[variant] assign and access the assigned value simultaneously - Boost - lists.stage.boost.cppalliance.org

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[variant] assign and access the assigned value simultaneously

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feverzsj

14 Mar 2014 14 Mar '14

6:22 a.m.

hi list, for common case: boost::variant var; var = val; Val& v = boost::get<Val>(var); why not supply a interface like: Val& v = var.assign(val); for simplity, and the unnecessary "get" is also elimited. regards -- View this message in context: http://boost.2283326.n4.nabble.com/variant-assign-and-access-the-assigned-va... Sent from the Boost - Dev mailing list archive at Nabble.com.

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Gavin Lambert

19 Mar 19 Mar

2:32 a.m.

On 14/03/2014 19:22, Quoth feverzsj:

for common case:

boost::variant var; var = val; Val& v = boost::get<Val>(var);

Why is that a common case? I would think that's an anti-pattern, personally. (But then I haven't played with variants much, so perhaps I'm missing something.)

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Antony Polukhin

7:13 a.m.

2014-03-14 10:22 GMT+04:00 feverzsj : <...>

why not supply a interface like:

Val& v = var.assign(val);

for simplity, and the unnecessary "get" is also elimited.

Such interface is not common for C++. Most programmers are used to the following: class_name& class_name::operator=(const class_name&); or class_name& class_name::operator=(const class_internal&); While you propose to implement `class_internal& class_name::operator=(const class_internal&);` This may be surprising for some users: boost::variant var; boost::variant& var2 = var = val; Code from the example above will have different behavior on different compilers (GCC won't compile it, MSVC will compile it but &var2 != &var1). This does not look good. -- Best regards, Antony Polukhin

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TONGARI J

7:26 a.m.

2014-03-19 15:13 GMT+08:00 Antony Polukhin :

2014-03-14 10:22 GMT+04:00 feverzsj : <...>

...
why not supply a interface like:

Val& v = var.assign(val);

for simplity, and the unnecessary "get" is also elimited.

Such interface is not common for C++. Most programmers are used to the following:

class_name& class_name::operator=(const class_name&); or class_name& class_name::operator=(const class_internal&);

While you propose to implement `class_internal& class_name::operator=(const class_internal&);`

I don't think he's proposing that for operator=, but a not-yet-exist assign method. Seems reasonable to me, sometimes we do want to do some immediate operations after the assignment.

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Steven Watanabe

3:16 p.m.

AMDG On 03/19/2014 12:26 AM, TONGARI J wrote:

2014-03-19 15:13 GMT+08:00 Antony Polukhin :

...
2014-03-14 10:22 GMT+04:00 feverzsj : <...>

...
why not supply a interface like:

Val& v = var.assign(val);

for simplity, and the unnecessary "get" is also elimited.

<snip>

<snip> Seems reasonable to me, sometimes we do want to do some immediate operations after the assignment.

I don't think it's worth cluttering the interface. It's one more function that has to be documented and tested. I don't see this as being a common use and there's nothing stopping anyone who wants such a function from implementing it himself as a non-member. In Christ, Steven Watanabe

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feverzsj

28 Mar 28 Mar

5:21 p.m.

hi list, Let me explain this in more detail. Say, if one want to renew an object of some type which may be hold by a variant, and not care about the object's old data, he may do somthing like: // assure variant hold object of type T if(var.which() != T's index) var = T(); // 2 visitation + copy T& t = boost::get<T>(var); // 1 visitation // process t If we have T& t = var.assign(T()), 1 visitation can be eliminated. If we have T& t = var.construct<T>(...), only 1 visitation and optionally 1 in place construction are needed. regars. -- View this message in context: http://boost.2283326.n4.nabble.com/variant-assign-and-access-the-assigned-va... Sent from the Boost - Dev mailing list archive at Nabble.com.

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participants (5)

Antony Polukhin
feverzsj
Gavin Lambert
Steven Watanabe
TONGARI J