Odd use of std::accumulate in a template

Question

I saw this code in an old version of C++ Cookbook, and it mystifies me. This seems to compile, but I have no clue as to why the code would be written this way.

What does the T() mean? This is the init argument for std::accumulate -- the value to start the sum. I wrote a version where I put double() in place of T() (to "hardwire" the template), and it compiles. What does double() mean?

#include <iostream>
#include <numeric> // accumulate
#include <vector>

template<class T, class Iter_T>
void computeAccum(Iter_T first, Iter_T last, T& accum)
{
    accum = std::accumulate(first, last, T()); // Why is init argument equal to T()?
}

int main()
{
    std::vector<double> vd;
    vd.push_back(1.11);
    vd.push_back(2.22);
    vd.push_back(4.44);
    vd.push_back(3.33);
    double sum1;
    std::cout << "Here is the vector:" << std::endl;
    std::vector<double> ::iterator it;
    for (it = vd.begin(); it != vd.end(); ++it) {
        std::cout << *it << std::endl;
    }

    computeAccum2(vd.begin(), vd.end(), sum1);
    std::cout << "Answer: " << sum1 << std::endl;
}

Answer 1

It's basically zero initialization. Consider this example for clarification

#include <bits/stdc++.h> 
using namespace std;

int main(){
    cout<<"Int: "<<int()<<endl;
    cout<<"Float: "<<float()<<endl;
    cout<<"String: "<<string();
}

Output:

Int: 0
Float: 0
String: