//[ 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/mpl/int.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; // Will be used to define the placeholders _1 and _2 template<int I> struct placeholder {}; // For expressions in the calculator domain, operator () // will be special; it will evaluate the expression. struct calculator_domain; // 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> { typedef proto::extends<Expr, calculator_expression<Expr>, calculator_domain> base_type; explicit calculator_expression(Expr const &expr = Expr()) : base_type(expr) {} using base_type::operator =; // 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); } }; // Tell proto how to generate expressions in the calculator_domain struct calculator_domain : proto::domain<proto::generator<calculator_expression> > {}; // 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; } //]