composetry.cpp

C语言数值算法程序大全（第二版）,课本文件

//: C06:ComposeTry.cpp
// From "Thinking in C++, 2nd Edition, Volume 2"
// by Bruce Eckel & Chuck Allison, (c) 2003 MindView, Inc.
// Available at www.BruceEckel.com.
// A first attempt at implementing function composition
#include <cassert>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <string>
using namespace std;

template<typename R, typename E, typename F1, typename F2>
class unary_composer {
   F1 f1;
   F2 f2;
public:
   unary_composer(F1 fone, F2 ftwo) : f1(fone), f2(ftwo) {}
   R operator()(E x) {
      return f1(f2(x));
   }
};
template<typename R, typename E, typename F1, typename F2>
unary_composer<R, E, F1, F2> compose(F1 f1, F2 f2) {
   return unary_composer<R, E, F1, F2>(f1, f2);
}
int main()
{
  double x =
    compose<double, const string&>(atof, 
      mem_fun_ref(&string::c_str))("12.34");
  assert(x == 12.34);
} ///:~