使用bindsecond()的奇怪编译器错误:“已经定义或声明的成员函数”而不是“引用引用”

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

我最近花了相当多的时间来理解在这段代码中调用func()时的错误消息:

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:

当func()这样声明时，代码编译良好:

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

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

当我使用这个声明时(为了提高效率)，我得到了一个编译错误:

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,

我期望看到的错误类似于引用到引用的错误，因为binder2的算子()的定义，

result_type operator()(const argument_type& _Left) const

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

相反，令我惊讶的是，Visual c++ (VS2012)编译器给我的错误是:

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

错误C2535: 'void std::binder2 <_Fn2>:::operator ()(const std:::vector<_Ty> &) const':成员函数已经定义或声明

which I cannot decipher.

我无法破译。

Can you explain under which mechanism the operator() is already defined?
您能解释一下操作符()是在什么机制下定义的吗?

The full error I got was:

我得到的全部错误是:

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.

2 个解决方案

#1

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

这种行为是定义良好的(每个正确的c++编译器都不能编译代码)。

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

从class template binder2的standard (N3376)节D.9.3中可以看出，算子()有两个定义:

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.

如果first_argument_type已经是const &，那么它们将相互冲突。

#2

This isn't an answer, but I just want to record the modern C++11 solution, where all the small bind helpers are deprecated in favour of the universal std::bind:

这并不是一个答案，但我只想记录现代的c++ 11解决方案，在这个解决方案中，所有的小绑定助手都被弃用，取而代之的是通用的std::bind: bind:

#include <functional>
#include <vector>
#include <algorithm>

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

int main()
{
    std::vector<std::vector<double>> v;
    double sum = 0;
    std::for_each(v.begin(), v.end(), std::bind(func, std::placeholders::_1, &sum));
}

The variadic templates of C++11, as well as a more comprehensive collection of type-modifying traits, give std::bind much stronger deduction abilities than the previous components in <functional>.

c++ 11的变量模板，以及更全面的类型修改特性集合，使std::绑定比之前中的组件具有更强的推理能力。

#1