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)
//[ vec3
#include <boost/yap/yap.hpp>
#include <array>
#include <iostream>
struct take_nth
{
auto operator() (boost::yap::terminal<boost::yap::expression, std::array<int, 3>> const & expr)
{
int x = boost::yap::value(expr)[n];
// The move forces the terminal to store the value of x, not a
// reference.
return boost::yap::make_terminal(std::move(x));
}
std::size_t n;
};
// Since this example doesn't constrain the operators defined on its
// expressions, we can just use boost::yap::expression<> as the expression
// template.
using vec3_terminal = boost::yap::expression<
boost::yap::expr_kind::terminal,
boost::hana::tuple<std::array<int, 3>>
>;
// Customize the terminal type we use by adding index and assignment
// operations.
struct vec3 : vec3_terminal
{
explicit vec3 (int i = 0, int j = 0, int k = 0)
{
(*this)[0] = i;
(*this)[1] = j;
(*this)[2] = k;
}
explicit vec3 (std::array<int, 3> a)
{
(*this)[0] = a[0];
(*this)[1] = a[1];
(*this)[2] = a[2];
}
int & operator[] (std::ptrdiff_t i)
{ return boost::yap::value(*this)[i]; }
int const & operator[] (std::ptrdiff_t i) const
{ return boost::yap::value(*this)[i]; }
template <typename T>
vec3 & operator= (T const & t)
{
decltype(auto) expr = boost::yap::as_expr(t);
(*this)[0] = boost::yap::evaluate(boost::yap::transform(expr, take_nth{0}));
(*this)[1] = boost::yap::evaluate(boost::yap::transform(expr, take_nth{1}));
(*this)[2] = boost::yap::evaluate(boost::yap::transform(expr, take_nth{2}));
return *this;
}
void print() const
{
std::cout << '{' << (*this)[0]
<< ", " << (*this)[1]
<< ", " << (*this)[2]
<< '}' << std::endl;
}
};
// This is a stateful transform that keeps a running count of the terminals it
// has seen.
struct count_leaves_impl
{
auto operator() (vec3_terminal const & expr)
{
value += 1;
return expr;
}
int value = 0;
};
template <typename Expr>
int count_leaves (Expr const & expr)
{
count_leaves_impl impl;
boost::yap::transform(boost::yap::as_expr(expr), impl);
return impl.value;
}
int main()
{
vec3 a, b, c;
c = 4;
b[0] = -1;
b[1] = -2;
b[2] = -3;
a = b + c;
a.print();
vec3 d;
auto expr1 = b + c;
d = expr1;
d.print();
int num = count_leaves(expr1);
std::cout << num << std::endl;
num = count_leaves(b + 3 * c);
std::cout << num << std::endl;
num = count_leaves(b + c * d);
std::cout << num << std::endl;
return 0;
}
//]
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