本例摘抄自《Design Patterns in Modern C++》，但是用concepts重写了虚函数的实现部分。
原例子是说需要对产品按照不同的属性进行过滤。如果直接写死filter,就需要写多个filter。如果写一个可以用于继承的specification，所有的specification都有一个is_satifisfied(Product*)方法，这样就可以对specification进行组合，按照不同的specification进行过滤。添加一个过滤规则，就是添加一个specification，不需要修改原来的代码，直接添加specification即可。对扩展开放，对修改封闭。
代码结构如下，
image.png
test/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(open_close)
set(CMAKE_CXX_STANDARD 20)
add_definitions(-g)
find_package(ZLIB)
find_package(OpenCV REQUIRED )
find_package(Arrow CONFIG REQUIRED)
find_package(unofficial-brotli REQUIRED)
find_package(unofficial-utf8proc CONFIG REQUIRED)
find_package(Thrift CONFIG REQUIRED)
find_package(glog REQUIRED)
find_package(OpenSSL 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)
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/)
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 test_file_list ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp)
file( GLOB APP_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/../impl/*.cpp ${CMAKE_CURRENT_SOURCE_DIR}/../include/*.h ${CMAKE_CURRENT_SOURCE_DIR}/../../../include/arr_/impl/*.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} ${test_file})
target_link_libraries(${PROJECT_NAME}_lib ${Boost_LIBRARIES} ZLIB::ZLIB glog::glog DataFrame::DataFrame ${OpenCV_LIBS})
target_link_libraries(${PROJECT_NAME}_lib OpenSSL::SSL OpenSSL::Crypto libgtest.a pystring libyaml-cpp.a libgmock.a ${ODBC_LIBS} libnanodbc.a pthread dl backtrace libzstd.a libbz2.a libsnappy.a re2::re2 parquet lz4 unofficial::brotli::brotlidec-static unofficial::brotli::brotlienc-static unofficial::brotli::brotlicommon-static utf8proc thrift::thrift arrow arrow_dataset)
foreach( test_file ${test_file_list} )
file(RELATIVE_PATH filename ${CMAKE_CURRENT_SOURCE_DIR} ${test_file})
string(REPLACE ".cpp" "" file ${filename})
add_executable(${file} ${test_file})
target_link_libraries(${file} ${PROJECT_NAME}_lib)
endforeach( test_file ${test_file_list})
test/open_close_test.cpp
#include "products.hpp"
#include "death_handler/death_handler.h"
#include "json/json.hpp"
#include <glog/logging.h>
#include <gtest/gtest.h>
#include "df/df.h"
using json = nlohmann::json;
int main(int argc, char** argv) {
FLAGS_log_dir = "./";
FLAGS_alsologtostderr = true;
// 日志级别 INFO, WARNING, ERROR, FATAL 的值分别为0、1、2、3
FLAGS_minloglevel = 0;
Debug::DeathHandler dh;
google::InitGoogleLogging("./logs.log");
testing::InitGoogleTest(&argc, argv);
int ret = RUN_ALL_TESTS();
return ret;
}
GTEST_TEST(OpenCloseTests, OpenClose) {
Product apple {"Apple", Color::green, Size::small};
Product tree {"Tree", Color::green, Size::large};
Product house {"House", Color::blue, Size::large};
std::vector<Product*> items {&apple, &tree, &house};
// auto spec = AndSpec(ColorSpec(Color::green), SizeSpec(Size::large));
auto spec = ColorSpec(Color::green) && SizeSpec(Size::large);
auto bf = BetterFilter();
for(auto& item: bf.filter(items, spec)) {
std::cout << item->name << " is green and large\n";
}
}
include/products.hpp
#ifndef _FREDRIC_PRODUCT_HPP_
#define _FREDRIC_PRODUCT_HPP_
#include <string>
#include <iostream>
#include <concepts>
#include <utility>
#include <vector>
enum class Color {red, green, blue};
enum class Size {small, medium, large};
struct Product {
std::string name;
Color color;
Size size;
};
template <typename T>
concept SpecType = requires (T value){
{value.is_satisfied(std::declval<Product*>())} -> std::convertible_to<bool>;
};
struct BetterFilter {
template <SpecType Specification>
std::vector<Product*> filter(std::vector<Product*> const& items,Specification& spec) {
std::vector<Product*> result;
for(auto& item: items) {
if(spec.is_satisfied(item)) {
result.push_back(item);
}
}
return result;
}
};
struct ColorSpec {
Color color;
ColorSpec(Color color_): color{color_} {}
bool is_satisfied(Product* item) {
return item->color == color;
}
};
struct SizeSpec {
Size size;
SizeSpec(Size size_): size{size_} {}
bool is_satisfied(Product* item) {
return item->size == size;
}
};
template <SpecType Spec1, SpecType Spec2>
struct AndSpec {
Spec1 first;
Spec2 second;
AndSpec(Spec1 first_, Spec2 second_): first{first_}, second{second_} {}
bool is_satisfied(Product* item) {
return first.is_satisfied(item) && second.is_satisfied(item);
}
};
template <SpecType Spec1, SpecType Spec2>
AndSpec<Spec1, Spec2> operator&&(Spec1 const& spec1, Spec2 const& spec2) {
return AndSpec<Spec1, Spec2>(spec1, spec2);
}
#endif
程序输出如下，
image.png
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