Parenthesis after variable name C++

Question

Working with the below source code (it is open source) and I've never seen parenthesis after a variable name. UDefEnergyH is definitely a variable as can be seen in line 1. Can anyone tell me what these parenthesis are doing? Don't really know how to Google this. Thanks.

bins[0] = UDefEnergyH.GetLowEdgeEnergy(size_t(0));
vals[0] = UDefEnergyH(size_t(0)); //Don't know what this does???
sum = vals[0];
for (ii = 1; ii < maxbin; ii++) {
    bins[ii] = UDefEnergyH.GetLowEdgeEnergy(size_t(ii));
    vals[ii] = UDefEnergyH(size_t(ii)) + vals[ii - 1];
    sum = sum + UDefEnergyH(size_t(ii));
}

And it is declared here in the header file:

G4PhysicsOrderedFreeVector UDefEnergyH;

Answer 1

It appears operator() is overloaded for the tyupe of UDefEnerfyH.

One way to do this is this solution

#include <iostream>
using namespace std;

struct MJ {
    void GetLowEdgeEnergy(size_t arg) {
        cout << "GetLowEdgeEnergy, arg = " << arg << endl;
    }
    void operator ()(size_t arg) {
        cout << "operator (), arg = " << arg << endl;
    }
};

int main() {
    MJ UDefEnergyH;
    UDefEnergyH.GetLowEdgeEnergy(42);
    UDefEnergyH(42);
    return 0;
}

Answer 2

It seems you are referring to the field in the class G4SPSEneDistribution. Its type is G4PhysicsOrderedFreeVector. And have a look at its members here. As you can see there is operator() overloaded and apparently this is what is called on the second line. It is not very easy to find out what that does, but if you have a look at the comment in the header file for G4PhysicsVector, you will see:

00100          // Returns simply the value in the bin specified by 'binNumber'
00101          // of the dataVector. The boundary check will not be Done. If 
00102          // you want this check, use the operator [].