的remove_if（）编译错误，在VS2010

Question

编译错误：

 
c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(1840): error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const triangle' (or there is no acceptable conversion) 
h:\kingston_backup\ocv\ocv\delaunay.h(281): could be 'triangle &triangle::operator =(const triangle &)' 
while trying to match the argument list '(const triangle, const triangle)' 

c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(1853) : see reference to function template instantiation '_FwdIt std::_Remove_if,_Pr>(_FwdIt,_FwdIt,_Pr)' being compiled 
with 
[ 
    _FwdIt=std::_Tree_unchecked_const_iterator,std::allocator,true>>>, 
    _Mytree=std::_Tree_val,std::allocator,true>>, 
    _Pr=triangleIsCompleted 
] 

h:\kingston_backup\ocv\ocv\delaunay.cpp(272) : see reference to function template instantiation '_FwdIt std::remove_if,triangleIsCompleted>(_FwdIt,_FwdIt,_Pr)' being compiled 
with 
[ 
    _FwdIt=std::_Tree_const_iterator,std::allocator,true>>>, 
    _Mytree=std::_Tree_val,std::allocator,true>>, 
    _Pr=triangleIsCompleted 
] 

我认为这个问题是在传递参数给STL的remove_if()，由编译器错误的建议。我已经添加了以下注释行：

//**** ERROR LINE 

---

class triangleIsCompleted 
{ 
public: 
    triangleIsCompleted(cvIterator itVertex, triangleSet& output, const vertex SuperTriangle[3]) 
     : m_itVertex(itVertex) 
     , m_Output(output) 
     , m_pSuperTriangle(SuperTriangle) 
    {} 

    bool operator()(const triangle& tri) const 
    { 
     bool b = tri.IsLeftOf(m_itVertex); 

     if (b) 
     { 
      triangleHasVertex thv(m_pSuperTriangle); 
      if (! thv(tri)) m_Output.insert(tri); 
     } 
     return b; 
    } 
}; 

// ... 

triangleSet workset; 
workset.insert(triangle(vSuper)); 

for (itVertex = vertices.begin(); itVertex != vertices.end(); itVertex++) 
{ 
    tIterator itEnd = remove_if(workset.begin(), workset.end(), triangleIsCompleted(itVertex, output, vSuper)); //**** ERROR LINE 
    // ... 
} 

Answer 1

remove_if不会删除任何东西（在擦除的意义上）。它复制周围的值，以便所有剩余的值最终在范围的开始处（并且范围的其余部分处于或多或少未指定的状态）。

由于关联容器中的键是不可变的，因此无法将值从一个地方复制到另一个地方的另一个地方，因此remove_if无法为其工作。

标准库似乎没有包含set的remove_if，所以你不得不推出自己的。这可能是这样的：

#include <set> 

template <class Key, class Compare, class Alloc, class Func> 
void erase_if(std::set<Key, Compare, Alloc>& set, Func f) 
{ 
    for (typename std::set<Key, Compare, Alloc>::iterator it = set.begin(); it != set.end();) { 
     if (f(*it)) { 
      set.erase(it++); //increment before passing to erase, because after the call it would be invalidated 
     } 
     else { 
      ++it; 
     } 
    } 
}