static_cast (expression)
该运算符把expression转换为type-id类型，但没有运行时类型检查来保证转换的安全性。它主要有如下几种用法：
①用于类层次结构中基类（父类）和派生类（子类）之间指针或引用的转换。
进行上行转换（把派生类的指针或引用转换成基类表示）是安全的；
进行下行转换（把基类指针或引用转换成派生类表示）时，由于没有动态类型检查，所以是不安全的。
②用于基本数据类型之间的转换，如把int转换成char，把int转换成enum。这种转换的安全性也要开发人员来保证。
③把空指针转换成目标类型的空指针。
④把任何类型的表达式转换成void类型。
boost::noncopyable
boost::noncopyable比较简单, 主要用于单例的情况.
通常情况下, 要写一个单例类就要在类的声明把它们的构造函
数, 赋值函数, 析构函数, 复制构造函数隐藏到private或者
protected之中, 每个类都这么做麻烦.
有noncopyable类, 只要让单例类直接继承noncopyable.
protected权限，这样子类可以调用，但是外面的类不能调用，
那么当子类需要定义构造函数的时候不至于通不过编译。
但是最关键的是noncopyable把复制构造函数和复制赋值函数做
成了private，这就意味着除非子类定义自己的copy构造和赋值函
数，否则在子类没有定义的情况下，外面的调用者是不能够通
过赋值和copy构造等手段来产生一个新的子类对象的。
__thread
__thread是GCC内置的线程局部存储设施，存取效率可以和
全局变量相比。
__thread变量每一个线程有一份独立实体，各个线程的值互不
干扰。可以用来修饰那些带有全局性且值可能变，但是又不
值得用全局变量保护的变量。
__thread使用规则：只能修饰POD类型(类似整型指针的标
容可以任意复制memset,memcpy,且内容可以复原)，不能修饰class类型，因为无法自动调用构造函数和析构函数，
可以用于修饰全局变量，函数内的静态变量，不能修饰函数的局
部变量或者class的普通成员变量，且__thread变量值只能初始化为编译器常量(值在编译器就可以确定const int i=5,
运行期常量是运行初始化后不再改变const int i=rand()).
#include<iostream>
#include<pthread.h>
#include<unistd.h>
using namespace std;
const int i=5;
__thread int var=i;//两种方式效果一样
//__thread int var=5;//
void* worker1(void* arg);
void* worker2(void* arg);
int main(){
pthread_t pid1,pid2;
//__thread int temp=5;
static __thread int temp=10;//修饰函数内的static变量
pthread_create(&pid1,NULL,worker1,NULL);
pthread_create(&pid2,NULL,worker2,NULL);
pthread_join(pid1,NULL);
pthread_join(pid2,NULL);
cout<<temp<<endl;//输出10
return 0;
}
void* worker1(void* arg){
cout<<++var<<endl;//输出 6
}
void* worker2(void* arg){
sleep(1);//等待线程1改变var值，验证是否影响线程2
cout<<++var<<endl;//输出6
}linux下syscall函数，SYS_gettid，SYS_tgkill
NAME
syscall - 间接系统调用
SYNOPSIS
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h> /* For SYS_xxx definitions */
int syscall(int number, ...); Linux中，每个进程有一个pid，类型pid_t，由getpid()取得。Linux下的POSIX线程也有一个id，类型 pthread_t，由pthread_self()取得，该id由线程库维护，其id空间是各个进程独立的（即不同进程中的线程可能有相同的id）。Linux中的POSIX线程库实现的线程其实也是一个进程（LWP），只是该进程与主进程（启动线程的进程）共享一些资源而已，比如代码段，数据段等。
有时候我们可能需要知道线程的真实pid。比如进程P1要向另外一个进程P2中的某个线程发送信号时，既不能使用P2的pid，更不能使用线程的pthread id，而只能使用该线程的真实pid，称为tid。
有一个函数gettid()可以得到tid，但glibc并没有实现该函数，只能通过Linux的系统调用syscall来获取。
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
pid_t tid;
tid = syscall(SYS_gettid);
printf("tid : %d\n",tid);
tid = syscall(SYS_tgkill, getpid(), tid, SIGHUP);
printf("tid : %d ...\n",tid);
