concepts终极版，一个模拟游戏角色系统的应用

本例是concepts应用的终极版，结合std::variant实现一个简易的模拟游戏角色系统的应用。
程序代码如下，
CMakeLists.txt

cmake_minimum_required(VERSION 2.6)

if(APPLE)
    message(STATUS "This is Apple, do nothing.")
    set(CMAKE_MACOSX_RPATH 1)
    set(CMAKE_PREFIX_PATH /Users/aabjfzhu/software/vcpkg/ports/cppwork/vcpkg_installed/x64-osx/share )
elseif(UNIX)
    message(STATUS "This is linux, set CMAKE_PREFIX_PATH.")
    set(CMAKE_PREFIX_PATH /vcpkg/ports/cppwork/vcpkg_installed/x64-linux/share)
endif(APPLE)

project(game_engine)

set(CMAKE_CXX_STANDARD 20)

add_definitions(-g)

find_package(ZLIB)

find_package(glog REQUIRED)

find_package(OpenCV REQUIRED )

find_package(OpenSSL REQUIRED)

find_package(restclient-cpp REQUIRED)

find_package(Boost REQUIRED COMPONENTS
    system
    filesystem
    serialization
    program_options
    thread
    )

find_package(DataFrame REQUIRED)

if(APPLE)
    MESSAGE(STATUS "This is APPLE, set INCLUDE_DIRS")
set(INCLUDE_DIRS ${Boost_INCLUDE_DIRS} /usr/local/include /usr/local/iODBC/include /opt/snowflake/snowflakeodbc/include/ ${CMAKE_CURRENT_SOURCE_DIR}/../../include/ ${CMAKE_CURRENT_SOURCE_DIR}/include/ ${CMAKE_CURRENT_SOURCE_DIR}/)
elseif(UNIX)
    MESSAGE(STATUS "This is linux, set INCLUDE_DIRS")
    set(INCLUDE_DIRS ${Boost_INCLUDE_DIRS} /usr/local/include ${CMAKE_CURRENT_SOURCE_DIR}/../../include/ ${CMAKE_CURRENT_SOURCE_DIR}/include/ ${CMAKE_CURRENT_SOURCE_DIR}/)
endif(APPLE)


if(APPLE)
    MESSAGE(STATUS "This is APPLE, set LINK_DIRS")
    set(LINK_DIRS /usr/local/lib /usr/local/iODBC/lib /opt/snowflake/snowflakeodbc/lib/universal)
elseif(UNIX)
    MESSAGE(STATUS "This is linux, set LINK_DIRS")
    set(LINK_DIRS ${Boost_INCLUDE_DIRS} /usr/local/lib /vcpkg/ports/cppwork/vcpkg_installed/x64-linux/lib)
endif(APPLE)

if(APPLE)
    MESSAGE(STATUS "This is APPLE, set ODBC_LIBS")
    set(ODBC_LIBS iodbc iodbcinst)
elseif(UNIX)
    MESSAGE(STATUS "This is linux, set LINK_DIRS")
    set(ODBC_LIBS odbc odbcinst ltdl)
endif(APPLE)

include_directories(${INCLUDE_DIRS})
LINK_DIRECTORIES(${LINK_DIRS})

file( GLOB main_file_list ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp) 

file( GLOB APP_SOURCES  ${CMAKE_CURRENT_SOURCE_DIR}/*.h ${CMAKE_CURRENT_SOURCE_DIR}/*.hpp ${CMAKE_CURRENT_SOURCE_DIR}/utils/*.h ${CMAKE_CURRENT_SOURCE_DIR}/utils/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../include/http1/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../include/yaml/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../include/df/impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../include/death_handler/impl/*.cpp  ${CMAKE_CURRENT_SOURCE_DIR}/include/utils/*.h ${CMAKE_CURRENT_SOURCE_DIR}/include/utils/*.hpp ${CMAKE_CURRENT_SOURCE_DIR}/impl/utils/*.cpp)

