从std :: list清除元素的顺序是什么？

Question

I want to clear the contents of some std::list . The order of removing of elements is important for me. According to output of the following test program, the order is from first to last element. Is it guaranteed to be so? It was not clear for me from C++2003 standard.

#include <list>
#include <iostream>

struct A
{
  A(int i) : I(i) {}
  ~A() { std::cout << I << std::endl; }
  int I;
};

int main()
{
  std::list<A> l;
  l.push_back(A(1));
  l.push_back(A(2));
  l.push_back(A(3));

  std::cout << "clearing list" << std::endl;
  l.clear();
}

ideone link

Answer 1

不，它没有定义，你不应该依赖它。

Answer 2

No it is not defined.

The standard only specifys that whenever you call a.clear() it will be resolved as a.erase(q1,q2) and it just specify's the erase will erase all the elements in the range [q1,q2) but it does not specify the order in which it will do so.

Answer 3

Just for completeness, the C++11 standard does not determine the order of destruction of any sequence containers , of which std::list is a member. It only states that all the elements are destroyed, all references, pointers and iterators referring to the elements are invalidated, and the past the end iterator may be invalidated. Concerning clear() , it makes no mention of erase() , begin() or end() , unlike hte C++03 standard.

Answer 4

From c++03 standard:

Table 67—Sequence requirements (in addition to container) ...

a.clear()

assertion/note: void erase(begin(), end())

post: size() == 0.

Since it starts deleting elements from "begin" I think is safe to infers that they will be deleted in order. Otherwise will be a performance penalty to random access list elements.