内積

template <class T>
struct vector2 {
    T x, y;
    vector2(const T& x, const T& y) : x(x), y(y) {}
};

template <class T>
T inner_product(const vector2<T>& v, const vector2<T>& u)
{
    return v.x * u.x + v.y * u.y;
}


#include <iostream>

int main()
{
    vector2<float> v(-1, 1);
    vector2<float> u(1, -2);

    std::cout << inner_product(v, u) << std::endl;
}

-3