Re: [boost] Would a construct_cast<T>(u..) be useful?

Subject: Re: [boost] Would a construct_cast<T>(u..) be useful?
From: Vicente BOTET (vicente.botet_at_[hidden])
Date: 2011-04-17 14:06:04

> Message du 17/04/11 18:36
> De : "Johannes Schaub (litb)" 
> A : boost_at_[hidden]
> Copie à : 
> Objet : [boost] Would a construct_cast(u..) be useful?
> 
> I was wondering whether it makes sense to have another kind of cast in 
> boost, which behaves like the following:
> 
> template
> T construct_cast(U &&... u) {
> T t(std::forward(u)...);
> return t;
> }
> 
> This cast works if std::is_constructible::value is true, and fails 
> if it is false. 
> 
> It presents a slightly more explicit form of boost::implicit_cast in that 
> explicit constructors of T are allowed, and for the case of a single-
> elemented parameter pack, explicit conversion functions of U converting to 
> "T" are allowed. It does not allow non-implicit conversions like void* -> 
> nonvoid*, Base* -> Derived*. 
> 
> Advantages over not having construct_cast:
> 
> - Converting expressions to "bool" by use of "contextually converted to 
> bool", without using things like "!!x" or "x ? true : false" (the first may 
> trigger user defined opeators for "operator!", which may not be desired). 
> - No need for introducing a temporary named variable like the above "t". 
> - If all you want is to consider contexual conversions, explicit 
> constructors and conversion functions, this can be used. All these 
> conversion seem "safe", compared to the set of conversions allowed by T(u) 
> and even static_cast(u). 
> 
> What do you think about it?

Hi,

happy to see that someone else has this need. The library Boost.Conversion [1] on the review schedule defines a convert_to function that behaves like your construct_cast. In addition the library defines a framework that allows the user to convert types that are unrelated. 

I will really appreciate if you can take a look and tell me if you find there what you are locking for or if you want something else.

Best,
Vicente

Boost.Conversion ([1]https://svn.boost.org/svn/boost/sandbox/conversion/libs/conversion_ext/doc/html/index.html).