}下面是一些调用函数：
/* Generated at libc build time from kernel syscall list. */
#ifndef _SYSCALL_H
# error "Never use <bits/syscall.h> directly; include <sys/syscall.h> instead."
#endif
#include <bits/wordsize.h>
#define SYS__sysctl __NR__sysctl
#define SYS_access __NR_access
#define SYS_acct __NR_acct
#define SYS_add_key __NR_add_key
#define SYS_adjtimex __NR_adjtimex
#define SYS_afs_syscall __NR_afs_syscall
#define SYS_alarm __NR_alarm
#define SYS_brk __NR_brk
#define SYS_capget __NR_capget
#define SYS_capset __NR_capset
#define SYS_chdir __NR_chdir
#define SYS_chmod __NR_chmod
#define SYS_chown __NR_chown
#define SYS_chroot __NR_chroot
#define SYS_clock_getres __NR_clock_getres
#define SYS_clock_gettime __NR_clock_gettime
#define SYS_clock_nanosleep __NR_clock_nanosleep
#define SYS_clock_settime __NR_clock_settime
#define SYS_clone __NR_clone
#define SYS_close __NR_close
#define SYS_creat __NR_creat
#define SYS_create_module __NR_create_module
#define SYS_delete_module __NR_delete_module
#define SYS_dup __NR_dup
#define SYS_dup2 __NR_dup2
#define SYS_epoll_create __NR_epoll_create
#define SYS_epoll_ctl __NR_epoll_ctl
#define SYS_epoll_wait __NR_epoll_wait
#define SYS_eventfd __NR_eventfd
#define SYS_execve __NR_execve
#define SYS_exit __NR_exit
#define SYS_exit_group __NR_exit_group
#define SYS_faccessat __NR_faccessat
#define SYS_fadvise64 __NR_fadvise64
#define SYS_fallocate __NR_fallocate
#define SYS_fchdir __NR_fchdir
#define SYS_fchmod __NR_fchmod
#define SYS_fchmodat __NR_fchmodat
#define SYS_fchown __NR_fchown
#define SYS_fchownat __NR_fchownat
#define SYS_fcntl __NR_fcntl
#define SYS_fdatasync __NR_fdatasync
#define SYS_fgetxattr __NR_fgetxattr
#define SYS_flistxattr __NR_flistxattr
#define SYS_flock __NR_flock
#define SYS_fork __NR_fork
#define SYS_fremovexattr __NR_fremovexattr
#define SYS_fsetxattr __NR_fsetxattr
#define SYS_fstat __NR_fstat
#define SYS_fstatfs __NR_fstatfs
#define SYS_fsync __NR_fsync
#define SYS_ftruncate __NR_ftruncate
#define SYS_futex __NR_futex
#define SYS_futimesat __NR_futimesat
#define SYS_get_kernel_syms __NR_get_kernel_syms
#define SYS_get_mempolicy __NR_get_mempolicy
#define SYS_get_robust_list __NR_get_robust_list
#define SYS_get_thread_area __NR_get_thread_area
#define SYS_getcwd __NR_getcwd
#define SYS_getdents __NR_getdents
#define SYS_getdents64 __NR_getdents64
#define SYS_getegid __NR_getegid
#define SYS_geteuid __NR_geteuid
#define SYS_getgid __NR_getgid
#define SYS_getgroups __NR_getgroups
#define SYS_getitimer __NR_getitimer
#define SYS_getpgid __NR_getpgid
#define SYS_getpgrp __NR_getpgrp
#define SYS_getpid __NR_getpid
#define SYS_getpmsg __NR_getpmsg
#define SYS_getppid __NR_getppid
#define SYS_getpriority __NR_getpriority
#define SYS_getresgid __NR_getresgid
#define SYS_getresuid __NR_getresuid
#define SYS_getrlimit __NR_getrlimit
#define SYS_getrusage __NR_getrusage
#define SYS_getsid __NR_getsid
#define SYS_gettid __NR_gettid
#define SYS_gettimeofday __NR_gettimeofday
#define SYS_getuid __NR_getuid
