时间:: 2019年
#include<boost/tokenizer.hpp>
using namespace boost;
bool tokenizer_test()
{
string s = "This is a test";
tokenizer<> token1(s);
for(auto item:token1){
cout<<item<<endl;//空格划分
}
//第二种:
string str = ";;I|love||-the--one;is;you|";
char_separator<char> sep("-;|");
tokenizer<char_separator<char>> token2(str, sep);
//escaped_list_separator<char>
//offset_separator
for(auto item:token2){
cout<<item<<" ";//I love the one is you
}
cout<<endl;
//第三种:
string s1 = "12252001";
int offsets[] = {2,2,4};
offset_separator f(offsets, offsets+3);
tokenizer<offset_separator> tok(s1,f);
for(auto item:tok){
cout<<item <<" "; //12 25 2001
}
cout<<endl;
return true;
}
#include<boost/pool/pool.hpp>
using namespace boost;
//
//内存池操作:head-only library
//std:vector使用:pool_allocator
//std:list使用:fast_pool_allocator
//
#include<boost/pool/singleton_pool.hpp>
#include<boost/pool/object_pool.hpp>
#include<boost/pool/pool_alloc.hpp>
#include<boost/pool/poolfwd.hpp>
#include<boost/pool/simple_segregated_storage.hpp>
struct X{int i;};
bool pool_test()
{
//形式1
boost::pool<> p(sizeof(int));
for (int i = 0; i < 10000; ++i)
{
void * const t = p.malloc();
}
//形式2
boost::object_pool<X> p1;
for (int i = 0; i < 10000; ++i){X * const t = p1.malloc();}
//形式3
typedef boost::singleton_pool<X, sizeof(int)> my_pool;
for (int i = 0; i < 10000; ++i){
void * const t = my_pool::malloc();
}
my_pool::purge_memory();
return true;
}
// The Safe Numerics library i
//C++14,依赖较多;溢出报错
//
#include <boost/safe_numerics/safe_integer.hpp>
using namespace boost::safe_numerics;
safe<int> func(safe<int> x, safe<int> y){
return x + y;
}
-lboost_filesystem
#include<boost/filesystem.hpp>
using namespace boost;
//前面必须有#inlcude<boost/*>
//否则:error: ‘boost’ is not a namespace-name
//文件系统
// absolute canonical
// copy copy_directory copy_file copy_symlink
// create_directories create_directory create_hard_link create_symlink
// exists[存在性] equivalent hard_link_count
// initial_path. is_directory, is_empty
// is_other is_regular_file is_symlink
// last_write_time permissions read_symlink relative
// remove remove_all rename resize_file space
// status status_known symlink_status system_complete
bool fs_test()
{
auto filename="log.txt";
filesystem::path p1{"/usr/bin"};
cout<<"文件大小为:"<<filesystem::file_size(filename)<<"字节"<<endl;
cout<<"常规文件:"<<filesystem::is_regular_file(filename)<<endl;
cout<<"是目录:"<<filesystem::is_directory(p1)<<endl;
int i=0;
for (filesystem::directory_entry& item : filesystem::directory_iterator(p1)){
//cout<<item.path()<<endl; //文件的路径
cout<<item.path().filename()<<endl; //只有文件名
i++;
if(i>10)break;
}
cout<<"根路径:"<< p1.root_path() <<endl;
cout<<"名字"<<p1.stem()<<endl;
cout<<"后缀"<<p1.extension()<<endl;
cout<<"为空:"<<p1.empty()<<endl;
cout<<"绝对路径:"<<p1.is_absolute()<<endl;
cout<<"有root_name: "<<(p1.has_root_name()?"true":"false")<<endl;
cout<<"有根目录:"<<p1.has_root_directory()<<endl;
cout<<"有根路径:"<<p1.has_root_path()<<endl;
cout<<"有相对路径:"<<p1.has_relative_path()<<endl;
cout<<"有父目录:"<<p1.has_parent_path()<<endl;
cout<<"有文件名:"<<p1.has_filename()<<endl;
cout<<"有stem;"<<p1.has_stem()<<endl;
cout<<"有后缀;"<<p1.has_extension()<<endl;
cout<<"string表示:"<<p1.string()<<endl;
cout<<"通用string表示:"<<p1.generic_string()<<endl;
auto p2=filesystem::current_path();
cout<<"当前目录为:"<<p2.string()<<endl;
cout<<"状态:"<<status(p2).type()<<endl;
cout<<"状态:"<<status(p2).permissions()<<endl;
cout<<"最后修改时间:"<<last_write_time(p2)<<endl;
filesystem::rename(filename,"log_record.txt");
remove("log_record.txt");
return true;
}
//
//uint128_t,uint256_t,uint512_t,uint1024_t;
//int128_t ...
