Re: [boost] variant - off topic

Subject: Re: [boost] variant - off topic
From: Robert Ramey (ramey_at_[hidden])
Date: 2019-04-08 06:29:58

On 4/7/19 10:59 PM, degski via Boost wrote:
> On Sun, 7 Apr 2019 at 18:44, Robert Ramey via Boost <boost_at_[hidden]>
> wrote:
> 
>> template<T ...>
>> struct safe_variant : public std::variant<T...> {
>>       // for each T static assert that is_trivially_constructible<T> == true
>> };
>>
>> Which gets trickier.  But would be cool if someone wanted to make this.
>>
> 
> 
> #include <type_traits>
> #include <variant>
> 
> struct Foo {
>      Foo ( ) = delete;
> };
> 
> template<typename... Args>
> struct safe_variant : public std::variant<Args...> {
>      static_assert (
> std::conjunction<std::is_trivially_constructible<Args>...>::value, "is not
> trivially constructible" );
> };
> 
> int main ( ) {
> 
>      safe_variant<int, double, bool> sv1; // compiles
>      // safe_variant<int, double, bool, Foo> sv2; // does not compile
> }
> 
> Unless I mis-understand what you mean, this does the trick.
> 
> degski
> 

LOL - Looks good to me!  So now we have std::variant that is guaranteed 
to have a valid state with no double buffer BS and no extra overhead.

I'll be curious to hear what otherothers have to say about this.

Robert Ramey