73ef4ff3
 Hu Chunming
提交三方库
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// Copyright (C) 2016-2018 T. Zachary Laine
//
// 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)
//[ calc3
#include <boost/yap/expression.hpp>
#include <boost/hana/maximum.hpp>
#include <iostream>
// Look! A transform! This one transforms the expression tree into the arity
// of the expression, based on its placeholders.
//[ calc3_get_arity_xform
struct get_arity
{
// Base case 1: Match a placeholder terminal, and return its arity as the
// result.
template <long long I>
boost::hana::llong<I> operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>,
boost::yap::placeholder<I>)
{ return boost::hana::llong_c<I>; }
// Base case 2: Match any other terminal. Return 0; non-placeholders do
// not contribute to arity.
template <typename T>
auto operator() (boost::yap::expr_tag<boost::yap::expr_kind::terminal>, T &&)
{
using namespace boost::hana::literals;
return 0_c;
}
// Recursive case: Match any expression not covered above, and return the
// maximum of its children's arities.
template <boost::yap::expr_kind Kind, typename... Arg>
auto operator() (boost::yap::expr_tag<Kind>, Arg &&... arg)
{
return boost::hana::maximum(
boost::hana::make_tuple(
boost::yap::transform(
boost::yap::as_expr(std::forward<Arg>(arg)),
get_arity{}
)...
)
);
}
};
//]
int main ()
{
using namespace boost::yap::literals;
// These lambdas wrap our expressions as callables, and allow us to check
// the arity of each as we call it.
auto expr_1 = 1_p + 2.0;
auto expr_1_fn = [expr_1](auto &&... args) {
auto const arity = boost::yap::transform(expr_1, get_arity{});
static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
return evaluate(expr_1, args...);
};
auto expr_2 = 1_p * 2_p;
auto expr_2_fn = [expr_2](auto &&... args) {
auto const arity = boost::yap::transform(expr_2, get_arity{});
static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
return evaluate(expr_2, args...);
};
auto expr_3 = (1_p - 2_p) / 2_p;
auto expr_3_fn = [expr_3](auto &&... args) {
auto const arity = boost::yap::transform(expr_3, get_arity{});
static_assert(arity.value == sizeof...(args), "Called with wrong number of args.");
return evaluate(expr_3, args...);
};
// Displays "5"
std::cout << expr_1_fn(3.0) << std::endl;
// Displays "6"
std::cout << expr_2_fn(3.0, 2.0) << std::endl;
// Displays "0.5"
std::cout << expr_3_fn(3.0, 2.0) << std::endl;
// Static-asserts with "Called with wrong number of args."
//std::cout << expr_3_fn(3.0) << std::endl;
// Static-asserts with "Called with wrong number of args."
//std::cout << expr_3_fn(3.0, 2.0, 1.0) << std::endl;
return 0;
}
//]
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