// Example program
struct Foo {
    int bar;
};

Foo func()
{
    return Foo(); // returning a temporary value
}

int main()
{
    func().bar += 3; // trying to increment a temporary value.
}

13:16: error: using temporary as lvalue [-fpermissive]

Aside from the fact that you're not returning references, another issue with these functions is that not all control branches return a value.

polinom queue::front()
{
    if(first != NULL)
        return first->val;
}
polinom queue::back()
{
    if(last != NULL)
        return last->val;
}

Nothing is returned if last == NULL... And you can't have null references by design in C++. There's no perfect solution to this. You could throw an exception. The standard library's containers will just create undefined behavior if you try to access an empty container's back() or front().

http://www.cplusplus.com/reference/vector/vector/front/

Exception safety
If the container is not empty, the function never throws exceptions (no-throw guarantee).
Otherwise, it causes undefined behavior.

Edit:

struct polinom
{
    int koef;
    int stepen;
}c;
struct polinom1
{
    int koef;
    int stepen;
}o;

Please note there that you're creating global objects for polinom and polinom1, which you don't even use, and also these are shadowed by the inputs to your functions (like queue::push).
In general, you should avoid using global variables. And use DESCRIPTIVE variable names. "o" and "c" are not good variable names. Also, the difference between "polinom" and "polinom1" is non-existent as far as the data goes, why do you have two structs?