爲什麼std :: remove_if認爲shared_ptr <T>是一個謂詞？

Question

所以，我有以下簡單的例子：

#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; 
} 

爲什麼這個remove_if認爲f2是謂語，而不是我在找一個價值？我真的必須在這裏提供一個謂詞來使這個工作 - 或者我在這裏做錯了什麼？

（注：編譯器：GCC 4.8.2 -std = C++ 11）

編輯：應該有RTFM！無論如何 - 這裏是編譯器輸出：

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

因爲這就是remove_if所做的;第三個參數是測試是否刪除元素的謂詞。據推測，你的代碼無法編譯，因爲shared_ptr不能被稱爲函數。

如果要刪除具有特定值的元素，請使用remove。