用String和ISTrimaMyter实现C++编译错误

尝试编译以下内容时:

#include <string>
#include <iterator>
#include <iostream>
using namespace std;
int main() {
  string s(istream_iterator<char>(cin), istream_iterator<char>());
  return s.size();
}

main.cc: In function ‘int main()’:
main.cc:6: error: request for member ‘size’ in ‘s’, which is of non-class type ‘std::string(std::istream_iterator<char, char, std::char_traits<char>, int>, std::istream_iterator<char, char, std::char_traits<char>, int> (*)())’

根据 libstdc++ docs ,string有一个接受开始/结束迭代器对的构造函数。那么,我为什么会犯这个错误呢?