//cpp_rational
//
#include<boost/multiprecision/cpp_int.hpp> //慢,gmp_int快
#include<boost/multiprecision/cpp_bin_float.hpp>
#include<boost/multiprecision/cpp_dec_float.hpp>
//#include<boost/multiprecision/gmp.hpp> //mpf_float,mpf_float_50,mpf_float_500,mpf_float_1000
//#include<boost/multiprecision/mpfr.hpp> //mpfr_float,mpfr_float_100,mpfr_float_1000
#include<boost/multiprecision/float128.hpp>
#include<boost/multiprecision/cpp_complex.hpp>
bool multiprecision_func()
{
multiprecision::cpp_int big_int=1234254;//任意精度int
big_int=big_int*big_int*big_int*big_int*big_int;
cout<<"int大数为:"<<big_int<<endl;
multiprecision::cpp_bin_float_100 big_float=0.234345456;
big_float*=big_float;
cout<<"float大数为:"<<big_float<<endl;
multiprecision::cpp_complex_100 complex1={123,234};
cout<<"实部"<<complex1.real()<<endl;
cout<<"虚部"<<imag(complex1)<<endl;
multiprecision::cpp_rational rat_val=123;
for(unsigned i=1;i<1000;i++ ){
rat_val*=i;
}
cout<<std::cout.precision(10)<<endl;//设置小数精度
cout<<"有理数:"<<rat_val<<endl;
return true;
}
// 运行时间测量函数
// -lboost_timer -lboost_chrono
// 还有timer类:用于定时等
#include <boost/timer/timer.hpp>
bool sub_func()
{
timer::auto_cpu_timer sub_t;
cout<<"子函数调用时间"<<endl;
return true;
}
bool timer_test()
{
timer::auto_cpu_timer t;//定义时开始计时
cout<<"计时间过程"<<endl;
vector<string> asd{"qweasd","asdxcv","dfgyert","tyugvhbn"};
for(auto i:asd){
cout<<i<<endl;
}
sub_func();
return true;
}
//
// 各种分布:仍待探索
//
#include<boost/random/bernoulli_distribution.hpp>
#include<boost/random/beta_distribution.hpp>
#include<boost/random/binomial_distribution.hpp>
#include<boost/random/cauchy_distribution.hpp>
#include<boost/random/chi_squared_distribution.hpp>
#include<boost/random/discrete_distribution.hpp>
#include<boost/random/exponential_distribution.hpp>
#include<boost/random/extreme_value_distribution.hpp>
#include<boost/random/gamma_distribution.hpp>
#include<boost/random/fisher_f_distribution.hpp>
#include<boost/random/generate_canonical.hpp>
#include<boost/random/geometric_distribution.hpp>
#include<boost/random/hyperexponential_distribution.hpp>
#include<boost/random/laplace_distribution.hpp>
#include<boost/random/lognormal_distribution.hpp>
#include<boost/random/negative_binomial_distribution.hpp>
#include<boost/random/non_central_chi_squared_distribution.hpp>
#include<boost/random/normal_distribution.hpp>
#include<boost/random/piecewise_constant_distribution.hpp>
#include<boost/random/piecewise_linear_distribution.hpp>
#include<boost/random/poisson_distribution.hpp>
#include<boost/random/student_t_distribution.hpp>
#include<boost/random/triangle_distribution.hpp>
#include<boost/random/uniform_int_distribution.hpp>
#include<boost/random/uniform_real_distribution.hpp>
#include<boost/random/weibull_distribution.hpp>
#include<boost/random/uniform_01.hpp>
#include<boost/random/uniform_smallint.hpp>
#include<boost/random/uniform_on_sphere.hpp>
//
// 生成器
//
#include<boost/random/linear_congruential.hpp> //minstd_rand0 minstd_rand rand48
#include<boost/random/additive_combine.hpp> //ecuyer1988
#include<boost/random/shuffle_order.hpp> //knuth_b, kreutzer1986
#include<boost/random/taus88.hpp> //taus88
