apply boost::fusion::for_each to boost::fusion::vector with mutable function object

Question

I am trying to use boost::fusion::vector. However, I am in trouble with the very simple problem.

#include <iostream>
#include <string>

#include <boost/fusion/container/vector.hpp>
#include <boost/fusion/algorithm.hpp>

using namespace std;

struct A{
    template <class T>
    void operator()(const T& t) {
        x++;
        cout << t << endl;
    }

    int x = 0;
};

int main(){
    A a;
    boost::fusion::vector<int, int, int> tuple{3,4,5};
    boost::fusion::for_each(tuple, a);
}

Note that the operator() of struct A modifies x in struct A. gcc 4.7.2 warns that ...\include\boost\fusion\algorithm\iteration\detail\for_each.hpp:77: error: passing 'const A' as 'this' argument of 'void A::operator()(const T&) [with T = int]' discards qualifiers [-fpermissive]

Is there a solution for this?

Answer 1

Well, I have not used boost::fusion, but from error message and example from docs, it seems that for_each expects const object. I.e. operator() should be const as well. However you will be unable to mutate object.

Try this:

void operator()(const T& t) const {
    cout << t << endl;
}

Edit:

I've checked the sources (v. 1.53) and the declaration is for_each(Sequence& seq, F const& f). So there is really no way to modify object itself. The only options I see are

either use static variables: static int x;

or use pointers:

struct A {
    template <class T>
    void operator()(const T& t) const {
        (*x)++;
        std::cout << t << std::endl;
    }

    int* x;
};

int main()
{
    A a;
    a.x = new int;
    *(a.x) = 0;
    //...

In this case be careful with coping A instances, as pointers will all point to the same location.