C++ overloading + operator runs into Problem with Constants

I defined a Vec2 object that should represent a 2d-vector. I tried to overload the + operator to implement vectoraddition but I get following error in Line 40:

error: no match for 'operator+' (operand types are 'const Vec2' and 'const Vec2')

Here's my code:

#include <iostream>

using namespace std;

class Vec2 {
private:
    double X;
    double Y;
public:
    Vec2(double x, double y){
        X = x;
        Y = y;
    }

double getX() const {
    return X;
}

double getY() const {
    return Y;
}

Vec2 operator+(Vec2 v ){
    Vec2 res(v.getX()+X, v.getY()+Y);
    return res;
}
};

ostream& operator<<(ostream& s , const Vec2& my_vec)
{
s << " ( "<< my_vec.getX()<< " , "<< my_vec.getY() <<" ) " ;
return s ;
}

int main()
{
    Vec2 const a(3,4);      cout << a << endl;
    Vec2 const b(3,4);      cout << b << endl;

    Vec2 c(a+b);       cout << c << endl;

    return 0;
}

Interestingly when you remove the "const" keyword from a in line 37 everything works, but I don't wan't to fix the error by changing the content of the main function.
I am new to operator overloading, so thanks for the help.

The left operand, a, is const but your operator+ is not const qualified.

You simply need to make it so. Also, instead of taking the right operand, b, by value, you can (optionally) take it by const reference:

Vec2 operator+(const Vec2& v) const {
//                            ^^^^^

Demo

An alternative is to only define operator+= as a member function and then to make operator+ a free function:

class Vec2 {
public:
//...
    Vec2& operator+=(const Vec2& v) { // this changes *this, so not const qual.
        X += v.X;
        Y += v.Y;
        return *this;
    }
};

Vec2 operator+(const Vec2& lhs, const Vec2& rhs) {
    Vec2 result(lhs);
    result += rhs;     // uses the member function operator+=
    return result;
}

Demo

Alternatively:

Vec2 operator+(Vec2 lhs, const Vec2& rhs) {
    return lhs += rhs; // uses the member function operator+=
}