#define SYS_getxattr __NR_getxattr
#define SYS_init_module __NR_init_module
#define SYS_inotify_add_watch __NR_inotify_add_watch
#define SYS_inotify_init __NR_inotify_init
#define SYS_inotify_rm_watch __NR_inotify_rm_watch
#define SYS_io_cancel __NR_io_cancel
#define SYS_io_destroy __NR_io_destroy
#define SYS_io_getevents __NR_io_getevents
#define SYS_io_setup __NR_io_setup
#define SYS_io_submit __NR_io_submit
#define SYS_ioctl __NR_ioctl
#define SYS_ioperm __NR_ioperm
#define SYS_iopl __NR_iopl
#define SYS_ioprio_get __NR_ioprio_get
#define SYS_ioprio_set __NR_ioprio_set
#define SYS_kexec_load __NR_kexec_load
#define SYS_keyctl __NR_keyctl
#define SYS_kill __NR_kill
#define SYS_lchown __NR_lchown
#define SYS_lgetxattr __NR_lgetxattr
#define SYS_link __NR_link
#define SYS_linkat __NR_linkat
#define SYS_listxattr __NR_listxattr
#define SYS_llistxattr __NR_llistxattr
#define SYS_lookup_dcookie __NR_lookup_dcookie
#define SYS_lremovexattr __NR_lremovexattr
#define SYS_lseek __NR_lseek
#define SYS_lsetxattr __NR_lsetxattr
#define SYS_lstat __NR_lstat
#define SYS_madvise __NR_madvise
#define SYS_mbind __NR_mbind
#define SYS_migrate_pages __NR_migrate_pages
#define SYS_mincore __NR_mincore
#define SYS_mkdir __NR_mkdir
#define SYS_mkdirat __NR_mkdirat
#define SYS_mknod __NR_mknod
#define SYS_mknodat __NR_mknodat
#define SYS_mlock __NR_mlock
#define SYS_mlockall __NR_mlockall
#define SYS_mmap __NR_mmap
#define SYS_modify_ldt __NR_modify_ldt
#define SYS_mount __NR_mount
#define SYS_move_pages __NR_move_pages
#define SYS_mprotect __NR_mprotect
#define SYS_mq_getsetattr __NR_mq_getsetattr
#define SYS_mq_notify __NR_mq_notify
#define SYS_mq_open __NR_mq_open
#define SYS_mq_timedreceive __NR_mq_timedreceive
#define SYS_mq_timedsend __NR_mq_timedsend
#define SYS_mq_unlink __NR_mq_unlink
#define SYS_mremap __NR_mremap
#define SYS_msync __NR_msync
#define SYS_munlock __NR_munlock
#define SYS_munlockall __NR_munlockall
#define SYS_munmap __NR_munmap
#define SYS_nanosleep __NR_nanosleep
#define SYS_nfsservctl __NR_nfsservctl
#define SYS_open __NR_open
#define SYS_openat __NR_openat
#define SYS_pause __NR_pause
#define SYS_personality __NR_personality
#define SYS_pipe __NR_pipe
#define SYS_pivot_root __NR_pivot_root
#define SYS_poll __NR_poll
#define SYS_ppoll __NR_ppoll
#define SYS_prctl __NR_prctl
#define SYS_pread64 __NR_pread64
#define SYS_pselect6 __NR_pselect6
#define SYS_ptrace __NR_ptrace
#define SYS_putpmsg __NR_putpmsg
#define SYS_pwrite64 __NR_pwrite64
#define SYS_query_module __NR_query_module
#define SYS_quotactl __NR_quotactl
#define SYS_read __NR_read
#define SYS_readahead __NR_readahead
#define SYS_readlink __NR_readlink
#define SYS_readlinkat __NR_readlinkat
#define SYS_readv __NR_readv
#define SYS_reboot __NR_reboot
#define SYS_remap_file_pages __NR_remap_file_pages
#define SYS_removexattr __NR_removexattr
#define SYS_rename __NR_rename
#define SYS_renameat __NR_renameat
#define SYS_request_key __NR_request_key
#define SYS_restart_syscall __NR_restart_syscall
