cpp中的迭代器

之前写 python 的时候我一直很喜欢的一个东西就是迭代器，用起来真的很方便，最接近开始写 cpp 觉得没有这个东西不行，研究了一下，cpp正好支持这样的特性（虽然还是冗长且不好用）。

我们先看一下 cppreference 上关于这一块说明

template<
    class Category,
    class T,
    class Distance = std::ptrdiff_t, // (deprecated in C++17)
    class Pointer = T*,
    class Reference = T&
> struct iterator;
 
// std::iterator is the base class provided to simplify definitions of the required types for iterators.
 
// Template parameters
// Category	-	the category of the iterator. Must be one of iterator category tags.
// T	-	the type of the values that can be obtained by dereferencing the iterator. This type should be void for output iterators.
// Distance	-	a type that can be used to identify distance between iterators
// Pointer	-	defines a pointer to the type iterated over (T)
// Reference	-	defines a reference to the type iterated over (T)
// Member types

下面我直接引用 cppreference 的示例代码了，对照着上面的文档还是挺好懂的。

#include <iostream>
#include <algorithm>
 
template <long FROM, long TO>
class Range {
public:
    // member typedefs provided through inheriting from std::iterator
    class iterator : public std::iterator<
            std::input_iterator_tag, // iterator_category
            long, // value_type
            long, // difference_type
            const long*, // pointer
            long // reference
        > {
        long num = FROM;
 
    public:
        explicit iterator(long _num = 0) : num(_num) {}
 
        iterator& operator++() {
            num = TO >= FROM ? num + 1 : num - 1;
            return *this;
        }
 
        iterator operator++(int) {
            iterator retval = *this;
            ++(*this);
            return retval;
        }
 
        bool operator==(iterator other) const { return num == other.num; }
        bool operator!=(iterator other) const { return !(*this == other); }
        reference operator*() const { return num; }
    };
 
    iterator begin() { return iterator(FROM); }
    iterator end() { return iterator(TO >= FROM ? TO + 1 : TO - 1); }
};
 
int main() {
    // std::find requires an input iterator
    auto range = Range<15, 25>();
    auto itr = std::find(range.begin(), range.end(), 18);
    std::cout << *itr << '\n'; // 18
 
    // Range::iterator also satisfies range-based for requirements
    for (long l : Range<3, 5>()) {
        std::cout << l << ' '; // 3 4 5
    }
    std::cout << '\n';
}