Compiling C++ Templates as opposed to Preprocessing them.
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From: Felipe Magno de Almeida on 4 Aug 2010 04:18 On Aug 4, 1:30 am, Walter Bright <newshou...(a)digitalmars.com> wrote: > Nikolay Ivchenkov wrote: > > A declaration cannot consist of arbitrary sequence of tokens (see C+ > > +03 - [dcl.dcl]). There are no special exceptions for templates in > > this respect. Thus, the syntax of any template must be completely > > checked regardless of whether the template is instantiated. > > This is the template definition part which is stored as a sequence of tokens > until instantiation time, not the declaration. The declarations are always > parsed and checked, even if never used. > > My reading of C++98 was that the compiler was not required to issue a diagnostic > for a malformed definition if it was never instantiated. Unless I'm missing something, I think it is required to diagnose. After all, if you implement Two Phase Lookup then you must make the first lookup when you first parse the template. > -- Regards, -- Felipe Magno de Almeida [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Nikolay Ivchenkov on 4 Aug 2010 04:18 On 4 Aug, 08:30, Walter Bright <newshou...(a)digitalmars.com> wrote: > Nikolay Ivchenkov wrote: > > A declaration cannot consist of arbitrary sequence of tokens (see C+ > > +03 - [dcl.dcl]). There are no special exceptions for templates in > > this respect. Thus, the syntax of any template must be completely > > checked regardless of whether the template is instantiated. > > This is the template definition part which is stored as a sequence of tokens > until instantiation time, not the declaration. Every template definition is a template declaration. For example, in the following template definition template <class T> void f() { T::g(); } every its token is a part of the template-declaration. According to the syntactic rules, the following sequence of tokens template <class T> void f() { T + 1; } cannot be considered as a template-declaration at all. > My reading of C++98 was that the compiler was not required to issue a diagnostic > for a malformed definition if it was never instantiated. What's the malformed definition? (I want to see normative criteria if they exist). For example, can we consider the following sequence of tokens %+*!^~ as a malformed template definition (according to your interpretation)? -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Walter Bright on 4 Aug 2010 11:15 Nikolay Ivchenkov wrote: > On 4 Aug, 08:30, Walter Bright <newshou...(a)digitalmars.com> wrote: >> My reading of C++98 was that the compiler was not required to issue a diagnostic >> for a malformed definition if it was never instantiated. > > What's the malformed definition? One that doesn't follow the grammar. > (I want to see normative criteria if > they exist). For example, can we consider the following sequence of > tokens > > %+*!^~ > > as a malformed template definition (according to your interpretation)? Yes. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Walter Bright on 4 Aug 2010 11:15 Felipe Magno de Almeida wrote: > On Aug 4, 1:30 am, Walter Bright <newshou...(a)digitalmars.com> wrote: >> My reading of C++98 was that the compiler was not required to issue a diagnostic >> for a malformed definition if it was never instantiated. > > Unless I'm missing something, I think it is required to diagnose. At this point, I'll require a specific quote from C++98 <g>. > After all, if you implement Two Phase Lookup then you must make the > first lookup > when you first parse the template. The Digital Mars C++ does implement two phase lookup correctly, so preparsing the template definitions is not required in order to get that right. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: CornedBee on 4 Aug 2010 18:13
On Aug 4, 7:15 pm, Walter Bright <newshou...(a)digitalmars.com> wrote: > Felipe Magno de Almeida wrote: > >> On Aug 4, 1:30 am, Walter Bright <newshou...(a)digitalmars.com> wrote: >>> My reading of C++98 was that the compiler was not required to issue a diagnostic >>> for a malformed definition if it was never instantiated. > >> Unless I'm missing something, I think it is required to diagnose. > > At this point, I'll require a specific quote from C++98 <g>. [temp.res]p8 gives an explicit example of completely nonsensical code (not even syntactically correct), with the note that it *may* be diagnosed even if the template is never instantiated. In other words, as long as a template is not instantiated, you can have whatever you want in there, as long as the parser is still able to find the end of the template. Getting diagnostics early is simply a QoI issue. Clang aims at diagnosing things as early as possible and as thoroughly as possible (there are many things in the area that are ill-formed, no diagnostic required; we want to diagnose them), so blindly caching tokens is simply not an option. We would have to cache them and parse them at definition time, and then parse them again at instantiation time. (For every instantiation, of course.) Now *that* is a waste of effort. Sebastian -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |