source: trunk/third/gcc/cp/templates.texi @ 8834

@node Templates
@chapter The Template Implementation

@cindex templates
@cindex function templates
@cindex class templates
@cindex parameterized types
@cindex types, parameterized
The C++ template@footnote{Class templates are also known as
@dfn{parameterized types}.} facility, which effectively allows use of
variables for types in declarations, is one of the newest features of
the language.

@sc{gnu} C++ is one of the first compilers to implement many
of the template facilities currently defined by the @sc{ansi} committee.

Nevertheless, the template implementation is not yet complete.  This
chapter maps the current limitations of the @sc{gnu} C++ template
implementation.

@menu
* Template limitations:: Limitations for function and class templates
* Function templates::   Limitations for function templates
* Class templates::      Limitations for class templates
* Template debugging::   Debugging information for templates
@end menu

@node Template limitations
@section Limitations for function and class templates

@cindex template limitations
@cindex template bugs
@cindex bugs, templates
These limitations apply to any use of templates (function templates or
class templates) with @sc{gnu} C++:

@table @emph
@item Template definitions must be visible
When you compile code with templates, the template definitions must come
first (before the compiler needs to expand them), and template
definitions you use must be visible in the current scope.
@c FIXME! Is this a defined property of templates, rather than a
@c temporary limitation?
@c ANSWER: It's a limitation, but it's hard to say why it's a limitation
@c to someone.  We need an infinite link-cycle, in one camp, to
@c accomplish things so you don't need the template definitions around.

@cindex static data in template classes
@cindex template classes, static data in
@item Individual initializers needed for static data
Templates for static data in template classes do not work.  @xref{Class
templates,,Limitations for class templates}.
@end table

@node Function templates
@section Limitations for function templates

@cindex function template limitations
Function templates are implemented for the most part.  The compiler can
correctly determine template parameter values, and will delay
instantiation of a function that uses templates until the requisite type
information is available.

@noindent
The following limitations remain: 

@itemize @bullet
@cindex template vs declaration, functions
@cindex declaration vs template, functions
@cindex function declaration vs template
@item
Narrowed specification: function declarations should not prevent
template expansion.  When you declare a function, @sc{gnu} C++
interprets the declaration as an indication that you will provide a
definition for that function.  Therefore, @sc{gnu} C++ does not use a
template expansion if there is also an applicable declaration.  @sc{gnu}
C++ only expands the template when there is no such declaration.

The specification in Bjarne Stroustrup's @cite{The C++ Programming
Language, Second Edition} is narrower, and the @sc{gnu} C++
implementation is now clearly incorrect.  With this new specification, a
declaration that corresponds to an instantiation of a function template
only affects whether conversions are needed to use that version of the
function.  It should no longer prevent expansion of the template
definition.

For example, this code fragment must be treated differently:

@smallexample
template <class X> X min (X& x1, X& x2) @{ @dots{} @}
int min (int, int);
@dots{}
int i; short s;
min (i, s); // @r{should call} min(int,int)
            // @r{derived from template}
@dots{}
@end smallexample

@item
The compiler does not yet understand function signatures where types are
nested within template parameters.  For example, a function like the
following produces a syntax error on the closing @samp{)} of the
definition of the function @code{f}:

@smallexample
template <class T> class A @{ public: T x; class Y @{@}; @};
template <class X> int f (A<X>::Y y) @{ @dots{} @}
@end smallexample

@cindex @code{inline} and function templates
@cindex function templates and @code{inline}
@item
If you declare an @code{inline} function using templates, the compiler
can only inline the code @emph{after} the first time you use
that function with whatever particular type signature the template
was instantiated.

Removing this limitation is akin to supporting nested function
definitions in @sc{gnu} C++; the limitation will probably remain until the
more general problem of nested functions is solved.

@item
All the @emph{method} templates (templates for member functions) for a
class must be visible to the compiler when the class template is
instantiated. 
@end itemize

@node Class templates
@section Limitations for class templates

@cindex class template limitations
@ignore
FIXME!!  Include a comprehensible version of this if someone can explain it.
         (Queried Brendan and Raeburn w/full orig context, 26may1993---pesch)
   - [RHP: I don't understand what the following fragment refers to.  If it's
     the "BIG BUG" section in the original, why does it say "overriding class
     declarations" here when the more detailed text refers to *function*
     declarations?  Here's the fragment I don't understand:] 
     there are problems with user-supplied overriding class declarations (see
     below). 
@end ignore

@itemize @bullet
@ignore
@cindex static data, not working in templates
@item
Templates for static data in template classes do not work.
Currently, you must initialize each case of such data
individually. 
@c FIXME!! Brendan to see if still true.
@c ANSWER: This section presumes that it's incorrect to have to
@c initialize for each type you instantiate with.  It's not, it's the
@c right way to do it.
@end ignore

Unfortunately, individual initializations of this sort are likely to be
considered errors eventually; since they're needed now, you might want to
flag places where you use them with comments to mark the need for a
future transition.

@cindex nested type results vs templates
@item
Member functions in template classes may not have results of nested
type; @sc{gnu} C++ signals a syntax error on the attempt.  The following
example illustrates this problem with an @code{enum} type @code{alph}:

@smallexample
template <class T> class list @{
  @dots{}
  enum alph @{a,b,c@};
  alph bar();
  @dots{}
@};

template <class T>
list<int>::alph list<int>::bar()  // @i{Syntax error here}
@{
@dots{}
@}
@end smallexample

@cindex preprocessor conditionals in templates
@cindex conditionals (preprocessor) in templates
@item
A parsing bug makes it difficult to use preprocessor conditionals within
templates.  For example, in this code:

@smallexample
template <class T>
class list @{
  @dots{}
#ifdef SYSWRONG
  T x;
#endif
  @dots{}
@}
@end smallexample

The preprocessor output leaves sourcefile line number information (lines
like @samp{# 6 "foo.cc"} when it expands the @code{#ifdef} block.  These
lines confuse the compiler while parsing templates, giving a syntax
error.

If you cannot avoid preprocessor conditionals in templates, you can
suppress the line number information using the @samp{-P} preprocessor
option (but this will make debugging more difficult), by compiling the
affected modules like this:

@smallexample
g++ -P foo.cc -o foo
@end smallexample

@cindex parsing errors, templates
@item
Parsing errors are reported when templates are first
@emph{instantiated}---not on the template definition itself.  In
particular, if you do not instantiate a template definition at all, the
compiler never reports any parsing errors that may be in the template
definition.
@end itemize

@node Template debugging
@section Debugging information for templates

@cindex templates and debugging information
@cindex debugging information and templates
Debugging information for templates works for some object code formats,
but not others.  It works for stabs@footnote{Except that insufficient
debugging information for methods of template classes is generated in
stabs.} (used primarily in @sc{a.out} object code, but also in the Solaris 2
version of @sc{elf}), and the @sc{mips} version of @sc{coff} debugging
format.

@sc{dwarf} support is currently minimal, and requires further
development.