#include<boost/random/inversive_congruential.hpp>//hellekalek1995
#include<boost/random/mersenne_twister.hpp> //mt11213b, mt19937
#include<boost/random/lagged_fibonacci.hpp>
//lagged_fibonacci607, lagged_fibonacci1279, lagged_fibonacci2281
//lagged_fibonacci3217, lagged_fibonacci4423, lagged_fibonacci9689
//lagged_fibonacci19937, lagged_fibonacci23209, lagged_fibonacci44497
#include<boost/random/ranlux.hpp>
//ranlux3, ranlux4, ranlux64_3,ranlux64_4, ranlux3_01, ranlux4_01
//ranlux64_3_01, ranlux64_4_01, ranlux24, ranlux48
///
///其他
///
#include<boost/random/seed_seq.hpp>
#include<boost/random/random_number_generator.hpp>
#include<boost/random/generate_canonical.hpp>
random::mt19937 rng;
int rand_one(int start=1,int end=600){
//random::uniform_int_distribution<> diset(start,end);
//random::student_t_distribution<> diset;
random::normal_distribution<> diset(start,end);
return diset(rng);
}
int gailv(){
double probabilities[] = {0.1, 0.2, 0.1, 0.3, 0.1, 0.2};
random::discrete_distribution<> dist(probabilities);
return dist(rng)+1;
}
#include<boost/random/random_device.hpp>
random::random_device rng1;
bool mima(){
//该函数需要:-lboost_random
string table="abcdefghijklmnopqrstuvwxyz1234567890";
random::uniform_int_distribution<> index_dist(0, 35);
for(int i=0;i<10;i++){
cout<<table[index_dist(rng1)]<<",";
}
cout<<"mima"<<endl;
return true;
}
bool rand_test()//再试试其他的分布和生成器
{
int s=0,s1=0,m=0,e1=0,e=0;
int start=1;
int end=600;
for(int i=0;i<1000;i++){
int tmp=rand_one();
//int tmp=gailv();
if(tmp<= (end/5))s++;
else if(tmp<= (end*0.4))s1++;
else if(tmp<= (end*0.6))m++;
else if(tmp<= (end*0.8))e1++;
else e++;
}
cout<<"从小到达的数量:"
<<s<<"-"
<<s1<<"-"
<<m<<"-"
<<e1<<"-"
<<e<<endl;
//mima();
return true;
}
//
// coroutine2 :provides asymmetric coroutines.
// 协程: 相当于python的yield 程序在任一位置中断或恢复执行
// 关键 就是执行的流程控制
// The implementation uses Boost.Context for context switching
// 适合事件驱动模型
// 实现: fcontext_t:默认实现,基于汇编,有平台依赖,性能最好
// ucontext_t:跨平台,性能稍差
// 对称协程:显示yield;非对称协程:隐式转移控制权[本库实现方式]
// 编译选项:-lboost_context
//
#include <boost/coroutine2/all.hpp>
bool called_func(coroutines2::coroutine<void>::push_type & qwe)//无值传递
{
cout << "一 ";
qwe();
cout << "二 ";
qwe();
cout << "三 ";
return true;
}
bool call_func()
{
//push_type 与 pull_type 的位置可互换 从push开始向下执行
coroutines2::coroutine<void>::pull_type goto_qwe(called_func);
cout << "1 ";
goto_qwe();
cout << "2 ";
goto_qwe();
cout << "3 "<<endl;
return true;
}
bool called_func1(coroutines2::coroutine<unsigned int>::push_type& qwe)//有值传递
{
for(int i=0;i<10;i++){
cout <<i<< " ";
qwe(++i);
}
return true;
}
bool call_func1()
{
coroutines2::coroutine<unsigned int>::pull_type goto_qwe(called_func1);
/*
//第一种方法:
unsigned int i=1;
for(;i<10;i++){
cout <<goto_qwe.get()<<" ";
goto_qwe();
}
//第二种方法
using iter = coroutines2::coroutine<unsigned int>::pull_type::iterator;
for (iter start = begin(goto_qwe); start != end(goto_qwe); ++start) {
cout<< *start << " ";
}
*/
//第三种方法:
for(auto val:goto_qwe){
cout<<val<<" ";
}
cout <<endl;
return true;
}
#include<iostream>
#include<string>
#include<boost/algorithm/string_regex.hpp>
#include<boost/algorithm/string.hpp> // split join replace
