C 是一门古老的语言， C++在为C 引入了面向对象和泛型，也引入了许多的复杂性，例如多重继承，模板的特化，等等。同时 C++ 本身除了 STL 标准库，缺少高质量的并发和网络软件包，每个C++程序员或多或少都造过大大小小的轮子。

例如我就写过很多 string 处理的函数, 比如 Trim, LowerCase, UpperCase, 等等，还有基于引用计数的智能指针，包装 pthread 函数的线程类，林林总总，各种轮子都要自己和团队手工打造。有时有点成就感，可是显然也拖慢了开发速度，自己的轮子虽然够用，可是并非考虑周全，异常保护不够，经常会挖坑，也让别人踩坑。

所以近年来，逐渐在项目中引入 boost 库来提高开发效率，也从 C++ 98 向 C++11, 14, 17, 乃至 C++ 20 过渡。

在 C++ 11、14、17、20 中我们可以看到很多 boost library 的影子， 例如 boost::shared_ptr, boost::thread , boost::random, 等等。

C++ 11 新特性

改进的对象构造

• 继承基类的构造函数
• 默认的成员值
• 委托构造函数
• overwrite 关键字
• final 关键字
• 统一的初始化 {}

其他语言增强

• 基于范围的 for 循环
• long long int 类型 (64位)
• lambda 函数
• 移动语义和右值引用
• 强类型枚举: enum 增加了类型检查
• 智能指针
• 原始字符串字面量
• 静态断言
• 可变参数模板
• 改善了右尖括号的处理

新的关键字

• auto 自动类型推导
• constexpr: 表达式常量
• decltype: 声明类型
• nullptr: 空指针
• thread_local: 线程局部存储

对 C++ 库的增强

• stl 容器的列表初始化
• 随机库
• 正则表达式库
• 无序容器(hash)
• 附加的算法
• 元组模板
• 其他的新类型: chrono(时间测量), ratio(有理数分数), complex(复数)

作为一个 Java/C++ 都时常用到的老程序员，委托构造函数，智能指针，lambda 等许多东西都不新鲜，唯独需要重点提到的是右值。

C++ 本来就已经很复杂了，为什么又要弄出来右值这个东西来烧脑呢？ 原因还是在于对于压榨出高性能的需求，就象Linux 系统命令 cp 和 mv, 原先的值拷贝好比 cp, 现在的右值用于移动语义好比 mv。

什么是右值，一句话，有名字的是左值，没名字的右值，就因为右值没有名字，它们都是一个临时的变量，对于它们的复制可以简化为移动，反正临时的无名变量没人会用到。

例子代码

• 头文件
  run_example.h

#ifndef RUN_EXAMPLE_H_
#define RUN_EXAMPLE_H_#include <stdio.h>
#include <stdint.h>#include <string>
#include <map>
#include <iostream>
#include <memory>#include <unordered_map>
#include <boost/log/trivial.hpp>
#include <boost/program_options.hpp>
#include <boost/core/noncopyable.hpp>
#include <boost/assert.hpp>//old function pointer
typedef int (*exam_func_ptr)(int argc, char** argv);
//new function object
typedef std::function<int(int, char**)> exam_func_t;template<typename T, typename... Ts>
std::unique_ptr<T> my_make_unique(Ts&&... params) {return std::unique_ptr<T>(new T(std::forward<Ts>(params)...));
}class Command
{
public:Command();Command(std::string name);Command(const Command& other);Command& operator=(const Command& other);Command(Command&& other);Command& operator=(Command&& other);virutal ~Command();void setName(const std::string& name);void setParameter(const std::string& name, const std::string& value);void setData(const uint8_t* pData, size_t length);friend std::ostream& operator<<(std::ostream&, const Command& obj);private:std::string m_name = "";std::map<std::string, std::string> m_parameters;size_t m_length = 0;uint8_t* m_data = nullptr;
};class ExampleRunner: boost::noncopyable {
public:ExampleRunner();//no need by noncopyable//ExampleRunner(const ExampleRunner& rhs);//ExampleRunner& operator=(const ExampleRunner& rhs);virtual ~ExampleRunner();void init();size_t size() const;void register_example(const std::string& name, const exam_func_t &exam);int execute_example(const std::string& name, int argc, char** argv) const;
private:volatile uint32_t m_example_count = 0;std::unordered_map<std::string, exam_func_t> m_func_examples;
};#endif

