为什么std :: remove_if认为shared_ptr <T>是一个谓词？

Question

所以，我有以下简单的例子：

#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是谓语，而不是我在找一个价值？我真的必须在这里提供一个谓词来使这个工作 - 或者我在这里做错了什么？

（注：编译器：GCC 4.8.2 -std = C++ 11）

编辑：应该有RTFM！无论如何 - 这里是编译器输出：

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;第三个参数是测试是否删除元素的谓词。据推测，你的代码无法编译，因为shared_ptr不能被称为函数。

如果要删除具有特定值的元素，请使用remove。