cpp 排序函数的行为是什么，通过对 vector<vector<int>> 使用默认重载更大的函数

Question

sort(begin(ans), end(ans)) 与 sort(begin(ans), end(ans), std::greater<std::vector<std::vector<int> > >())

之间的区别

as sort(begin(ans), end(ans)) 如预期的那样完美工作，因为指定更大的不起作用。

  vector<int> S = {12,13, 34, 9,10};
  vector<vector<int> >ans;
  vector<int> currSet;
  subset(0, S, currSet, ans);
  sort(begin(ans), end(ans), std::greater<std::vector<std::vector<int> > >());
  cout << ans.size();
}

报错

In file included from subset.cpp:1:
In file included from /usr/local/include/bits/stdc++.h:52:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/ccomplex:21:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/complex:247:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/sstream:174:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/ostream:138:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/ios:216:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/__locale:15:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/string:505:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/string_view:176:
In file included from /Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/__string:57:
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:3832:17: error: no
      matching function for call to object of type
      'std::__1::greater<std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >,
      std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > > >'
            if (__comp(*--__last, *__first))
                ^~~~~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:4018:5: note: in
      instantiation of function template specialization
      'std::__1::__sort<std::__1::greater<std::__1::vector<std::__1::vector<int,
      std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int,
      std::__1::allocator<int> > > > > &, std::__1::vector<int, std::__1::allocator<int> >
      *>' requested here
    __sort<_Comp_ref>(__first, __last, __comp);
    ^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:4052:12: note: in
      instantiation of function template specialization
      'std::__1::sort<std::__1::vector<int, std::__1::allocator<int> > *,
      std::__1::greater<std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >,
      std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > > > &>'
      requested here
    _VSTD::sort<_Tp*, _Comp_ref>(__first.base(), __last.base(), __comp);
           ^
subset.cpp:33:3: note: in instantiation of function template specialization
      'std::__1::sort<std::__1::vector<int, std::__1::allocator<int> >,
      std::__1::greater<std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >,
      std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > > > >'
      requested here
  sort(all(ans), greater<vector<vector<int> > >());
  ^

Answer 1

std::greater<std::vector<std::vector<int> > > 用于比较 std::vector<std::vector<int> > 类型的对象。但是向量 ans 不包含该类型的对象。它包含 std::vector<int> 类型的对象。因此，您需要的比较器是 std::greater<std::vector<int>>。或者，您可以简单地使用 std::greater<>，它更易于阅读、编写并且不太可能导致错误。

Answer 2

std::greater 的模板参数需要是迭代器指向的元素类型。由于 std::vector<std::vector<int>> 的迭代器具有 std::vector<int> 的 value_type（这是 ans 中元素的类型），这就是您所需要的。那给你

sort(begin(ans), end(ans), std::greater<std::vector<int> >());

随心所欲。

另请注意，模板参数列表中的结束 > 不再需要 space。您可以使用

sort(begin(ans), end(ans), std::greater<std::vector<int>>());

这将在 C++11+ 兼容的编译器中编译。

cpp 排序函数的行为是什么，通过对 vector<vector<int>> 使用默认重载更大的函数

What is behaviour of cpp sort function, by using default overloaded greater function for vector<vector<int>>

c++

vector

clang++