c++ - 为什么 std::remove_if 认为 shared_ptr 是谓词？

Question

所以我有下面这个简单的例子:

#include <iostream>
#include <memory>
#include <vector>
#include <algorithm>

using namespace std;

struct foo
{};


int main(void)
{
  vector<shared_ptr<foo>> v;

  auto f1 = make_shared<foo>();
  auto f2 = make_shared<foo>();
  auto f3 = make_shared<foo>();
  auto f4 = make_shared<foo>();

  v.push_back(f1);
  v.push_back(f2);
  v.push_back(f3);
  v.push_back(f4);

  cout << v.size() << endl;

  v.erase(remove_if(begin(v), end(v), f2), end(v));

  cout << v.size() << endl;
}

为什么这个 remove_if 认为 f2 是谓词而不是我正在寻找的值？我真的必须在这里提供一个谓词才能让它工作 - 或者我在这里做错了什么？

(注意:编译器:gcc 4.8.2 -std=c++11)

编辑:应该有 rtfm!无论如何 - 这是编译器输出:

In file included from /usr/local/gcc/4.8.2/include/c++/4.8/algorithm:62:0,
                 from remove_if.cpp:4:
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h: In instantiation of '_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = __gnu_cxx::__normal_iterator<std::shared_ptr<foo>*, std::vector<std::shared_ptr<foo> > >; _Predicate = std::shared_ptr<foo>]':
remove_if.cpp:29:41:   required from here
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:1150:33: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
         if(!bool(__pred(*__first)))
                                 ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h: In instantiation of '_RandomAccessIterator std::__find_if(_RandomAccessIterator, _RandomAccessIterator, _Predicate, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<std::shared_ptr<foo>*, std::vector<std::shared_ptr<foo> > >; _Predicate = std::shared_ptr<foo>]':
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:4465:41:   required from '_IIter std::find_if(_IIter, _IIter, _Predicate) [with _IIter = __gnu_cxx::__normal_iterator<std::shared_ptr<foo>*, std::vector<std::shared_ptr<foo> > >; _Predicate = std::shared_ptr<foo>]'
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:1144:64:   required from '_FIter std::remove_if(_FIter, _FIter, _Predicate) [with _FIter = __gnu_cxx::__normal_iterator<std::shared_ptr<foo>*, std::vector<std::shared_ptr<foo> > >; _Predicate = std::shared_ptr<foo>]'
remove_if.cpp:29:41:   required from here
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:214:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:218:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:222:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:226:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:234:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:238:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))
                       ^
/usr/local/gcc/4.8.2/include/c++/4.8/bits/stl_algo.h:242:23: error: no match for call to '(std::shared_ptr<foo>) (std::shared_ptr<foo>&)'
    if (__pred(*__first))

Answer 1

因为这就是 remove_if 所做的；第三个参数是一个谓词，用于测试是否删除一个元素。据推测，您的代码无法编译，因为 shared_ptr 不能像函数一样被调用。

如果要删除具有特定值的元素，请使用 remove .