Update of /cvsroot/boost/boost/libs/xpressive/proto/doc
In directory sc8-pr-cvs3.sourceforge.net:/tmp/cvs-serv26581
Modified Files:
extensibility.qbk
Log Message:
fix lazy_vector example
Index: extensibility.qbk
===================================================================
RCS file: /cvsroot/boost/boost/libs/xpressive/proto/doc/extensibility.qbk,v
retrieving revision 1.5
retrieving revision 1.6
diff -u -d -r1.5 -r1.6
--- extensibility.qbk 15 Apr 2007 06:46:57 -0000 1.5
+++ extensibility.qbk 17 Apr 2007 23:19:41 -0000 1.6
@@ -221,7 +221,6 @@
{
typedef typename proto::result_of::arg<Expr>::type::value_type
result_type;
- template<typename Expr>
result_type operator()( Expr const & expr, lazy_subscript_context
& ctx ) const
{
return proto::arg( expr )[ ctx.subscript_ ];
@@ -260,7 +259,7 @@
typedef typename proto::terminal< std::vector<T> >::type expr_type;
lazy_vector( std::size_t size = 0, T const & value = T() )
- : lazy_vector_expr( expr_type::make( std::vector<T>( size, value ) )
)
+ : lazy_vector_expr<expr_type>( expr_type::make( std::vector<T>(
size, value ) ) )
{}
template< typename Expr >
@@ -305,19 +304,26 @@
double d1 = ( v2 + v3 )[ 2 ]; // Look ma, no temporaries!
// Subtract two vectors and add the result to a third vector.
- v1 += v2 - v3; // Still no temporaries!
+ v1 += v2 - v3; // Hmm, trouble here.
- // But what does this do?
+ // What does this do?
(v2 + v3) += v1;
-The last line is clearly a bug. It is nonsensical to add two vectors and then
-assign to the result. But strangely, this code compiles! And even more
-strangely, it had no effect! It is building an expression template and then
+The line `v1 += v2 - v3` is somewhat ambiguous. Clearly we want it to use the
+`lazy_vector<>::operator+=` we defined above, but it could also mean to
+construct an even larger expression template using proto's `operator+=`. At
+least one compiler actually believes the call to be ambiguous! We have to tell
+the compiler which.
+
+And the last line is clearly a bug. It is nonsensical to add two vectors and
+then assign /to/ the result. But strangely, this line of code compiles! And
even
+more strangely, it has no effect! It is building an expression template and
then
discarding it. We would like this to be a compile error, instead. We can make
-it an error by disabling Proto's `operator+=` overload, and all the other
-overloaded operators that don't make sense in our domain. To do that, we define
-the grammar for our domain. Let's say we want to allow addition and subtraction
-of our vector terminals. Our grammar looks like this:
+it an error, and solve the ambiguity problem, by disabling Proto's `operator+=`
+overload, and all the other overloaded operators that don't make sense in our
+domain. To do that, we define the grammar for our domain. Let's say we want to
+allow addition and subtraction of our vector terminals. Our grammar looks like
+this:
using proto::_;
@@ -341,7 +347,7 @@
: proto::domain< LazyVectorGrammar >
{};
-And that's it! Now, all operators that do not produce valid lazy-vector
+And that's it! Now, all operators that do not produce valid lazy vector
expressions are automatically disabled.
[endsect]
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