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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:\consoleapplica

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