• 实现文件
  run_example.cpp

#include "run_example.h"using namespace std;
namespace po = boost::program_options;extern int function_demo(int argc, char** argv);
extern int lambda_demo(int argc, char* argv[]);
extern int rvalue_demo(int argc, char* argv[]);
extern int smart_ptr_demo(int argc, char** argv);const int CASE_COUNT =4;const char* usage = R"name(please specify example name:function_demoor lambda_demoor rvalue_demoor smart_ptr_demo
)name";//C++11: delegate constructor
Command::Command():Command("") {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". construct: " << m_name << "@" <<this;
}Command::Command(string name):m_name(name) {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". construct: " << m_name << "@" <<this;
}Command::Command(const Command& other) {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". copy construct: " << m_name << "@" <<this;m_name = other.m_name;for (const auto& kv : other.m_parameters) {BOOST_LOG_TRIVIAL(trace) << kv.first << " has value " << kv.second;m_parameters.insert(kv);}std::copy(other.m_data, other.m_data + m_length, m_data);
}Command& Command::operator=(const Command& other) {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". operator =: " << other.m_name;if (this != &other){this->m_name = other.m_name;for (const auto& kv : other.m_parameters) {std::cout << kv.first << " has value " << kv.second << std::endl;m_parameters.insert(kv);}// Free the existing resource.if(m_data) {delete[] m_data;}m_length = other.m_length;m_data = new uint8_t[m_length];std::copy(other.m_data, other.m_data + m_length, m_data);}return *this;
}Command::Command(Command&& other) {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< " move copy construct: " << other.m_name << "@" <<this;m_name = std::move(other.m_name);m_parameters = std::move(other.m_parameters);// Copy the data pointer and its length from the source object.m_data = other.m_data;m_length = other.m_length;// Release the data pointer from the source object so that// the destructor does not free the memory multiple times.other.m_data = nullptr;other.m_length = 0;}Command& Command::operator=(Command&& other)
{BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< " move operator ==" << other.m_name;if (this != &other){m_name = std::move(other.m_name);m_parameters = std::move(other.m_parameters);// Free the existing resource.delete[] m_data;// Copy the data pointer and its length from the// source object.m_data = other.m_data;m_length = other.m_length;// Release the data pointer from the source object so that// the destructor does not free the memory multiple times.other.m_data = nullptr;other.m_length = 0;}return *this;
}Command::~Command() {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". destruct: " << m_name ;
}void Command::setName(const string& name) {m_name = name;
}
void Command::setParameter(const string& name, const string& value) {m_parameters[name] = value;
}void Command::setData(const uint8_t* pData, size_t length) {BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__<< ". setData: " << m_name;if(nullptr != m_data) {delete[] m_data;}m_length = length;m_data = new uint8_t[length];std::copy(pData, pData + length, m_data);
}ostream& operator<<(ostream& os, const Command& obj)
{os << obj.m_name;os << ": ";//for (map<string, string>::iterator it = obj.m_parameters.begin(); it != obj.m_parameters.end(); ++it)for (const auto& kv : obj.m_parameters){os << kv.first << "=" << kv.second << endl;}for(size_t i=0; i< obj.m_length; ++i) {os << *(obj.m_data + i);}return os;
}ExampleRunner::ExampleRunner(): m_example_count(0),m_func_examples() {BOOST_LOG_TRIVIAL(trace)<<"* ExampleRunner construct: " ;
}ExampleRunner::~ExampleRunner() {BOOST_LOG_TRIVIAL(trace)<<"* ExampleRunner destruct: ";
}void ExampleRunner::init() {register_example("function_demo", function_demo);register_example("smart_ptr_demo", smart_ptr_demo);register_example("lambda_demo", lambda_demo);register_example("rvalue_demo", rvalue_demo);
}void ExampleRunner::register_example(const string& name, const exam_func_t &exam)
{m_example_count++;m_func_examples[name] = exam;
}int ExampleRunner::execute_example(const string& name, int argc, char** argv) const
{auto it = m_func_examples.find(name);if(it != m_func_examples.end()) {BOOST_LOG_TRIVIAL(trace) << "execute "<< it->first;exam_func_t func = it->second;return func(argc, argv);}BOOST_LOG_TRIVIAL(trace) << "not registered "<< name;return -1;
}size_t ExampleRunner::size() const {return m_func_examples.size();
}int main(int argc, char** argv)
{unique_ptr<ExampleRunner> runner = my_make_unique<ExampleRunner>();runner->init();BOOST_ASSERT_MSG(runner->size()==CASE_COUNT, "example count should be 2");//c++11 R"raw string"po::options_description desc("Allowed options:");desc.add_options()("help,h", "produce help message")("name,n", po::value<string>(), usage);po::variables_map vm;po::store(po::parse_command_line(argc, argv, desc), vm);po::notify(vm);    if (vm.count("help")) {BOOST_LOG_TRIVIAL(trace) << desc << "\n";return 1;}if (vm.count("name")) {BOOST_LOG_TRIVIAL(trace) << "* example name is "<< vm["name"].as<string>() << ".";runner->execute_example(vm["name"].as<string>(), argc, argv);} else {BOOST_LOG_TRIVIAL(trace) << "example name was not set.";BOOST_LOG_TRIVIAL(trace) << desc ;}return 0;
}

