Problem with the copy constructor of an class holden in boost::variant

Question

I've tried to construct a tree structure using boost::variant and now I have some problems. I implemented a copy constructor in the class holden in boost::variant and call the copy constructor just like this code:

#include <iostream>
#include <boost/variant.hpp>


struct Hoge{
int i;
Hoge(int j) : i(j){};
Hoge(Hoge const& hoge) : i(hoge.i) {}
};

using var = boost::variant<int,Hoge>;

int main()
{
var hoge = Hoge(1);
var hogehoge = Hoge(hoge);
}

A compile failed because of no matching function for call to ‘Hoge::Hoge(var&)’.

How to fix this code? Should the copy constructor has an argument boost::variant? Or can I fix this code with an brilliant boost technique?

Answer 1

You're not copy constructing: that would be like saying:

var hogehoge = var(hoge); // elided assign of copy-ctor

Or indeed just

var hogehoge(hoge);
var hogehoge = hoge;

So, what you need is to SAY you want a Hoge to copy,

var hogehoge = boost::get<Hoge>(hoge); // may throw if `hoge` holds int

So you copy-construct Hoge, which you then convert-construct to the variant type.

See it Live On Coliru

#include <boost/variant.hpp>
#include <iostream>

struct Hoge {
    int i;
    Hoge(int j) : i(j){};
    Hoge(Hoge const& hoge) : i(hoge.i) {}
};

using var = boost::variant<int, Hoge>;

int main() {
    var hoge = Hoge(1);
    var hogehoge = boost::get<Hoge>(hoge);
}