diff options
Diffstat (limited to 'gcc/doc/extend.texi')
| -rw-r--r-- | gcc/doc/extend.texi | 31 |
1 files changed, 30 insertions, 1 deletions
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 2132b1e5602..599dee3c3bf 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -153,7 +153,7 @@ the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable. If you don't know the type of the operand, you can still do this, but you -must use @code{typeof} (@pxref{Typeof}). +must use @code{typeof} or @code{__auto_type} (@pxref{Typeof}). In G++, the result value of a statement expression undergoes array and function pointer decay, and is returned by value to the enclosing @@ -755,6 +755,35 @@ Thus, @code{array (pointer (char), 4)} is the type of arrays of 4 pointers to @code{char}. @end itemize +In GNU C, but not GNU C++, you may also declare the type of a variable +as @code{__auto_type}. In that case, the declaration must declare +only one variable, whose declarator must just be an identifier, the +declaration must be initialized, and the type of the variable is +determined by the initializer; the name of the variable is not in +scope until after the initializer. (In C++, you should use C++11 +@code{auto} for this purpose.) Using @code{__auto_type}, the +``maximum'' macro above could be written as: + +@smallexample +#define max(a,b) \ + (@{ __auto_type _a = (a); \ + __auto_type _b = (b); \ + _a > _b ? _a : _b; @}) +@end smallexample + +Using @code{__auto_type} instead of @code{typeof} has two advantages: + +@itemize @bullet +@item Each argument to the macro appears only once in the expansion of +the macro. This prevents the size of the macro expansion growing +exponentially when calls to such macros are nested inside arguments of +such macros. + +@item If the argument to the macro has variably modified type, it is +evaluated only once when using @code{__auto_type}, but twice if +@code{typeof} is used. +@end itemize + @emph{Compatibility Note:} In addition to @code{typeof}, GCC 2 supported a more limited extension that permitted one to write |