重载ostream＆<<以从类中以相反顺序输出向量

Question

I have defined a class with a public vector and I am trying to output each member of the vector in the reverse order using reverse iterators.

std::ostream& operator<<(std::ostream& os, const bignum& num)
{
   for (std::vector<char>::reverse_iterator p = num.vec.rbegin(); p != num.vec.rend(); ++p)
      os << static_cast<int>(*p);
   return os;
}

This thing doesn't compile and I am a bit confused on what is wrong. Thanks.

Answer 1

You have to use const_reverse_iterator:

std::ostream & operator<<( std::ostream &os, const bignum &num ) 
{
    for ( std::vector<char>::const_reverse_iterator p = num.vec.rbegin();  
          p != num.vec.rend();  ++p )
        os << static_cast<int>(*p);

    return os;
}

Also you could use standard algorithm, std::copy . For example

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>

//...

std::ostream & operator<<( std::ostream &os, const bignum &num ) 
{
    std::copy( num.vec.rbegin(), num.vec.rend(),
               std::ostream_iterator<int>( os ) );

    return os;
}

Answer 2

Since you're receiving a reference to a const bignum , you'd need a const_reverse_iterator , not just a reverse_iterator .

I'd just use std::copy instead though:

std::copy(num.vec.rbegin(), num.vec.rend(), std::ostream_iterator<int>(os));

Answer 3

All the answers shown provide great ways of doing it, I'll just throw in my 2 cents:

std::for_each(num.vec.rbegin(),num.vec.rend(),[](int x){
  std::cout << x << std::endl; // or any delimeter you wish
 });