Operator overloading as friend function error

Question

I'm trying to use + to add 2 vector (mathematical vector). Here's my code:

class Vector{
    double v[Max_size];
    int dim;
public:
    int getDim() const;
    Vector();
    Vector(int n);
    Vector(const Vector& a);
    Vector add(const Vector&b);
    friend Vector operator+(Vector summand1, Vector summand2);
};

Operator overloading:

Vector operator+(Vector summand1, Vector summand2){
    int dim1 = summand1.getDim();
    int dim2 = summand2.getDim();
    assert(dim1 == dim2);
    Vector sum(dim1);
    int i;
    for(i = 0; i < dim1; i++){
        sum.v[i] = summand1.v[i] + summand2.v[i];
    }
    return sum;
}

And how I use it:

Vector m = v+t;

When I run the code, it always shows that m is (0,0) (2D vector), which is the default value generated by the constructor. What's wrong with it? Thanks!

Your problem isn't in the code posted, can you post more details that will enable us run the code? — WhiZTiM, Apr 25 '16 at 09:42
BTW... for efficiency reasons, I suggest you read this: http://stackoverflow.com/a/414260/1621391 — WhiZTiM, Apr 25 '16 at 09:43
@T.C. I think it would use the default copy constructor. My code posted above has another way to add 2 vector, and it seems to work properly without manually creating the copy constructor. — Minh Nghĩa, Apr 25 '16 at 10:03
On a side note, had you used `std::vector` instead of a C-style array, you wouldn't have needed the copy constructor for the code to work properly. — pzelasko, Apr 25 '16 at 10:10
To avoid this question being closed, you must post your problem in the form of an [MCVE]. — Lightness Races in Orbit, Apr 25 '16 at 18:44

rems4e · Answer 1 · 2016-04-25T18:42:10.007

Your copy constructor:

Vector::Vector(const Vector& a){
    dim = a.dim;
    Vector(dim);
}

correctly sets the value of the dim member, but has not other side effect.

You should have a variant of the following code:

Vector::Vector(const Vector& a) : dim(a.dim) {
    std::copy(std::begin(a.v), std::end(a.v), v);
}

This will actually copy the data present in the parameter, and you will see the correct behavior for the code:

// Copy constructor called here, but did not correctly copy the data before.
Vector m = v + t;

For a better (by that I intend simpler and safer) Vector class, if you have access to a compiler that is at least C++11 compliant, you can write:

class Vector{
    std::array<double, Max_size> v; // Note the std::array here.
    int dim;
public:
    int getDim() const;
    Vector();
    Vector(int n);
    Vector(const Vector& a);
    Vector add(const Vector&b);
    friend Vector operator+(Vector summand1, Vector summand2);
};

The std::array will take care of everything, provided you write your copy constructor like this:

Vector::Vector(const Vector& a) : v(a.v), dim(a.dim) {

}

Or, even better, you could then let the compiler generate the copy constructor itself, with the same behavior.

dim(a.dim) constructs the dim member with the value a.dim. In C++, it is preferred, more optimized and sometimes mandatory to initialize class members like that. Search "member initialization list" for more info on this syntax. — rems4e, Apr 25 '16 at 18:35

Operator overloading as friend function error

1 Answers1