Re: [Boost-users] [variant] how to in-place construct a variant

Subject: Re: [Boost-users] [variant] how to in-place construct a variant
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2009-09-07 18:46:57

Stefan Strasser wrote:
> I can't figure out how to in-place construct the contents of a boost::variant. 

Why do you think that is supported? I see no such thing in the 
documentation.

> so why does the following fail:
> 
> struct A : noncopyable{
> 	A(int);
> 	A &operator=(int);
> };
> struct B{};
> 
> 
> int main(){
> 	A a(5); //ok
> 	a=5; //ok
> 	variant<A,B> var(5); //error
> 	var=5; //error
> }

Because variant requires all types it can contain to be 
CopyConstructible, in particular because it has no support for in-place 
construction or move semantics.