c – const,span和iterator麻烦

之后,我尝试写一个span< T>在一个容器上.
对于不是const的类型T,两个const span< T>并且跨度< T>允许更改容器的内容.
const span< const T>和span< const T>禁止更改容器的内容.

代码无法编译,因为：

// *this is const within a const method
    // But It<self_type> requires a non-const *this here.
    // So the code does not compile
    It<self_type> begin() const { return It<self_type>(*this,0); }

如果我使It的构造函数接受一个const容器,它看起来不正确,因为迭代器可以修改容器的内容.

如果我摆脱了方法的const,那么对于非const类型T,const span< T>无法修改容器.

它继承自Const_it,以允许在模板实例化期间从It到Const_it的隐式转换.

我在迭代器(const C * container_;)中使用指针而不是引用来允许将一个迭代器分配给另一个迭代器.

我怀疑这里有些不对劲,因为我甚至想到：

Does cast away const of *this cause undefined behavior?

但我不知道如何解决它.

测试：

#include <vector>
#include <numeric>
#include <iostream>

template<typename C>
class Const_it {
    typedef Const_it<C> self_type;
public:
    Const_it(const C& container,const int ix)
            : container_(&container),ix_(ix) {}
    self_type& operator++() {
        ++ix_;
        return *this;
    }

    const int& operator*() const {
        return ref_a()[ix_];
    }

    bool operator!=(const self_type& rhs) const {
        return ix_ != rhs.ix_;
    }

protected:
    const C& ref_a() const { return *container_; }
    const C* container_;
    int ix_;
};

template<typename C>
class It : public Const_it<C> {
    typedef Const_it<C> Base;
    typedef It<C> self_type;
public:
    //It(const C& container.
    It(C& container,const int ix)
            : Base::Const_it(container,ix) {}
    self_type& operator++() {
        ++ix_;
        return *this;
    }

    int& operator*() const {
        return mutable_a()[ix_];
    }

private:
    C& mutable_a() const { return const_cast<C&>(ref_a()); }
    using Base::ref_a;
    using Base::container_;
    using Base::ix_;
};


template <typename V>
class span {
    typedef span<V> self_type;
public:
    explicit span(V& v) : v_(v) {}
    It<self_type> begin() { return It<self_type>(*this,0); }
    // *this is const within a const method
    // But It<self_type> requires a non-const *this here.
    // So the code does not compile
    It<self_type> begin() const { return It<self_type>(*this,0); }
    It<self_type> end() { return It<self_type>(*this,v_.size()); }
    It<self_type> end() const { return It<self_type>(*this,v_.size()); }

    int& operator[](const int ix) {return v_[ix];}
    const int& operator[](const int ix) const {return v_[ix];}
private:
    V& v_;
};


int main() {
    typedef std::vector<int> V;
    V v(10);
    std::iota(v.begin(),v.end(),0);
    std::cout << v.size() << "n";
    const span<V> s(v);
    for (auto&& x : s) {
        x = 4;
        std::cout << x << "n";
    }
}