// C++11 std::function test
// g++ -std=c++0x std_function.cpp
#include <iostream>
#include <functional>
#include <cmath>

using namespace std;

double integrate(function<double(double)> f,const double &a,const double &b){
  // rough Riemann sum integration
  const int N=1000000;
  double res = 0, x = a;
  double dx = (b-a)/(N-1);
  for (int i=0; i<N; ++i) {
    res += f(x);
    x   += dx;  
  }
  return dx*res;
}

double fun(const double &x){
  return sqrt(x);
}

int main()
{
  function<double(double)> g;
  g = fun;
  cout<<"sqrt          "<<integrate(g,0.0,1.0)<<endl;;
  cout<<"x*x           "<<integrate([](double x){return x*x;},0.0,1.0)<<endl;;
  cout<<"sqrt (lambda) "<<integrate([](double x){return sqrt(x);},0.0,1.0)<<endl;;
  return 0;
}