#include <boost/lexical_cast.hpp>
#include <boost/convert.hpp>
#include <boost/regex.hpp>
using namespace boost;
void show_str(string one_str){cout<<one_str+";"<<endl;}
bool str_opt()
{
string one_str=" 啊撒了123看到 年份,12314s gdfg1bf. SDFG345SDFUK ";
cout<<"1、size:"<<one_str.size()<<endl;
cout<<"2、length:"<<one_str.length()<<endl;
//to_lower(),to_upper(one_str); show_str(one_str);
cout<<"3、转为小写"<<to_lower_copy(one_str) <<endl;
// 去掉开头结尾空格trim(),trim_left,trim_right,trim_left_copy
//trim_copy_if(str, boost::is_alnum());
cout<<"4、去开头空格："<<trim_left_copy(one_str)<<endl;
cout<<"5、starts_with:"<<starts_with(" BBCSDRFG","BBC")<<endl;
cout<<"6、ends_with："<<ends_with("踩踩julia","julia") <<endl;
cout<<"7、contains(包含 in)："<<contains("123zhong456","zhong") <<endl;
cout<<"8、equals（串相等）"<<equals("julia123","julia123") <<endl;
cout<<"9、字典序比较："<< lexicographical_compare("abvfg","bwerf")<<endl;
//cout<<"10、all(元素相同):"<<all("aaaaa",[](char i){return i=='a'?true:false;}) <<endl;
cout<<"10、all(元素相同):"<<all("aaaaa",is_any_of("a") ) <<endl;
//find_last,find_nth,
cout<<"11、find_first:"<<find_first(one_str,"123") <<endl; //怎么用?返回123不是索引
//if(find_first(one_str,"看到")){cout<<"找到了"<<endl;}else{cout<<"未找到"<<endl;}
cout<<"12、检索头"<<find_head(one_str,6) <<endl; //find_tail
iterator_range<string::iterator> iter1=find_token(one_str,is_any_of("312"));
cout<<"13、find_token:"<<string(iter1.begin(),iter1.end()) <<endl; //怎么用???
regex re1{"[0-9]+"};
iterator_range<string::iterator> iter=find_regex(one_str,re1);
cout<<"14、find_regex:"<<string(iter.begin(),iter.end()) <<endl;
//cout<<"15、find："<<find(one_str,first_finder("查找的串",is_iequal()) ) <<endl;
//{replace | erase}_{first | last | all | nth | head | tail | regex}_{copy|""}
// 28中组合
replace_first(one_str,"啊撒了","Julia");show_str(one_str);
replace_last(one_str,"年份","天狼星");show_str(one_str);
replace_all(one_str,"gdf","aab");show_str(one_str);
erase_all(one_str,"1");show_str(one_str); //删除所有的1
//replace_all_regex_copy()
//find_all,find_all_regex,iter_find,iter_split
vector<string> neirong;
find_all_regex(neirong,one_str,re1);
cout<<"16、find_all_regex 个数"<<neirong.size()<<" "<<neirong[1]<<endl; //****常用****
find_all(neirong,one_str,"23");
cout<<"17、find_all: "<<neirong[1]<<endl;
vector<string> str_vec;
regex e{"\\s+"};
//split(str_vec,one_str,is_any_of(" "));
split_regex(str_vec,one_str,e);
for(auto val:str_vec){cout<<val+"+"<<endl;}
//join,join_if（比join多个谓词）
string single_str=join(str_vec,"+");
cout<<"18、join连接："<<single_str<<endl;
unsigned int y=142;
cout<<"19、字面量转换 : "<<lexical_cast<string>(y)<<endl; //类型转换142->"142"
//string s2 = convert<string>(100).value();
//cout<<"20.convert: "<< s2<<endl;
//finder
//{first,last,nth,head,tail,token,range,regex}_finder
//Formatters 格式化器
//const_formatter,identity_formatter,empty_formatter,regex_formatter
//迭代器
//find_iterator,split_iterator
return true;
}
#include <boost/circular_buffer.hpp>
bool buffer()
{
circular_buffer<string> buffer5(5);
buffer5.push_back("1.sdf");
buffer5.push_back("2.啊撒");
buffer5.push_back("3.撒地方了");
buffer5.push_back("4.333");
buffer5.push_back("5.让他鱼");
string example=buffer5[3];
