Providing leaf::result with forwarding constructor and emplace method

[fedraco-gh]

Would it be possible to expose a leaf::result constructor to directly initialize its value from a parameter pack as it is provided by constructor 9 of std::expected?

I think it would be useful when dealing with return types that are both expensive to copy and move. For the same reason an emplace method might come in handy.

[zajo]

You do this by invoking new_error, which does take a parameter pack:

result<int> f()
{
    if( .... )
        return leaf::new_error(e1, e2, ...);
}

Note that in LEAF result<> types never hold any error objects, so moving an object of a result<> type is efficient even when e1, e2, ... are extremely expensive to move.

[fedraco-gh]

Thank you for your reply. I was referring to the object that is returned if no error arises.

Please refer to this example on Compiler Explorer: https://godbolt.org/z/7nefnxnrv.

With throwing_foo there are no copies being made thanks to prvalue materialization rules. With noexcept_foo, which returns a leaf::result, one copy takes place.

If it was possible to initialize leaf::result directly with the T constructor parameters the copy would not happen.