关于智能指针和移动拷贝的例子

• boost_memory.cpp

#include "run_example.h"using namespace std;int rvalue_demo(int argc, char* argv[])
{//move constructorCommand cmd1(Command("c1"));//copy constructorCommand cmd2 = cmd1;shared_ptr<Command> sharedPtr = make_shared<Command>("update");sharedPtr->setParameter("user", "alice");uint8_t nBytes[3]   = { 0x00,0x01,0x02 };sharedPtr->setData(nBytes, 3);shared_ptr<Command> sharedPtr2 = sharedPtr;BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << ". sharedPtr=" << sharedPtr.get() << ", "<< sharedPtr.use_count() <<", command="<< *sharedPtr;BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << ". sharedPtr2=" << sharedPtr2.get() << ", "<< sharedPtr2.use_count() <<", command="<< *sharedPtr2;return 0;
}int smart_ptr_demo(int argc, char* argv[])
{BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << " -- unique_ptr --";//unique_ptr<Command> ptr = unique_ptr<Command>(new Command());unique_ptr<Command> uniquePtr = my_make_unique<Command>();uniquePtr->setName("create");uniquePtr->setParameter("user", "walter");BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << ". unique pointer=" << uniquePtr.get() <<", command="<< *uniquePtr;BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << ". -- shared_ptr --";shared_ptr<Command> sharedPtr = make_shared<Command>();sharedPtr->setName("retrieve");sharedPtr->setParameter("user", "walter");BOOST_LOG_TRIVIAL(trace) << "line " <<__LINE__ << ". shared pointer=" << sharedPtr.get() << ", "<< sharedPtr.use_count() <<", command="<< *sharedPtr;return 0;
}

运行结果

./bin/run_example -n rvalue_demo
[2020-10-07 13:59:40.303527] [0x0000000107d9edc0] [trace]   * ExampleRunner construct:
[2020-10-07 13:59:40.304168] [0x0000000107d9edc0] [trace]   * example name is rvalue_demo.
[2020-10-07 13:59:40.304179] [0x0000000107d9edc0] [trace]   execute rvalue_demo
[2020-10-07 13:59:40.304198] [0x0000000107d9edc0] [trace]   line 27. construct: c1@0x7ffeed644df0
[2020-10-07 13:59:40.304204] [0x0000000107d9edc0] [trace]   line 61 move copy construct: c1@0x7ffeed644db0
[2020-10-07 13:59:40.304210] [0x0000000107d9edc0] [trace]   line 100. destruct:
[2020-10-07 13:59:40.304234] [0x0000000107d9edc0] [trace]   line 31. copy construct: @0x7ffeed644df0
[2020-10-07 13:59:40.304255] [0x0000000107d9edc0] [trace]   line 27. construct: update@0x7faa5c407408
[2020-10-07 13:59:40.304264] [0x0000000107d9edc0] [trace]   line 112. setData: update
[2020-10-07 13:59:40.304281] [0x0000000107d9edc0] [trace]   line 19. sharedPtr=0x7faa5c407408, 2, command=update: user=alice[2020-10-07 13:59:40.304289] [0x0000000107d9edc0] [trace]   line 20. sharedPtr2=0x7faa5c407408, 2, command=update: user=alice[2020-10-07 13:59:40.304295] [0x0000000107d9edc0] [trace]   line 100. destruct: update
[2020-10-07 13:59:40.304315] [0x0000000107d9edc0] [trace]   line 100. destruct: c1
[2020-10-07 13:59:40.304319] [0x0000000107d9edc0] [trace]   line 100. destruct: c1
[2020-10-07 13:59:40.304327] [0x0000000107d9edc0] [trace]   * ExampleRunner destruct:

参考资料

• https://www.modernescpp.com/index.php/what-is-modern-c
• https://github.com/lefticus/cppbestpractices
• https://github.com/isocpp/CppCoreGuidelines