add_library(${PROJECT_NAME}_lib SHARED ${APP_SOURCES})
target_link_libraries(${PROJECT_NAME}_lib ${Boost_LIBRARIES} ZLIB::ZLIB glog::glog DataFrame::DataFrame ${OpenCV_LIBS})
target_link_libraries(${PROJECT_NAME}_lib  OpenSSL::SSL OpenSSL::Crypto restclient-cpp libgtest.a pystring libyaml-cpp.a libgmock.a ${ODBC_LIBS} libnanodbc.a pthread dl)

foreach( main_file ${main_file_list} )
    file(RELATIVE_PATH filename ${CMAKE_CURRENT_SOURCE_DIR} ${main_file})
    string(REPLACE ".cpp" "" file ${filename})
    add_executable(${file}  ${main_file})
    target_link_libraries(${file} ${PROJECT_NAME}_lib)
endforeach( main_file ${main_file_list})

main.cpp

#include <utility>
#include <iostream>
#include <variant>
#include <vector>
#include <typeinfo>

namespace game_engine {
    struct rendering {};
};

namespace game::entity::concepts {
    template <typename T>
    concept movable = requires (T value) {
        value.velocity;
        value.position;
    };

    template <typename T>
    concept renderable = requires (const T value) {
        value.render_to(std::declval<game_engine::rendering&>());
    };

    template <typename T>
    concept has_behavior = requires (T value) {
        value.behave();
    };
};

namespace game::entity::implementations {
    struct character { // renderable movable has_behavior
        void render_to(game_engine::rendering&) const {}
        void behave() {}
        const int velocity = 2;
        const std::pair<int, int> position = {0, 0};
    };

    struct tree { // renderable
        void render_to(game_engine::rendering&) const {}
        const std::pair<int, int> position = {0, 0};
    };

    struct hidden_magic_spike {  // has_behavior
        void behave() {}
    };
};

// usage
// 用于std::variant遍历的operator操作符重载
// Refer to https://www.cppstories.com/2019/02/2lines3featuresoverload.html/
template <typename ... Ts>
struct overload: Ts... {
    using Ts::operator()...;
};

// 根据 overload(Ts..) 推导出 lambda表达式类型
template <typename ... Ts>
overload(Ts...) -> overload<Ts...>;

auto main(int argc, char** argv) -> int {
    using namespace game::entity::concepts;
    using namespace game::entity::implementations;

    // entities 
    using entity_type = std::variant<character, tree, hidden_magic_spike>;
    using entity_collection = std::vector<entity_type>;

    auto entities = entity_collection {
        character{},
        tree{},
        hidden_magic_spike{}
    }; 

    // visitors: 
    const auto move_visitor = overload {
        [](movable auto arg) {
            const auto [x, y] = arg.position;
            std::cout << "-" << typeid(decltype(arg)).name() << "\n" 
                << "    with velocity=" << arg.velocity <<
                " position={" << x << "," << y << "}\n";
        },
        [](auto) {/* default case */}
    };

    const auto rendering_visitor = overload {
        [](renderable auto arg) {
            std::cout << "-" << typeid(decltype(arg)).name() << "\n";
        },
        [](auto) {/* default case */}
    };

    const auto behave_visitor = overload {
        [](has_behavior auto arg) {
            std::cout << "-" << typeid(decltype(arg)).name() << "\n";
        },
        [](auto) {/* default case */}
    };

    // usage:
    std::cout << "move: \n";
    for(auto& value: entities) {
        std::visit(move_visitor, value);
    }

    std::cout << "render: \n";
    for(auto& value: entities) {
        std::visit(rendering_visitor, value);
    }

    std::cout << "behave: \n";
    for(auto& value: entities) {
        std::visit(behave_visitor, value);
    }
    return EXIT_SUCCESS;
}

程序输出如下，