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范围For 语句

程序员文章站 2022-03-16 19:14:22
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For (declaration: expression)

        Statement

Declaration:定义一个作为访问方式变量

Expression:指向需要操作的对象

Statement:需要执行的操作。

for(int& i : vec) //若需要修改对象中的值则要加引用(&)
{
    i++;    
}

for (auto i : vec)//推荐使用auto类型说明符
{
    cout << i << endl; // 输出更新后的数值
}

注:expression为一个“序列”,可进行自定义,需要对象满足一下条件:

指针或迭代器(*);

拥有能够返回迭代器的begin和end成员函数;

前缀自加操作符(++);

不等关系运算符(!=);

// a simple iterator sample.

#include <iostream>

using namespace std;

// forward-declaration to allow use in Iter
class IntVector;

class Iter
{
private:
    int _pos;
    const IntVector *_p_vec;

public:
    Iter(const IntVector* p_vec, int pos) : _pos(pos), _p_vec(p_vec){}

    // these three methods form the basis of an iterator for use with a range-based for loop
    bool operator!=(const Iter& other) const
    {
        return _pos != other._pos;
    }

    // this method must be defined after the definition of IntVector since it needs to use it
    int operator*() const;
    const Iter& operator++()
    {
        ++_pos;
        // although not strictly necessary for a range-based for loop
        // following the normal convention of returning a value from
        // operator++ is a good idea.
        return *this;
    }
};

class IntVector
{
private:
    int _data[100];

public:
    IntVector(){}

    int get(int col) const
    {
        return _data[col];
    }

    Iter begin() const
    {
        return Iter(this, 0);
    }

    Iter end() const
    {
        return Iter(this, 100);
    }

    void set(int index, int val)
    {
        _data[index] = val;
    }
};

int Iter::operator*() const
{
    return _p_vec->get(_pos);
}

// sample usage of the range-based for loop on IntVector
int main()
{
    IntVector v;

    for(int i = 0; i < 100; i++)
    {
        v.set(i,i);
    }
    for( int i : v)
    {
        cout << i << endl;
    }
}

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