Question

I have a container of large objects that are expensive to copy. I must sometimes iterate over the whole container normally, and sometimes in reverse. Once I determine the iteration direction, I don’t need to change mid-flight, i.e. no random access needed.

I’m hoping to do something like this pattern:

#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;
}

This is a C++11 program, but I don’t think that really helps me here. I’m just not seeing the best way to do this. Thanks for any help.

Answer 1

The C++ standard containers come with these things called “reverse iterators”. Use std::vector::rbegin() and std::vector::rend() to get an iterator that iterates backwards through the vector. C++03 can do this easily:

#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; 
} 

You may be able to do this, using lambdas in C++11:

#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; 
}