C++ 20 使用concept 实现静态多态

本例演示使用C++ 20提供的concept 实现简单的静态多态。
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(static_poly)

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

// 静态多态
#include <iostream>
#include <tuple>

// 希望模板类型 s 有个 draw方法
template <typename T>
concept Drawable = requires(T s) {
    s.draw();
};

class Rectangle {
    public:
        auto draw() const {
            std::cout << "Draw an rectangle\n";
        }
};

class Circle {
    public:
        auto draw() const {
            std::cout << "Draw a circle\n";
        }
};

class Square {
    public:
        auto draw() const {
            std::cout << "Draw a square\n";
        }
};

template <size_t Index=0, Drawable... Ts>
constexpr auto drawAll(std::tuple<Ts ...> const& shapes) {
    if constexpr(Index != sizeof...(Ts)) {
        std::get<Index>(shapes).draw();
        drawAll<Index+1>(shapes);
    }
}


int main(int argc, char* argv[]) {
    drawAll(std::make_tuple(Rectangle{}, Circle{}, Square{}));
    return EXIT_SUCCESS;
}

程序输出如下，