Re: [boost] variant - off topic

Subject: Re: [boost] variant - off topic
From: degski (degski_at_[hidden])
Date: 2019-04-08 07:45:48

On Mon, 8 Apr 2019 at 07:30, Gavin Lambert via Boost <boost_at_[hidden]>
wrote:

> Or did you mean to check is_nothrow_constructible rather than
> is_trivially_constructible?
>
> (Also, for robustness you should check assignment as well as construction.)
>

Just now I'm at it:

#include <type_traits>
#include <variant>

#include <boost/mp11.hpp>
#include <boost/variant.hpp>

namespace mp11 = boost::mp11;

struct Foo {
    Foo ( ) = delete;
};

template<typename... Args>
struct safe_std_variant : public std::variant<Args...> {
    static_assert (
std::conjunction<std::is_trivially_constructible<Args>...>::value, "is not
trivially constructible" );
};

template<typename... Args>
struct safe_boost_variant : public boost::variant<Args...> {
    static_assert (
mp11::mp_all<std::is_trivially_constructible<Args>...>::value, "is not
trivially constructible" );
};

template<typename... Args>
struct evensafer_std_variant : public std::variant<Args...> {
    static_assert (
std::conjunction<std::conjunction<std::is_nothrow_constructible<Args>...>,

std::conjunction<std::is_nothrow_copy_assignable<Args>...>>::value,
                    "is not trivially constructible or copy assignable" );
};

template<typename... Args>
struct evensafer_boost_variant : public boost::variant<Args...> {
    static_assert (
mp11::mp_all<mp11::mp_all<std::is_nothrow_constructible<Args>...>,

mp11::mp_all<std::is_nothrow_copy_assignable<Args>...>>::value,
                    "is not trivially constructible or copy assignable" );
};

int main ( ) {

    safe_std_variant<int, double, bool> sv1;        // compiles
    safe_boost_variant<int, double, bool> sv2;      // compiles
    evensafer_std_variant<int, double, bool> sv3;   // compiles
    evensafer_boost_variant<int, double, bool> sv4; // compiles
    // safe_std_variant<int, double, bool, Foo> sv5;        // does not
compile
    // safe_boost_variant<int, double, bool, Foo> sv6;      // does not
compile
    // evensafer_std_variant<int, double, bool, Foo> sv7;   // does not
compile
    // evensafer_boost_variant<int, double, bool, Foo> sv8; // does not
compile
}

degski
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