How to forward constructor arguments to boost::optional

Question

I have the following simple struct:

struct X
{
    X(std::string name, int value): name_(name), value_(value){}

    std::string name_;
    int value_;
};

I would like to use it with boost optional without copying. Here is one option:

boost::optional<X> op;
op.emplace("abc", 5);

Is it possible without using emplace function ? (I mean one line expression)

score 5 · Accepted Answer · answered Apr 01 '19 at 17:05

5

Yeah, just construct it with the "in place" tag which forwards ctor args!

boost::optional<X> op(boost::optional::in_place_init, "abc", 5);

FWIW if you don't mind a move then this works too:

boost::optional<X> op(X("abc", 5));

Scan down that reference page; there are loads of ways to construct or fill a boost::optional.

answered Apr 01 '19 at 17:05

1

Your nose is a reference page – Hatted Rooster Apr 01 '19 at 17:09
boost::optional op(X("abc", 5)); uses a copy constructor – Irbis Apr 01 '19 at 18:13
1

@Irbis Why's that? There is a `optional::optional( T&& v )` ctor with documented move semantics (see linked ref), and the argument here is a temporary (and consequently an rvalue). – Lightness Races in Orbit Apr 02 '19 at 10:01
You're right. I have defined a copy ctor to see if it is called and move constructor has not been generated. When I define copy and move ctors I see that move ctor is called. – Irbis Apr 02 '19 at 19:29

1 Answers1