Annex D (normative) Compatibility features [depr]

D.9 Binders [depr.lib.binders]

D.9.1 Class template binder1st [depr.lib.binder.1st]

template <class Fn> class binder1st : public unary_function<typename Fn::second_argument_type, typename Fn::result_type> { protected: Fn op; typename Fn::first_argument_type value; public: binder1st(const Fn& x, const typename Fn::first_argument_type& y); typename Fn::result_type operator()(const typename Fn::second_argument_type& x) const; typename Fn::result_type operator()(typename Fn::second_argument_type& x) const; };

The constructor initializes op with x and value with y.

operator() returns op(value,x).