奇怪的编译器错误使用bind2nd（）：“已定义或声明的成员函数”而不是“参考”。 [英] weird compiler error using bind2nd(): "member function already defined or declared" instead of "reference to reference"

问题描述

我最近在这段代码中调用 func（）时花了很多时间来理解错误信息：

  int main（）
 {
 vector< vector< double> > v; 
 
 double sum = 0; 
 for_each（v.begin（），v.end（），
 bind2nd（ptr_fun（func），& sum））; 
 
 return 0; $ c $ b} 
  

>被声明如此，代码编译得很好：

  void func（vector< double> v，double * sum）
 {
} 
  

当我使用这个声明编译器错误：

  void func（const vector< double>& v，double * sum）
 {
} 
  

我希望看到的错误类似于 reference-to-reference 错误，因为binder2nd的operator（）的定义，

  result_type operator（）（const argument_type& _Left ）const 
  

令我惊讶的是，Visual C ++（VS2012）编译器给出的错误是：

错误C2535：'void std :: binder2nd< _Fn2> :: operator（）（const
std :: vector< _Ty>&）const'：已定义的成员函数或
声明了

你可以解释一下 operator（）是什么机制已经

<$>

p $ p> 错误C2535：'void std :: binder2nd< _Fn2> :: operator（）（const std :: vector< _Ty> &）const'：已定义或声明的成员函数
with
[
_Fn2 = std :: pointer_to_binary_function< const std :: vector< double> &，double *，void，void（__cdecl *）（const std :: vector< double>&，double *）>，
_Ty = double
]
c：\ vc \include\xfunctional（319）：请参见声明'std :: binder2nd< _Fn2> :: operator（）'
with
[
_Fn2 = std :: pointer_to_binary_function< const std :: vector< double> &，double *，void，void（__cdecl *）（const std :: vector< double>&，double *）&
]
c：\consoleapplication1.cpp（31）：参见类模板实例化'std :: binder2nd< _Fn2>'被编译
with
[
_Fn2 = std :: pointer_to_binary_function< const std :: vector< double> &，double *，void，void（__cdecl *）（const std :: vector< double>&，double *）&
]

构建失败。


解决方案

从类模板的标准（N3376）部分 D.9.3 code> binder2nd ，这两个定义 operator（）存在：

  typename Fn :: result_type 
 operator（）（const typename Fn :: first_argument_type& x）const; 
 
 typename Fn :: result_type 
 operator（）（typename Fn :: first_argument_type& x）const; 
  

如果 first_argument_type $ c> const T& ，那么它们就会发生冲突。

I recently spent quite some time understanding the error message when calling func() in this piece of code:

int main()
{
    vector< vector<double> > v;

    double sum = 0;
    for_each( v.begin(), v.end(), 
        bind2nd( ptr_fun(func), &sum ) );

    return 0;
}

when func() was declared like so, the code compiled fine:

void func( vector<double> v, double *sum )
{
}

when I used this declaration (for efficiency), I got a compiler error:

void func( const vector<double> &v, double *sum )
{
}

The error I expected to see was something like a reference-to-reference error, because of the definition of operator() of binder2nd,

result_type operator()(const argument_type& _Left) const

Instead, to my surprise, the error the Visual C++ (VS2012) compiler gave me was:

error C2535: 'void std::binder2nd<_Fn2>::operator ()(const std::vector<_Ty> &) const' : member function already defined or declared

which I cannot decipher.

• Can you explain under which mechanism the operator() is already defined?

The full error I got was:

error C2535: 'void std::binder2nd<_Fn2>::operator ()(const std::vector<_Ty> &) const' : member function already defined or declared
with
[
     _Fn2=std::pointer_to_binary_function<const std::vector<double> &,double *,void,void (__cdecl *)(const std::vector<double> &,double *)>,
      _Ty=double
]
c:\vc\include\xfunctional(319) : see declaration of 'std::binder2nd<_Fn2>::operator ()' 
with
[
      _Fn2=std::pointer_to_binary_function<const std::vector<double> &,double *,void,void (__cdecl *)(const std::vector<double> &,double *)>
] 
c:\consoleapplication1.cpp(31) : see reference to class template instantiation 'std::binder2nd<_Fn2>' being compiled 
with 
[
       _Fn2=std::pointer_to_binary_function<const std::vector<double> &,double *,void,void (__cdecl *)(const std::vector<double> &,double *)>
]

Build FAILED.

解决方案

This behavior is well-defined (every correct C++ compiler will fail to compile your code).

From the standard (N3376) section D.9.3 on class template binder2nd, these two defintions of operator() exist:

typename Fn::result_type
operator()(const typename Fn::first_argument_type& x) const;

typename Fn::result_type
operator()(typename Fn::first_argument_type& x) const;

If first_argument_type is already a const T&, then they will be conflicting.

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