buffer5[3]="4.电饭锅";
buffer5.pop_back();
buffer5.pop_front();
for(auto i : buffer5){
cout<<"["<<i<<endl;
}
return true;
}
// === ========================
//
// 1. 非自定义对象:写个函数(文件名,对象,存储方式)直接存储
// 2. 自定义对象: 在类中添加方法
//
// 编译时添加lib库 : -lboost_serialization
//
// === ========================
#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_woarchive.hpp> //宽字符 utf8
#include <boost/archive/text_wiarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_woarchive.hpp>
#include <boost/archive/xml_wiarchive.hpp>
#include <boost/archive/binary_oarchive.hpp>
#include <boost/archive/binary_iarchive.hpp>
#include <boost/serialization/vector.hpp>
#include <boost/serialization/map.hpp>
#include <boost/serialization/set.hpp>
//
// 自定义类 的序列化
//
class self_def
{
public:
//类内数据定义
vector<float> aaa;
set<string> bbb;
string ccc;
friend class boost::serialization::access;
template<class Archive>
void serialize(Archive &ar,unsigned int version)
{
ar & aaa & bbb & ccc ;//读取,写入均可
}
self_def(vector<float>& val1,set<string>& val2,string val3){
this->aaa=val1;
this->bbb=val2;
this->ccc=val3;
}
self_def(){};
};
//
// 函数对象 实现序列化与反序列化
//
template<class object>
class Arch_out
{
public:
bool operator()(string filename,object& obj,string type="xml")
{
ofstream out_stream(filename);
if(type=="text"){
archive::text_oarchive out_ar(out_stream);
out_ar<<obj;
}
else if(type=="xml"){
archive::xml_oarchive out_ar(out_stream);
out_ar<<BOOST_SERIALIZATION_NVP(obj);
}
else{}
// out_ar && obj; //相同
return true;
}
friend class boost::serialization::access;
};
template<class object>
class Arch_in //该class代码未测试
{
public:
bool operator()(string filename,object& obj,string type="xml")
{
ifstream in_stream(filename);
if(type=="text"){
archive::text_iarchive in_ar(in_stream);
in_ar>>obj;
}
else if(type=="xml"){
archive::xml_iarchive in_ar(in_stream);
in_ar>>BOOST_SERIALIZATION_NVP(obj);
}
else{}
return true;
}
friend class boost::serialization::access;
};
//
// 泛型 实现序列化与反序列化
//
template<class object>
bool arch_out(string filename,object& obj,string type="xml")
{
ofstream out_stream(filename);
//多试几种存储方式
if(type=="txt"){
archive::text_oarchive out_ar(out_stream);
out_ar<<obj;
}
else if(type=="xml"){
archive::xml_oarchive out_ar(out_stream);
out_ar<<BOOST_SERIALIZATION_NVP(obj);
}
else if(type=="bin"){
archive::binary_oarchive out_ar(out_stream);
out_ar<<obj;
}
/*对‘vtable for boost::archive::codecvt_null<wchar_t>’未定义的引用
// -lboost_wserialization 加上无效,难道依赖C++20的u8string
if(type=="wtxt"){
wofstream out_stream(filename);
archive::text_woarchive out_ar(out_stream);
out_ar<<obj;
}
else if(type=="wxml"){
wofstream out_stream(filename);
archive::xml_woarchive out_ar(out_stream);
out_ar<<BOOST_SERIALIZATION_NVP(obj);
} */
else{
//type参数的内容:未知
cerr<<"type 参数错误"<<endl;
}
out_stream.close();
return true;
}
template<class object>
bool arch_in(string filename,object& obj,string type="xml")
{
ifstream in_stream(filename);//stringstream is ok
if(type=="txt"){
archive::text_iarchive in_ar(in_stream);
in_ar>>obj;
}
else if(type=="xml"){
archive::xml_iarchive in_ar(in_stream);
in_ar>>BOOST_SERIALIZATION_NVP(obj);
}
