1 files changed, 103 insertions, 0 deletions
diff --git a/src/boost/libs/proto/example/calc2.cpp b/src/boost/libs/proto/example/calc2.cpp
new file mode 100644
index 00000000..6fc490bf
--- /dev/null
+++ b/src/boost/libs/proto/example/calc2.cpp
@@ -0,0 +1,103 @@
+//[ Calc2
+//  Copyright 2008 Eric Niebler. Distributed under the Boost
+//  Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// This example enhances the simple arithmetic expression evaluator
+// in calc1.cpp by using proto::extends to make arithmetic
+// expressions immediately evaluable with operator (), a-la a
+// function object
+
+#include <iostream>
+#include <boost/proto/core.hpp>
+#include <boost/proto/context.hpp>
+namespace proto = boost::proto;
+using proto::_;
+
+template<typename Expr>
+struct calculator_expression;
+
+// Tell proto how to generate expressions in the calculator_domain
+struct calculator_domain
+  : proto::domain<proto::generator<calculator_expression> >
+{};
+
+// Will be used to define the placeholders _1 and _2
+template<int I> struct placeholder {};
+
+// Define a calculator context, for evaluating arithmetic expressions
+// (This is as before, in calc1.cpp)
+struct calculator_context
+  : proto::callable_context< calculator_context const >
+{
+    // The values bound to the placeholders
+    double d[2];
+
+    // The result of evaluating arithmetic expressions
+    typedef double result_type;
+
+    explicit calculator_context(double d1 = 0., double d2 = 0.)
+    {
+        d[0] = d1;
+        d[1] = d2;
+    }
+
+    // Handle the evaluation of the placeholder terminals
+    template<int I>
+    double operator ()(proto::tag::terminal, placeholder<I>) const
+    {
+        return d[ I - 1 ];
+    }
+};
+
+// Wrap all calculator expressions in this type, which defines
+// operator () to evaluate the expression.
+template<typename Expr>
+struct calculator_expression
+  : proto::extends<Expr, calculator_expression<Expr>, calculator_domain>
+{
+    explicit calculator_expression(Expr const &expr = Expr())
+      : calculator_expression::proto_extends(expr)
+    {}
+
+    BOOST_PROTO_EXTENDS_USING_ASSIGN(calculator_expression<Expr>)
+
+    // Override operator () to evaluate the expression
+    double operator ()() const
+    {
+        calculator_context const ctx;
+        return proto::eval(*this, ctx);
+    }
+
+    double operator ()(double d1) const
+    {
+        calculator_context const ctx(d1);
+        return proto::eval(*this, ctx);
+    }
+
+    double operator ()(double d1, double d2) const
+    {
+        calculator_context const ctx(d1, d2);
+        return proto::eval(*this, ctx);
+    }
+};
+
+// Define some placeholders (notice they're wrapped in calculator_expression<>)
+calculator_expression<proto::terminal< placeholder< 1 > >::type> const _1;
+calculator_expression<proto::terminal< placeholder< 2 > >::type> const _2;
+
+// Now, our arithmetic expressions are immediately executable function objects:
+int main()
+{
+    // Displays "5"
+    std::cout << (_1 + 2.0)( 3.0 ) << std::endl;
+
+    // Displays "6"
+    std::cout << ( _1 * _2 )( 3.0, 2.0 ) << std::endl;
+
+    // Displays "0.5"
+    std::cout << ( (_1 - _2) / _2 )( 3.0, 2.0 ) << std::endl;
+
+    return 0;
+}
+//]