创建一个没有已知类型的对象

Question

我想知道如果这个对象的类型存储在一个字符串中，我该如何创建一个对象。我的目标是尽可能地优化它，这个算法会被迭代很多次。我知道 3 种可能的解决方案：

1. 创建可能类的列表（向量？）并迭代所有可能的变体（如果我有超过 10 个可能的类，速度会非常慢）
2. std::map 方法（创建地图并检查所有地图）（未检查性能）
3. 使用 Boost::PFR（一种用于 C++ 的“反射”库）（性能也未知）

您知道哪种方法是最好和最快的选择吗？以及如何实施？ 真诚的，卡罗尔

Answer 1

看来你的情况是反序列化。

解析和实例化将比类工厂的一个虚函数调度慢得多。这是一个草图，正在解析：

Point(3, 4.5);
Circle(Point(0,0), 42e3);
Rectangle(Point(7, -0.87654), Point (77, 9e2));

进入以下层次结构：

struct Shape {
    virtual ~Shape() = default;
    virtual void print(std::ostream&) const {};

    friend std::ostream& operator<<(std::ostream& os, Shape const& s) {
        s.print(os);
        return os;
    }
};

struct Point : Shape {
    double x, y;
    virtual void print(std::ostream& os) const override {
        os << "Point(" << x << "," << y << ")";
    }
};

struct Circle : Shape {
    Point  origin;
    double radius;
    virtual void print(std::ostream& os) const override {
        os << "Circle(" << origin << "," << radius << ")";
    }
};

struct Rectangle : Shape {
    Point topleft, bottomright;
    virtual void print(std::ostream& os) const override {
        os << "Rectangle(" << topleft << "," << bottomright << ")";
    }
};

解析器正在使用 Spirit X3：

namespace Parser {
    using namespace boost::spirit::x3;

    template <typename T>
    auto as        = [](auto expr) { return rule<struct _, T>{"rule"} = expr; };

    auto point     = as<Point>(eps >> "Point" >> "(" >> double_ >> ',' >> double_ >> ')');
    auto circle    = as<Circle>(eps >> "Circle" >> "(" >> point >> ',' >> double_ >> ')');
    auto rectangle = as<Rectangle>(eps >> "Rectangle" >> "(" >> point >> ',' >> point >> ')');
    auto shape     = point | circle | rectangle;

    auto push = [](auto& ctx) {
        auto vis = [&](auto& shape) { _val(ctx).insert(std::move(shape)); };
        apply_visitor(vis, _attr(ctx));
    };
    auto shapes = rule<struct _, Shapes>{}  //
                = skip(space)[shape[push] % ';'];
} // namespace Parser

让我们使用PolyCollection 来存储多态类型。如果您愿意，显然可以使用指针容器，但您提到性能是一个问题。

int main() {
    std::string const& input = R"(Point(3, 4.5);
    Circle(Point(0,0), 42e3);
    Rectangle(Point(7, -0.87654), Point (77, 9e2)))";

    Shapes parsed;
    if (parse(begin(input), end(input), Parser::shapes, parsed)) {
        for (auto& s : parsed) {
            std::cout << "Instantiated: " << s << "\n";
        }
    }
}

打印

Instantiated: Rectangle(Point(7,-0.87654),Point(77,900))
Instantiated: Circle(Point(0,0),42000)
Instantiated: Point(3,4.5)

现场演示

Live On Coliru

#include <iostream>
#include <typeinfo>

struct Shape {
    virtual ~Shape() = default;
    virtual void print(std::ostream&) const {};

    friend std::ostream& operator<<(std::ostream& os, Shape const& s) {
        s.print(os);
        return os;
    }
};

struct Point : Shape {
    double x, y;
    virtual void print(std::ostream& os) const override {
        os << "Point(" << x << "," << y << ")";
    }
};

struct Circle : Shape {
    Point  origin;
    double radius;
    virtual void print(std::ostream& os) const override {
        os << "Circle(" << origin << "," << radius << ")";
    }
};

struct Rectangle : Shape {
    Point topleft, bottomright;
    virtual void print(std::ostream& os) const override {
        os << "Rectangle(" << topleft << "," << bottomright << ")";
    }
};

#include <boost/poly_collection/base_collection.hpp>
using Shapes = boost::poly_collection::base_collection<Shape>;

#include <boost/fusion/adapted.hpp>
BOOST_FUSION_ADAPT_STRUCT(Point, x, y)
BOOST_FUSION_ADAPT_STRUCT(Circle, origin, radius)
BOOST_FUSION_ADAPT_STRUCT(Rectangle, topleft, bottomright)

#include <boost/spirit/home/x3.hpp>

namespace Parser {
    using namespace boost::spirit::x3;

    template <typename T>
    auto as        = [](auto expr) { return rule<struct _, T>{"rule"} = expr; };

    auto point     = as<Point>(eps >> "Point" >> "(" >> double_ >> ',' >> double_ >> ')');
    auto circle    = as<Circle>(eps >> "Circle" >> "(" >> point >> ',' >> double_ >> ')');
    auto rectangle = as<Rectangle>(eps >> "Rectangle" >> "(" >> point >> ',' >> point >> ')');
    auto shape     = point | circle | rectangle;

    auto push = [](auto& ctx) {
        auto vis = [&](auto& shape) { _val(ctx).insert(std::move(shape)); };
        apply_visitor(vis, _attr(ctx));
    };
    auto shapes = rule<struct _, Shapes>{}  //
                = skip(space)[shape[push] % ';'];
} // namespace Parser

int main() {
    std::string const& input = R"(Point(3, 4.5);
    Circle(Point(0,0), 42e3);
    Rectangle(Point(7, -0.87654), Point (77, 9e2)))";

    Shapes parsed;
    if (parse(begin(input), end(input), Parser::shapes, parsed)) {
        for (auto& s : parsed) {
            std::cout << "Instantiated: " << s << "\n";
        }
    }
}