类继承和运算符重载

Question

我正在通过SFML上的一个小项目来学习C ++，我想扩展sf::Vector2<T>类，该类表示2D数学向量。 此类的声明在此处提供 。 特别是，我想向类中添加一些方法，用于范数计算，旋转等。

这是我到目前为止所拥有的：

#include <cmath>
#include <SFML/System/Vector2.hpp>
#include <SFML/Graphics/Transform.hpp>
#include <ostream>

namespace dw::math {

template <typename T>
class Vector2 : public sf::Vector2<T> {
public:
    // Don't really know what does that exactly means, explainations are welcome !
    using sf::Vector2<T>::Vector2;

    template <typename U>
    explicit Vector2(const U &vector) : sf::Vector2<T>(vector) {}

    // This ctor is for implicit conversion from base, don't know if it's really useful ?
    Vector2(sf::Vector2<T> &vector) : sf::Vector2<T>(vector) {}

    // Some methods here ...

    friend std::ostream & operator<<(std::ostream &os, const math::Vector2<T>& right) {
        os << '{' << right.x << ", " <<right.y << '}';
        return os;
    }
};

在代码的后面，我声明了这样的结构：

struct Derivative {
    dw::math::Vector2f dPos;
    dw::math::Vector2f dVel;

    Derivative() = default;
    Derivative(const dw::math::Vector2f &dPos, const dw::math::Vector2f &dVel) : dPos(dPos), dVel(dVel) {}

    Derivative operator+(const Derivative &rhs) const {
        return Derivative(
            dPos + rhs.dPos,
            dVel + rhs.dVel
        );
    }
}

operator+ Derivative重载的返回部分不会编译：

/home/palra/Documents/Projets/dw/src/physics/World.cpp: In member function ‘Derivative Derivative::operator+(const Derivative&) const’:
/home/palra/Documents/Projets/dw/src/physics/World.cpp:24:18: error: invalid user-defined conversion from ‘sf::Vector2<float>’ to ‘const Vector2f& {aka const dw::math::Vector2<float>&}’ [-fpermissive]
             dPos + rhs.dPos,
             ~~~~~^~~~~~~~~~
In file included from /home/palra/Documents/Projets/dw/src/physics/Body.h:8:0,
                 from /home/palra/Documents/Projets/dw/src/physics/World.h:10,
                 from /home/palra/Documents/Projets/dw/src/physics/World.cpp:5:
/home/palra/Documents/Projets/dw/src/physics/../math/Vector2.h:30:5: note: candidate is: dw::math::Vector2<T, <template-parameter-1-2> >::Vector2(sf::Vector2<T>&) [with T = float; <template-parameter-1-2> = void] <near match>
     Vector2(sf::Vector2<T> &vector) : sf::Vector2<T>(vector) {}

我不明白为什么这行不通。 表达式dPos + rhs.dPos与sf::Vector2<float> operator +(const sf::Vector2<float>& left, const sf::Vector2<float>& right)的调用兼容，因为dw::math::Vector2<float> sf::Vector2<float> operator +(const sf::Vector2<float>& left, const sf::Vector2<float>& right) dw::math::Vector2<float> sf::Vector2<float>通过继承是sf::Vector2<float> 。 然后，该表达式应该产生一个sf::Vector2<float> ，这要归功于dw::math::Vector2f这要归功于我猜想的非显式构造函数。 我哪里错了？ 我应该如何使其工作？

Answer 1

sf::Vector<T>并非旨在用作派生类，因为更好的方法是编写自由函数来扩展sf::Vector<T> ，这对操作员而言是无缝的。 不幸的是，对于正常的函数调用，C ++不像C＃那样支持扩展。