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
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