C++ - Structure Methods Syntax - c++

Question

I get a weird error when I try and compile the following code:
I need to use structs (I was taught classes with the struct keywor, and am trying to learn it that way. I also need to put the function definitions outside the struct block.
#include <iostream>
#include <string>
using namespace std;
struct Box {
int l;
int w;
int area();
Box();
Box(int a, int b);
Box operator+(const Box a, const Box b);
};
Box::Box() {
l = 0;
w = 0;
}
Box::Box(int a, int b) {
l = a;
w = b;
}
Box Box::operator+(const Box a, const Box b) {
Box box(a.l + b.l, a.w + b.w);
return box;
}
int Box::area() {
return l * w;
}
int main() {
Box a(1, 2);
Box b;
b.l = 3;
b.w = 4;
Box c = a + b;
cout << "Total area is: " << a.area() << " + " << (b.area) << " = " << (c.area) << endl;
}
Could someone help me out? Thanks

Answer 1

operator+ which belongs to the class/struct should receive only one parameter of type Box (from the right side of +) which should be added to the current object (from the left side of +):
Box Box::operator+(const Box& a) {
Box box(a.l + l, a.w + w);
return box;
}
Also in the cout line it should be b.area() and c.area() instead of (b.area) and (c.area).

Answer 2

Here's your code modified a bit. I had to put the operator overload into the struct due to the compiler not using NRVO (See here)
#include <iostream>
#include <string>
using namespace std;
struct Box {
int l;
int w;
Box();
Box(int a, int b);
int area();
Box operator+(const Box a)
{
return Box(a.l + l, a.w + w);
}
};
Box::Box() {
l = 0;
w = 0;
}
Box::Box(int a, int b) {
l = a;
w = b;
}
int Box::area() {
return l * w;
}
int main() {
Box a(1, 2);
Box b;
b.l = 3;
b.w = 4;
Box c = a + b;
cout << "Total area is: " << a.area() << " + " << (b.area()) << " = " << (c.area()) << endl;
}
Result:
Total area is: 2 + 12 = 24