back_inserter（容器）的类型是什么

Question

My following code compiles but I do not know how to write this without using decltype. I have written my failed attempts in commented lines below the line which uses decltype.

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

using namespace std;

template<class Out>
class Fill{
public:
  Fill(){}
  void fill(Out x){
    for(int i = 0; i != 10; i++)*x++ = i;
  }
};

int main(){
  vector<int> v;

  Fill<decltype(back_inserter(v))> f; //works

  //does not work
  //Fill<vector<int>::iterator> g;

  //does not work
  //Fill<back_insert_iterator<vector<int>> h;


  f.fill(back_inserter(v));
  copy(v.begin(),v.end(),ostream_iterator<int>(cout," "));
}

Thanks

suresh

Answer 1

The type will be back_insert_iterater<vector<int> > . Your 3rd example doesn't work because you need a whitespace between the ">>". See http://www.cplusplus.com/reference/std/iterator/back_insert_iterator/ , it simply holds a pointer to the container type and redefines operator= to be container.push_back. I believe this is present so that std containers can work with std algorithms. But, since you have control the definition to Fill you could simply allow the Fill class to hold a container pointer and call push_back directly against the container.

Answer 2

vector<int> ints;
typedef back_insert_iterator<vector<int>> InserterType; // This is what you need.
Fill<InserterType> f;
f.fill(back_inserter(ints));
copy(ints.begin(), ints.end(), ostream_iterator<int>(std::cout, "\n"));

Answer 3

如果您使用的是c ++ 0x，答案是：'auto'：D

Answer 4

I figured out that the mistake in my original post was that I missed the 3rd right angled bracket in the definition of h . It should be corrected as

Fill<back_insert_iterator<vector<int>>> h;

Now the code would compile and work as intended.