Update of /cvsroot/boost/boost/libs/xpressive/proto/example
In directory sc8-pr-cvs3.sourceforge.net:/tmp/cvs-serv15367
Modified Files:
vec3.cpp
Log Message:
demonstrate new callable_context
Index: vec3.cpp
===================================================================
RCS file: /cvsroot/boost/boost/libs/xpressive/proto/example/vec3.cpp,v
retrieving revision 1.4
retrieving revision 1.5
diff -u -d -r1.4 -r1.5
--- vec3.cpp 26 Jun 2007 06:16:06 -0000 1.4
+++ vec3.cpp 1 Jul 2007 07:53:14 -0000 1.5
@@ -9,6 +9,7 @@
// evaluating expressions. It is a port of the Vec3 example
// from PETE (http://www.codesourcery.com/pooma/download.html).
+#include <cassert>
#include <iostream>
#include <functional>
#include <boost/mpl/int.hpp>
@@ -43,10 +44,30 @@
int i_;
};
-// This transform counts the number of int[3] terminals in an expression.
-// It accumulates a runtime value, rather than computing a compile-time
-// value, just to demonstrate the use of the std::plus<> function object
-// with the function2 transform.
+// Here is an evaluation context that counts the number
+// of Vec3 terminals in an expression.
+struct CountLeavesCtx
+ : callable_context< CountLeavesCtx, null_context >
+{
+ CountLeavesCtx()
+ : count(0)
+ {}
+
+ typedef void result_type;
+
+ void operator()(tag::terminal, int const(&)[3])
+ {
+ ++this->count;
+ }
+
+ int count;
+};
+
+// Here is a transform that does the same thing as the above context.
+// It demonstrates the use of the std::plus<> function object
+// with the function2 transform. With minor modifications, this
+// transform could be used to calculate the leaf count at compile
+// time, rather at runtime.
struct CountLeaves
: or_<
// match a Vec3 terminal, return 1
@@ -109,8 +130,16 @@
template<typename Expr>
int count_leaves(Expr const &expr)
{
+ // Count the number of Vec3 terminals using the
+ // CountLeavesCtx evaluation context.
+ CountLeavesCtx ctx;
+ eval(expr, ctx);
+
+ // This is another way to count the leaves using a transform.
int i = 0;
- return CountLeaves::call(expr, i, i);
+ assert( CountLeaves::call(expr, i, i) == ctx.count );
+
+ return ctx.count;
}
int main()
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