C++使用模板特化做类型提升

主题思想就是用IfThenElse模板在编译期选择类型。

CMakeLists.txt

cmake_minimum_required(VERSION 2.6)

if(APPLE)
    message(STATUS "This is Apple, do nothing.")
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(promote1)

add_definitions(-std=c++17)

add_definitions(-g)

find_package(ZLIB)

find_package(glog REQUIRED)

find_package(OpenCV 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}/../)
elseif(UNIX)
    MESSAGE(STATUS "This is linux, set INCLUDE_DIRS")
    set(INCLUDE_DIRS ${Boost_INCLUDE_DIRS} /usr/local/include ${CMAKE_CURRENT_SOURCE_DIR}/../../include/)
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}/main.cpp) 

file( GLOB APP_SOURCES  ${CMAKE_CURRENT_SOURCE_DIR}/*.h ${CMAKE_CURRENT_SOURCE_DIR}/*.hpp ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../../include/http/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)

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  ssl crypto libgtest.a pystring libyaml-cpp.a libgmock.a ${ODBC_LIBS} libnanodbc.a pthread dl backtrace)

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})

promotion1.hpp

#ifndef _FREDRIC_PROMOTION1_HPP_
#define _FREDRIC_PROMOTION1_HPP_

#include "ifthenelse.hpp"

// Promotion 对象逻辑，
// 如果 T1占用内存大 取 T1，否则取T2,
// 如果 T1 == T2， 取void
template <typename T1, typename T2>
class Promotion {
    public:
        using ResultT = typename IfThenElse<(sizeof(T1) > sizeof(T2)), 
                                            T1,
                            typename IfThenElse<(sizeof(T1) < sizeof(T2)),
                                            T2,
                                            void
                            >::ResultT>::ResultT;
};

#endif

promotion2.hpp

#ifndef _FREDRIC_PROMOTION2_HPP_
#define _FREDRIC_PROMOTION2_HPP_
#include "promotion1.hpp"

// 针对相同类型的特化
template <typename T>
class Promotion <T, T> {
    public:
        using ResultT = T;
};

#endif

promotion3.hpp

#ifndef _FREDRIC_PROMOTION3_HPP_
#define _FREDRIC_PROMOTION3_HPP_
#include "promotion1.hpp"

#define MK_PROMOTION(T1, T2, Tr)    \
    template <> \
    class Promotion<T1, T2> {   \
        public: \
            using ResultT = Tr; \
    };  \
    template <> \
    class Promotion<T2, T1> {   \
        public: \
            using ResultT = Tr; \
    };  \


MK_PROMOTION(bool, char, int)
MK_PROMOTION(bool, unsigned char, int)
MK_PROMOTION(bool, signed char, int)

#undef MK_PROMOTION

#endif

promotion_array.hpp

#ifndef _FREDRIC_PROMOTION_ARRAY_HPP_
#define _FREDRIC_PROMOTION_ARRAY_HPP_

#include "promotion1.hpp"
#include "promotion2.hpp"
#include "promotion3.hpp"

template <typename T>
class Array {};

// 针对Array的特化
template <typename T1, typename T2>
class Promotion<Array<T1>, Array<T2>> {
    public:
        using ResultT = Array<typename Promotion<T1, T2>::ResultT>;
};

// 针对同类型 Array的特化
template <typename T>
class Promotion<Array<T>, Array<T>> {
    public:
        using ResultT = Array<typename Promotion<T, T>::ResultT>;
};

#endif

ifthenelse.hpp

#ifndef _FREDRIC_IF_THEN_ELSE_HPP_
#define _FREDRIC_IF_THEN_ELSE_HPP_

// 总体模板
template <bool Bool, typename Ta, typename Tb>
class IfThenElse;

// 偏特化
template <typename Ta, typename Tb>
class IfThenElse<true, Ta, Tb> {
    public:
        using ResultT = Ta;
};

// false 偏特化
template <typename Ta, typename Tb>
class IfThenElse<false, Ta, Tb> {
    public:
        using ResultT = Tb;
};

#endif

main.cpp

#include "promotion_array.hpp"

#include <iostream>
#include <cstdlib>

#include <boost/type_index.hpp>

int main(int argc, char* argv[]) {
    typename Promotion<char, bool>::ResultT res_t1;
    typename Promotion<Array<double>, Array<int>>::ResultT res_t2;

    std::cout << boost::typeindex::type_id_runtime(res_t1) << std::endl;
    std::cout << boost::typeindex::type_id_runtime(res_t2) << std::endl;
    
    return EXIT_SUCCESS;
}

程序输出如下，

image.png