Passing in a function which increments elements of boost::fusion:vector - c++

Question

I found the following code which I am using as a basis. It describes how to populate a boost::fusion::vector with the values 6:
#include <boost/fusion/algorithm.hpp>
#include <boost/fusion/container.hpp>
struct F {
F(int blah): blah(blah){}
template <typename T>
void operator()(T& t) const {
t = blah;
}
int blah;
};
template <typename T>
void apply_for_each_to_assign(T &t)
{
boost::fusion::for_each(t, F(6));
}
int main() {
boost::fusion::vector<int, double, int> idi;
apply_for_each_to_assign(idi);
}
My question is this, instead of populating every element with the value 6- what would be the neatest way to have every element incrementing by one? So
v[0] = 1
v[1] = 2
etc? I presume I would need to write an increment function but I am not sure how I need to incorporate it in to the above code?

Answer 1

You can use fold.
fold takes a function A f(A, B) where B is your element type, and a intial A value and call it on all the elements. It is rougly equivalent to f(f(f(initial, first_elem), second_elem), third_elem).
You should declare the functor like this:
struct accumulator
{
typedef int result_type;
template<typename T>
int operator()(int value, T& t) const
{
t += value;
return value + 1;
}
};
And here's the usage:
template <typename T>
void apply_for_each_to_assign(T &t)
{
boost::fusion::fold(t, 1, accumulator());
}
Here a whole example with printing:
#include <boost/fusion/algorithm/iteration/fold.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/include/fold.hpp>
#include <boost/fusion/container.hpp>
#include <iostream>
struct accumulator
{
typedef int result_type;
template<typename T>
int operator()(int value, T& t) const
{
t += value;
return value + 1;
}
};
struct Print {
template <typename T>
void operator()(T& t) const {
std::cout << t << std::endl;
}
};
template <typename T>
void apply_for_each_to_assign(T &t)
{
boost::fusion::fold(t, 1, accumulator());
boost::fusion::for_each(t, Print());
}
int main()
{
boost::fusion::vector<int, double, int> idi;
apply_for_each_to_assign(idi);
}