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CPP foreach循环浅析

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概况

CPP foreach循环是CPP11引入的新特性，其能够很轻易地完成对可迭代对象的遍历，一般而言，一个典型的foreach循环结构如下：

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for (const auto &item : contain) {
    do_something(item);
}

可迭代对象

可迭代对象（上述示例中的contain）是一个可以使用迭代器遍历的对象，其要求满足以下条件：

  1. 实现了begin方法，并要求该方法返回一个迭代器且无参数，一个典型的声明如下：

    iterator begin() const;

  2. 实现了end方法，并要求该方法返回一个迭代器且无参数，一个典型的声明如下：

    iterator end() const;

而迭代器要求满足以下条件：

  1. 实现了相应的构造函数和赋值函数。

  2. 实现了operator++方法，一个典型的声明如下：

    iterator &operator++();

  3. 实现了operator!=方法，一个典型的声明如下：

    bool operator!=(const iterator &) const;

  4. 实现了operator*方法，一个典型的声明如下：

    T &operator*() const;

等价代码

实际上CPP中的foreach循环是一种语法糖，其等价于如下代码：

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{
    iterator first = contain.begin();
    iterator last = contain.end();
    while (first != last) {
        do_something(*first);
        ++first;
    }
}

代码示例

下面展示了一个对自定义可迭代对象进行迭代的Demo示例：

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#include <iostream>

class Range {
public:
    class RangeIterator {
    public:
        explicit RangeIterator(std::size_t i) :m_number_(i) {
            std::cout << __FILE__ << ':' << __LINE__ << " ictor" << std::endl;
        }

        RangeIterator(const RangeIterator &it) :RangeIterator(it.m_number_) {
            std::cout << __FILE__ << ':' << __LINE__ << " icopy_ctor" << std::endl;
        }

        RangeIterator(RangeIterator &&it) :RangeIterator(it.m_number_) {
            std::cout << __FILE__ << ':' << __LINE__ << " imove_ctor" << std::endl;
        }

        RangeIterator &operator=(const RangeIterator &it) {
            m_number_ = it.m_number_;
            std::cout << __FILE__ << ':' << __LINE__ << " icopy" << std::endl;
            return *this;
        }

        RangeIterator &operator=(RangeIterator &&it) {
            m_number_ = it.m_number_;
            std::cout << __FILE__ << ':' << __LINE__ << " imove" << std::endl;
            return *this;
        }

        bool operator!=(const RangeIterator &rit) const {
            std::cout << __FILE__ << ':' << __LINE__ << " operator!=" << std::endl;
            return m_number_ != rit.m_number_;
        }

        RangeIterator &operator++() {
            std::cout << __FILE__ << ':' << __LINE__ << " operator++" << std::endl;
            ++m_number_;
            return *this;
        }

        std::size_t operator*() const {
            std::cout << __FILE__ << ':' << __LINE__ << " *" << std::endl;
            return m_number_;
        }
    private:
        std::size_t m_number_;
    };
public:
    explicit Range(std::size_t first = 0, std::size_t last = SIZE_MAX) :
        m_first_(first), m_last_(last) {
        std::cout << __FILE__ << ':' << __LINE__ << " ctor" << std::endl;
    }

    Range(const Range &range) :
        m_first_(range.m_first_), m_last_(range.m_last_) {
        std::cout << __FILE__ << ':' << __LINE__ << " copy_ctor" << std::endl;
    }

    Range(Range &&range) :
        m_first_(std::move(range.m_first_)), m_last_(std::move(range.m_last_)) {
        std::cout << __FILE__ << ':' << __LINE__ << " move_ctor" << std::endl;
    }

    Range &operator=(const Range &range) {
        m_first_ = range.m_first_;
        m_last_ = range.m_last_;
        std::cout << __FILE__ << ':' << __LINE__ << " copy" << std::endl;
        return *this;
    }

    Range &operator=(Range &&range) {
        m_first_ = std::move(range.m_first_);
        m_last_ = std::move(range.m_last_);
        std::cout << __FILE__ << ':' << __LINE__ << " move_ctor" << std::endl;
        return *this;
    }

    RangeIterator begin() const {
        std::cout << __FILE__ << ':' << __LINE__ << " begin" << std::endl;
        return m_first_;
    }

    RangeIterator end() const {
        std::cout << __FILE__ << ':' << __LINE__ << " end" << std::endl;
        return m_last_;
    }
private:
    RangeIterator m_first_;
    RangeIterator m_last_;
};

int main() {
    for (const auto &i : Range(0, 4)) {
        std::cout << __FILE__ << ':' << __LINE__ << ' ' << i << std::endl;
    }
    return 0;
}

编译运行后的结果为：

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test0.cpp:8 ictor
test0.cpp:8 ictor
test0.cpp:52 ctor
test0.cpp:80 begin
test0.cpp:8 ictor
test0.cpp:12 icopy_ctor
test0.cpp:85 end
test0.cpp:8 ictor
test0.cpp:12 icopy_ctor
test0.cpp:32 operator!=
test0.cpp:43 *
test0.cpp:95 0
test0.cpp:37 operator++
test0.cpp:32 operator!=
test0.cpp:43 *
test0.cpp:95 1
test0.cpp:37 operator++
test0.cpp:32 operator!=
test0.cpp:43 *
test0.cpp:95 2
test0.cpp:37 operator++
test0.cpp:32 operator!=
test0.cpp:43 *
test0.cpp:95 3
test0.cpp:37 operator++
test0.cpp:32 operator!=