#define SYS_rmdir __NR_rmdir
#define SYS_rt_sigaction __NR_rt_sigaction
#define SYS_rt_sigpending __NR_rt_sigpending
#define SYS_rt_sigprocmask __NR_rt_sigprocmask
#define SYS_rt_sigqueueinfo __NR_rt_sigqueueinfo
#define SYS_rt_sigreturn __NR_rt_sigreturn
#define SYS_rt_sigsuspend __NR_rt_sigsuspend
#define SYS_rt_sigtimedwait __NR_rt_sigtimedwait
#define SYS_sched_get_priority_max __NR_sched_get_priority_max
#define SYS_sched_get_priority_min __NR_sched_get_priority_min
#define SYS_sched_getaffinity __NR_sched_getaffinity
#define SYS_sched_getparam __NR_sched_getparam
#define SYS_sched_getscheduler __NR_sched_getscheduler
#define SYS_sched_rr_get_interval __NR_sched_rr_get_interval
#define SYS_sched_setaffinity __NR_sched_setaffinity
#define SYS_sched_setparam __NR_sched_setparam
#define SYS_sched_setscheduler __NR_sched_setscheduler
#define SYS_sched_yield __NR_sched_yield
#define SYS_select __NR_select
#define SYS_sendfile __NR_sendfile
#define SYS_set_mempolicy __NR_set_mempolicy
#define SYS_set_robust_list __NR_set_robust_list
#define SYS_set_thread_area __NR_set_thread_area
#define SYS_set_tid_address __NR_set_tid_address
#define SYS_setdomainname __NR_setdomainname
#define SYS_setfsgid __NR_setfsgid
#define SYS_setfsuid __NR_setfsuid
#define SYS_setgid __NR_setgid
#define SYS_setgroups __NR_setgroups
#define SYS_sethostname __NR_sethostname
#define SYS_setitimer __NR_setitimer
#define SYS_setpgid __NR_setpgid
#define SYS_setpriority __NR_setpriority
#define SYS_setregid __NR_setregid
#define SYS_setresgid __NR_setresgid
#define SYS_setresuid __NR_setresuid
#define SYS_setreuid __NR_setreuid
#define SYS_setrlimit __NR_setrlimit
#define SYS_setsid __NR_setsid
#define SYS_settimeofday __NR_settimeofday
#define SYS_setuid __NR_setuid
#define SYS_setxattr __NR_setxattr
#define SYS_sigaltstack __NR_sigaltstack
#define SYS_splice __NR_splice
#define SYS_stat __NR_stat
#define SYS_statfs __NR_statfs
#define SYS_swapoff __NR_swapoff
#define SYS_swapon __NR_swapon
#define SYS_symlink __NR_symlink
#define SYS_symlinkat __NR_symlinkat
#define SYS_sync __NR_sync
#define SYS_sync_file_range __NR_sync_file_range
#define SYS_sysfs __NR_sysfs
#define SYS_sysinfo __NR_sysinfo
#define SYS_syslog __NR_syslog
#define SYS_tee __NR_tee
#define SYS_tgkill __NR_tgkill
#define SYS_time __NR_time
#define SYS_timer_create __NR_timer_create
#define SYS_timer_delete __NR_timer_delete
#define SYS_timer_getoverrun __NR_timer_getoverrun
#define SYS_timer_gettime __NR_timer_gettime
#define SYS_timer_settime __NR_timer_settime
#define SYS_times __NR_times
#define SYS_tkill __NR_tkill
#define SYS_truncate __NR_truncate
#define SYS_umask __NR_umask
#define SYS_umount2 __NR_umount2
#define SYS_uname __NR_uname
#define SYS_unlink __NR_unlink
#define SYS_unlinkat __NR_unlinkat
#define SYS_unshare __NR_unshare
#define SYS_uselib __NR_uselib
#define SYS_ustat __NR_ustat
#define SYS_utime __NR_utime
#define SYS_utimes __NR_utimes
#define SYS_vfork __NR_vfork
#define SYS_vhangup __NR_vhangup