else if(type=="bin"){//binary
archive::binary_iarchive in_ar(in_stream);
in_ar>>obj;
}
/*
else if(type=="wtxt"){
wifstream in_stream(filename);
archive::text_wiarchive in_ar(in_stream);
in_ar>>obj;
}
else if(type=="wxml"){
wifstream in_stream(filename);
archive::xml_wiarchive in_ar(in_stream);
in_ar>>BOOST_SERIALIZATION_NVP(obj);
} */
else{
//wait to fill
}
in_stream.close();
return true;
}
//
// 序列化测试函数
//
#include<map>
#include<set>
bool Serial_test()
{
//1. 多试几种类型 map set vector<int string long>
vector<long> example_obj{234,456,2342456,6784523,7897435,123457};
map<string,long> map_tmp{{"C++",1},{"julia",2},{"python",3},{"Perl",4}};
set<string> set_tmp{"喀什的愤怒","速度快了女","sdlvn","所得率几年","送到了房间"};
set<string> tmp;
//2. 多次类型
auto filename="archive";
//方法1 函数对象
//Arch_out<vector<long>> CC;
//CC(filename,example_obj);
//方法2 泛型
//*
vector<string> type{"txt","bin","xml"};
for(auto ll:type){
arch_out(filename,set_tmp,ll);
arch_in(filename,tmp,ll);
//for(auto i:tmp){cout<<i.first<<"="<<i.second<<" ";} //map
for(auto i:tmp){cout<<i<<" ";}
cout<<ll<<"_tested"<<endl;
}//
//方法3 自定义类的序列化
vector<float> val1={3.12443,5.2347,2345.3456,2345.789};
set<string> val2={"喀什的愤怒","速度快了女","sdlvn","所得率几年","送到了房间"};
string val3="撒的开ak sdnf发那可asdf怜检索2354;多次";
self_def item1{val1,val2,val3};
self_def item2;
ofstream out_stream(filename);
archive::text_oarchive out_ar(out_stream);
out_ar<<item1;
out_stream.close();
ifstream in_stream(filename);
archive::text_iarchive in_ar(in_stream);
in_ar>>item2;
/*
arch_out<self_def>(filename,item1,"txt"); //该部分出现的错误,尚未解决
arch_in <self_def>(filename,item2,"txt"); //暂时使用上面七行代码解决
*/
cout<<item2.aaa[0]<<endl;;
cout<<item2.bbb.size()<<endl;
cout<<item2.ccc<<endl;
return true;
}
-lboost_serialization 编译时需要添加的命令后缀
using namespace std;
#include<vector>
#include<fstream>
bool writelines(vector<string> log_record)
{
ofstream log_file("log.txt",ios::app);
for(auto item:log_record){
log_file<<item<<endl;
}
return true;
}
vector<string> log_record{"项目执行日志\n","时间:2019- \n"};
bool log_add(string log_info,bool write2file=false)
{
log_record.push_back(log_info);
if(write2file==true){
writelines(log_record);
log_record.clear();
}
return true;
}
//调用方式:
// log_add("项目执行结束.",true);
//将上述代码放到main函数前
//
// this code from boost doc
// print("为恶哦","asdf","啊撒地方",123);
//
void print(){}
template<class T, class... Ts>
void print(const T& x, const Ts&... xs)
{
cout << x;
print(xs...);
}
//=========================================
// boost 的 相关库
//
// Range:增强stl 的可读性
// core:核心工具,无依赖,元编程的小函数,
// tribool: <boost/logic/tribool.hpp>: 三值:true,false,indeterminate
// beast:网络编程:request,repose
// Graph:图论中的图,
// icl:区间,interval_set,interval_map
//
//
//
//
//===================================
//
//Higher-order functions
//
//1.函数指针:&函数名 2.函数对象
//包含:BOOST_HOF_STATIC_FUNCTION,BOOST_HOF_STATIC_LAMBDA
// BOOST_HOF_LIFT(函数名)
//感觉没啥用啊
//
//
constexpr auto pi=3.141592657;
constexpr auto two_pi=2*pi;//常量表达式:编译时求值
cout<<"常量表达式"<<two_pi<<endl;
//什么时候用多线程?
// 1.大规模网络爬虫
// 2.大规模网络请求
// 3.密集的计算任务
//noexcept: 该关键字告诉编译器，函数中不会发生异常
// 确定函数不发生异常的根据,或判断标准是什么?
//token : an individual instance of a type of symbol
regex:有时间再看!!!!