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//[ Calc1
// 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 is a simple example of how to build an arithmetic expression
// evaluator with placeholders.
#include <iostream>
#include <boost/proto/core.hpp>
#include <boost/proto/context.hpp>
namespace proto = boost::proto;
using proto::_;
template<int I> struct placeholder {};
// Define some placeholders
proto::terminal< placeholder< 1 > >::type const _1 = {{}};
proto::terminal< placeholder< 2 > >::type const _2 = {{}};
// Define a calculator context, for evaluating arithmetic expressions
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 ];
}
};
template<typename Expr>
double evaluate( Expr const &expr, double d1 = 0., double d2 = 0. )
{
// Create a calculator context with d1 and d2 substituted for _1 and _2
calculator_context const ctx(d1, d2);
// Evaluate the calculator expression with the calculator_context
return proto::eval(expr, ctx);
}
int main()
{
// Displays "5"
std::cout << evaluate( _1 + 2.0, 3.0 ) << std::endl;
// Displays "6"
std::cout << evaluate( _1 * _2, 3.0, 2.0 ) << std::endl;
// Displays "0.5"
std::cout << evaluate( (_1 - _2) / _2, 3.0, 2.0 ) << std::endl;
return 0;
}
//]
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