[cfe-dev] [libcxx] Which headers must include other headers?
Craig, Ben via cfe-dev
cfe-dev at lists.llvm.org
Thu Mar 10 06:02:42 PST 2016
On 3/9/2016 4:47 PM, Eric Fiselier wrote:
> On Wed, Mar 9, 2016 at 3:20 PM, Craig, Ben via cfe-dev
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> From the C++14 spec...
> 220.127.116.11 Headers [res.on.headers]
> 1 A C++ header may include other C++ headers. A C++ header
> shall provide the declarations and definitions
> that appear in its synopsis. A C++ header shown in its
> synopsis as including other C++ headers shall provide
> the declarations and definitions that appear in the synopses
> of those other headers.
> I'm not 100% sure I know what this means. If a header (say...
> <system_error>) mentions a class (like std::string), is it
> required to provide the full definition for std::string, or is the
> forward declared template good enough?
> According to the standard <system_header> does not need <string> even
> though the synopsis mentions it. A forward declaration is good enough.
> Although <system_header> mentions string, the synopsis for that header
> doesn't declare or define std::string so [res.on.headers] doesn't apply.
> Right now, libcxx uses the forward declaration. What about
> entities that have specializations scattered all over the place,
> like std::hash? Is the unspecialized forward declaration good
> enough, or does the full bulk of all the std::hash specializations
> have to come along too?
> I don't think all of the specialization need to come along. I'm not
> sure what the QoI implications are though. Could you provide some
> motivation as to why you asked this question?
My motivation is that I'm running a conformance test suite on my
platform (Hexagon + libcxx + libcxxabi + modified Dinkumware C
library). For the portion covering <system_error>, I get a lot of
failures because the test suite tries to use / instantiate std::string,
and it believes that it should be able to get the template definition
transitively through <system_error>.
I like the compile time benefits of a forward string declaration, I just
want to be sure it's conformant. If not, I'm fine reporting the test as
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