C++ Traits和Policy演示累加器和累乘器

仿照 C++ Templates 《Traits and Policy Classes》这一章，改进了一下，造了一个泛型的累加器和累乘器出来。
大家平时不要这么搞，直接用STL自带的就行。
这个示例完全只是为了练手。
这个示例还有一种实现方法，把initia_val放在 Accum类的accum函数里面。但是没有直接放在模板参数里面好用。
放在模板参数里面，你可以直接使用默认实参。

CMakeLists.txt
其中有一些库是没有必要链接的，只需要标准C++就可以。
C++标准大于 C++11。

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(accum7)

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})

accum_traits7.hpp

#ifndef _FREDRIC_ACCUM_TRAITS6_HPP_
#define _FREDRIC_ACCUM_TRAITS6_HPP_

template <typename T, int initial_value=0>
class accumulation_traits;

#define MK_TRAITS(source_type, dest_type) \
template <int initial_value> \
class accumulation_traits<source_type, initial_value> { \
    public:  \
        using AccT = dest_type;     \
        static AccT init() {    \
            return initial_value;  \
        }                       \
};              \


MK_TRAITS(char, int)
MK_TRAITS(short, int)
MK_TRAITS(int, long)
MK_TRAITS(unsigned int, unsigned long)
MK_TRAITS(float, double)

#undef MK_TRAITS

#endif

sum_policy.hpp

#ifndef _FREDRIC_SUM_POLICY_HPP_
#define _FREDRIC_SUM_POLICY_HPP_

class sum_policy {
    public:
        template <typename T1, typename T2>
        static void accumulate(T1& total, T2 const& val) {
            total += val;
        }
};

#endif

mul_policy.hpp

#ifndef _FREDRIC_MUL_POLICY_HPP_
#define _FREDRIC_MUL_POLICY_HPP_

class mul_policy {
    public:
        template <typename T1, typename T2>
        static void accumulate(T1& total, T2 const& val) {
            total *= val;
        }
};
#endif

accum7.hpp

#ifndef _FREDRIC_ACCUM6_HPP_
#define _FREDRIC_ACCUM6_HPP_

#include "accum_traits7.hpp"
#include "sum_policy.hpp"
#include "mul_policy.hpp"

template <typename T,
    typename Policy = sum_policy,
    typename Traits = accumulation_traits<T>>
class Accum {
    public:
        using AccT = typename Traits::AccT;

        static AccT accum(T const* beg, T const* end) {
            // 有问题，这个zero是0，加法可以，乘法乘出来是错误的
            AccT total = Traits::init();
            while(beg != end) {
                Policy::accumulate(total, *beg);
                ++beg;
            }

            return total;
        }
};

template <typename T, int initial_val=1>
using MulAccum = Accum<T, mul_policy, accumulation_traits<T, initial_val>>;

template <typename T, int initial_val=0>
using SumAccum = Accum<T, sum_policy, accumulation_traits<T, initial_val>>;

#endif

main.cpp

#include "accum7.hpp"
#include "mul_policy.hpp"

#include <iostream>
#include <cstdlib>

int main(int argc, char* argv[]) {

    int nums[] {1, 2, 3, 4, 5};

    std::cout << "The average value of the integer value is: "
        << SumAccum<int, 15>::accum(&nums[0], &nums[5]) / 5
        << "\n";

    std::cout << "The product of integer values is: "
        << MulAccum<int>::accum(&nums[0], &nums[5])
        << "\n";

    char const name[] = "templates";
    int length = sizeof(name)-1;

    std::cout << "The average value of the character values is: "
        << SumAccum<char>::accum(&name[0], &name[length]) / length
        << std::endl;

    return EXIT_SUCCESS;
}

程序输出如下，

image.png