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https://github.com/boostorg/openmethod.git
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91 lines
2.4 KiB
C++
91 lines
2.4 KiB
C++
// Copyright (c) 2018-2025 Jean-Louis Leroy
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// Distributed under the Boost Software License, Version 1.0.
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// See accompanying file LICENSE_1_0.txt
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// or copy at http://www.boost.org/LICENSE_1_0.txt)
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// clang-format off
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// tag::content[]
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struct Node {
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virtual ~Node() {}
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virtual int value() const = 0;
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};
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struct Variable : Node {
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Variable(int value) : v(value) {}
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int value() const override { return v; }
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int v;
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};
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struct Plus : Node {
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Plus(const Node& left, const Node& right) : left(left), right(right) {}
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int value() const override { return left.value() + right.value(); }
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const Node& left; const Node& right;
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};
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struct Times : Node {
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Times(const Node& left, const Node& right) : left(left), right(right) {}
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int value() const override { return left.value() * right.value(); }
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const Node& left; const Node& right;
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};
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// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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// library code
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// =============================================================================
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// application code
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// vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
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#include <boost/openmethod.hpp>
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#include <boost/openmethod/initialize.hpp>
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#include <iostream>
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using boost::openmethod::virtual_ptr;
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BOOST_OPENMETHOD(postfix, (virtual_ptr<const Node> node, std::ostream& os), void);
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// tag::variable_overrider[]
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BOOST_OPENMETHOD_OVERRIDE(
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postfix, (virtual_ptr<const Variable> var, std::ostream& os), void) {
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os << var->v;
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}
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// end::variable_overrider[]
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// tag::plus_overrider[]
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BOOST_OPENMETHOD_OVERRIDE(
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postfix, (virtual_ptr<const Plus> plus, std::ostream& os), void) {
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postfix(plus->left, os);
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os << ' ';
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postfix(plus->right, os);
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os << " +";
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}
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// end::plus_overrider[]
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BOOST_OPENMETHOD_OVERRIDE(
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postfix, (virtual_ptr<const Times> times, std::ostream& os), void) {
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postfix(times->left, os);
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os << ' ';
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postfix(times->right, os);
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os << " *";
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}
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// tag::class_registration[]
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BOOST_OPENMETHOD_CLASSES(Node, Variable, Plus, Times);
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// end::class_registration[]
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int main() {
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boost::openmethod::initialize();
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Variable a{2}, b{3}, c{4};
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Plus d{a, b}; Times e{d, c};
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postfix(e, std::cout);
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std::cout << " = " << e.value() << "\n"; // 2 3 + 4 * = 20
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}
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// end::content[]
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void call_via_ref(const Node& node, std::ostream& os) {
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postfix(node, os);
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}
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void call_via_virtual_ptr(virtual_ptr<const Node> node, std::ostream& os) {
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postfix(node, os);
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}
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