double() function in c++ or not? -

i hava following c++ code

#include <cstdlib> #include <iostream> #include <fstream> #include <time.h> #include <math.h>  using namespace std;  #define pi 3.1415926  int n; double* a,*b; double function(double);  double a(int j) {     double s=0;     int ii;     (int i=-n;i<n+1;i++)     {         s=s+function(2*pi*double(i)/(2*n+1))*cos(2*pi*double(j)*double(i)/(2*n+1));     }     if (j==0)         return 1/double(2*n+1)*s;     return 2/double(2*n+1)*s; } 

and there isn't declared function double(), , looked function in math.h library , haven't found such. can explain it, lambda expression or delegate ?

double(...) thought of way of saying (double)(...). recall when have class constructor takes parameters can write

myclass(params) 

and produce instance of myclass. c++ standard expanded same syntax primitive types well. syntax shorter cast, because not require parentheses around double ^†.

note: in program above explicit double(i) unnecessary, because pi double, , multiplying int double produces double.

^† may not work primitive types names take multiple parts. example, unsigned int(12.4) will not compile under gcc.