Boost logo

Boost :

Subject: Re: [boost] [config] request for BOOST_NO_FWD_STD_DECLARATION
From: Christopher Jefferson (chris_at_[hidden])
Date: 2011-02-22 08:41:05

On 22 Feb 2011, at 13:28, Felipe Magno de Almeida wrote:

> On Wed, Feb 16, 2011 at 12:49 PM, Mathias Gaunard
> <mathias.gaunard_at_[hidden]> wrote:
>> On 16/02/2011 13:52, Christopher Jefferson wrote:
> [snip]
>>> The question is then if forward declarations of types in std:: are
>>> allowed. From in the current C++0x draft (there is identical
>>> text in the c++03 standard):
>>> "The behavior of a C++ program is undefined if it adds declarations or
>>> definitions to namespace std or to a namespace within namespace std unless
>>> otherwise specified."
>>> While people could read this in various ways, I would say it forbids these
>>> kinds of games, although they might work for particular standard libraries.
>> The standard also gives the reference of what the declarations of the
>> classes of the standard library must be, and all those declarations are in
>> namespace std.
> Isn't the standard library allowed to add template parameters with defaults?
> template <typename T, typename Allocator, typename _InternalParameter = int>
> struct vector;


The fine details of the standard aren't really important here anyway. In practice there are compilers where we have been able to write well-defined forward declarations for many years and people have found them useful. On other standard libraries, like libc++, forward declarations are hard to write and not advised.

Therefore we should support forward declarations where we can, and just include the standard headers where we can't.


Boost list run by bdawes at, gregod at, cpdaniel at, john at