So I am making a program that will do some operations with polynomials and I am using a vector full of a struct I defined to represent a polynomial. Part of what I am trying to do is to overload the +,-,*.+=,-= and *= operators.

term is a struct I defined.

I am trying to contain all of the overload methods within a class.

I have a single file that overloads it and works fine. The method is defined as:

vector<term> operator+(vector<term> t1, vector<term> t2)

When I try and put that in a c++ header file to use it in a class I get this error:

Method declaration in header:

vector<term> operator+(vector<term> v1,vector<term> v2);

Error I get:

poly.h:25:71: error: ‘std::vector<term> Polynomial::operator+(std::vector<term>, std::vector<term>)’ must take either zero or one argument
     vector<term> operator+(const vector<term> v1,const vector<term> v2);

Why would it work in one instance and not the other?

I am new to c++ so forgive if I miss any blatant mistakes.

From the error message:

std::vector Polynomial::operator+(std::vector, std::vector) [...]

I deduce that you declared operator +() as a member of class Polynomial. As a member of this class, it must take either zero or one argument (as the message says).

The overload taking zero argument is called when evaluating the expression +p for some p of type Polynomial. The overload taking one argument of type say, std::vector<term>, is used when evaluating the expression p + q where p has type Polynomial and q has type std::vector<term> (or anything implicitly convertible to it).

It looks like that you need this function

vector<term> operator+(vector<term> v1,vector<term> v2);

to be declared and defined outside any class. It might be either in the global namespace (not advisable) or inside the namespace containing term (preferable).

In addition, I would expect the correct declaration to be

vector<term> operator+(const vector<term>& v1, const vector<term>& v2);

since inside this function you (probably) don't need to make a copy of either operand to work on and you just need reading access to them.