c++ beginner - class and passing

I am trying to print these polynomials but I can not seem to print what I want. If my polynomial was 1*x + 2*x^2 then output would be 4*x. I want 1 + 4*x.

```void Poly :: derivative (){
term *z ;
if ( term_t == NULL)
return ;
term *temp1;
temp1 = term_t;
while ( temp1 != NULL ){
if ( term_t == NULL ){
term_t = new term ;
z = term_t ;
}
else{
z -> next = new term ;
z = z -> next ;
}
z-> coef = temp1 -> coef * temp1 -> exp;
z-> exp = temp1 -> exp - 1;
temp1 = temp1 -> next;
}
term_t=z;
}
```

I have a class poly and a struct term_t with coef and exp in them.

Personally, I'd have the derivative function return a Poly, rather than modify the existing one. Something like this

```Poly derivative() {
Poly res;
Term *temp1 = this->term_t;
while (temp1 != NULL) {
int nexp = temp1->getExp() - 1;
int ncoef = temp1->getCoef() * temp1->getExp();
temp1 = temp1->getNext();
}
return res;
}
```

As for the printing problem though, yeah, you haven't provided enough of your code to really know what's going on. So, I wrote my own... this is how I'd approach the problem. First a Term class like this

```class Term {
public:
Term(int coef, int exp) {
this->coef = coef;
this->exp = exp;
this->next = NULL;
}

std::string toString() const {
std::stringstream ss;
if (this->coef == 0) return "0";
if (this->exp == 0) ss << this->coef;
else {
if (this->coef != 1) ss << this->coef;
ss << "x";
if (this->exp != 1) ss << "^" << this->exp;
}
return ss.str();
}

int getCoef() const {return this->coef;}
int getExp() const {return this->exp;}
Term* getNext() const {return this->next;}
void setNext(Term* n) {this->next = n;}

private:
int coef;
int exp;
Term* next;
};
```

then a Poly class like this

```class Poly {
public:
Poly() {
this->term_t = NULL;
}
if (this->term_t == NULL) {
this->term_t = nt;
} else {
Term* t = this->term_t;
while(t->getNext() != NULL) t = t->getNext();
t->setNext(nt);
}
}
std::string toString() const {
std::stringstream ss;
Term* t = this->term_t;
while (t != NULL) {
ss << t->toString();
if (t->getNext() != NULL) ss << " + ";
t = t->getNext();
}
return ss.str();
}

Poly derivative() {
Poly res;
Term *temp1 = this->term_t;
while (temp1 != NULL){
int nexp = temp1->getExp() - 1;
int ncoef = temp1->getCoef() * temp1->getExp();
temp1 = temp1->getNext();
}
return res;
}

private :
Term* term_t;
};
```

That'd leave the memory management of the Term objects up to the class users, but you could also write a destructor for the Poly class that deletes them.