如何最好地控制迭代方向？

Question

我有一个大型对象的容器,复制起来很昂贵.我有时必须正常地迭代整个容器,有时反过来.一旦我确定了迭代方向,我就不需要改变飞行中途,即不需要随机访问.

我希望做这样的模式:

#include <iostream>
#include <vector>
using namespace std;
int main( int argc, char** )
{
    // pretend this is a vector of expensive objects
    vector<int> foo = {1,2,3,4,5};

    // calculate forward or backward iteration direction
    bool backwards = (argc > 1);

    if( backwards )
        // prepare backward iteration, but don't copy objects
    else
        // prepare forward iteration, but don't copy objects

    for( auto& i : /* either forward or backward */ )
    {
        // my loop body
        cout << i;
    }

    return 0;
}

这是一个C++ 11程序,但我不认为这对我有帮助.我只是没有看到最好的方法来做到这一点.谢谢你的帮助.

Answer 1

C++标准容器带有这些称为"反向迭代器"的东西.使用std::vector::rbegin()和std::vector::rend()获取迭代器向后迭代向量.C++ 03可以很容易地做到这一点:

#include <iostream> 
#include <vector>  

// Use const reference to pass expensive-to-copy types
void loop_body(const int& i)
{
    std::cout << i;
}

int main( int argc, char** ) 
{ 
    // pretend this is a vector of expensive objects 
    std::vector<int> foo = {1,2,3,4,5}; 

    // calculate forward or backward iteration direction 
    bool backwards = (argc > 1); 

    if( backwards ) { 
        std::for_each(foo.rbegin(), foo.rend(), &loop_body);
    } else { 
        std::for_each(foo.begin(), foo.end(), &loop_body);
    } 
    return 0; 
} 

你可以在C++ 11中使用lambdas来做到这一点:

#include <iostream> 
#include <vector> 

int main( int argc, char** ) 
{ 
    // pretend this is a vector of expensive objects 
    std::vector<int> foo = {1,2,3,4,5}; 

    // calculate forward or backward iteration direction 
    bool backwards = (argc > 1); 

    // Use const reference to pass expensive-to-copy types
    auto loop_body = [](const int& i)
    {
        std::cout << i;
    };

    if( backwards ) { 
        std::for_each(foo.rbegin(), foo.rend(), loop_body);
    } else { 
        std::for_each(foo.begin(), foo.end(), loop_body);
    } 
    return 0; 
}

Answer 2

你为什么不把你的算法放到模板函数中？然后用begin/end或rbegin/rend调用它是微不足道的.

template <class Iterator>
void do_stuff(Iterator first, Iterator last)
{
    // Your loop code here.
}

或者您可以使用lambda(因为它是C++ 11)以及std::for_each:

auto loop_body = [&](int &i) { std::cout << i << std::endl; } ;

if (backward)
  std::for_each(v.rbegin(), v.rend(), loop_body);
else
  std::for_each(v.begin(), v.end(), loop_body);