#define SYS_vmsplice __NR_vmsplice
#define SYS_vserver __NR_vserver
#define SYS_wait4 __NR_wait4
#define SYS_waitid __NR_waitid
#define SYS_write __NR_write
#define SYS_writev __NR_writev
#if __WORDSIZE == 64
#define SYS_accept __NR_accept
#define SYS_arch_prctl __NR_arch_prctl
#define SYS_bind __NR_bind
#define SYS_connect __NR_connect
#define SYS_epoll_ctl_old __NR_epoll_ctl_old
#define SYS_epoll_pwait __NR_epoll_pwait
#define SYS_epoll_wait_old __NR_epoll_wait_old
#define SYS_getpeername __NR_getpeername
#define SYS_getsockname __NR_getsockname
#define SYS_getsockopt __NR_getsockopt
#define SYS_listen __NR_listen
#define SYS_msgctl __NR_msgctl
#define SYS_msgget __NR_msgget
#define SYS_msgrcv __NR_msgrcv
#define SYS_msgsnd __NR_msgsnd
#define SYS_newfstatat __NR_newfstatat
#define SYS_recvfrom __NR_recvfrom
#define SYS_recvmsg __NR_recvmsg
#define SYS_security __NR_security
#define SYS_semctl __NR_semctl
#define SYS_semget __NR_semget
#define SYS_semop __NR_semop
#define SYS_semtimedop __NR_semtimedop
#define SYS_sendmsg __NR_sendmsg
#define SYS_sendto __NR_sendto
#define SYS_setsockopt __NR_setsockopt
#define SYS_shmat __NR_shmat
#define SYS_shmctl __NR_shmctl
#define SYS_shmdt __NR_shmdt
#define SYS_shmget __NR_shmget
#define SYS_shutdown __NR_shutdown
#define SYS_signalfd __NR_signalfd
#define SYS_socket __NR_socket
#define SYS_socketpair __NR_socketpair
#define SYS_timerfd_create __NR_timerfd_create
#define SYS_tuxcall __NR_tuxcall
#define SYS_utimensat __NR_utimensat
#else
#define SYS__llseek __NR__llseek
#define SYS__newselect __NR__newselect
#define SYS_bdflush __NR_bdflush
#define SYS_break __NR_break
#define SYS_chown32 __NR_chown32
#define SYS_fadvise64_64 __NR_fadvise64_64
#define SYS_fchown32 __NR_fchown32
#define SYS_fcntl64 __NR_fcntl64
#define SYS_fstat64 __NR_fstat64
#define SYS_fstatat64 __NR_fstatat64
#define SYS_fstatfs64 __NR_fstatfs64
#define SYS_ftime __NR_ftime
#define SYS_ftruncate64 __NR_ftruncate64
#define SYS_getcpu __NR_getcpu
#define SYS_getegid32 __NR_getegid32
#define SYS_geteuid32 __NR_geteuid32
#define SYS_getgid32 __NR_getgid32
#define SYS_getgroups32 __NR_getgroups32
#define SYS_getresgid32 __NR_getresgid32
#define SYS_getresuid32 __NR_getresuid32
#define SYS_getuid32 __NR_getuid32
#define SYS_gtty __NR_gtty
#define SYS_idle __NR_idle
#define SYS_ipc __NR_ipc
#define SYS_lchown32 __NR_lchown32
#define SYS_lock __NR_lock
#define SYS_lstat64 __NR_lstat64
#define SYS_madvise1 __NR_madvise1
#define SYS_mmap2 __NR_mmap2
#define SYS_mpx __NR_mpx
#define SYS_nice __NR_nice
#define SYS_oldfstat __NR_oldfstat
#define SYS_oldlstat __NR_oldlstat
#define SYS_oldolduname __NR_oldolduname
#define SYS_oldstat __NR_oldstat
#define SYS_olduname __NR_olduname
#define SYS_prof __NR_prof
#define SYS_profil __NR_profil
#define SYS_readdir __NR_readdir
#define SYS_sendfile64 __NR_sendfile64
#define SYS_setfsgid32 __NR_setfsgid32
#define SYS_setfsuid32 __NR_setfsuid32
#define SYS_setgid32 __NR_setgid32
#define SYS_setgroups32 __NR_setgroups32
#define SYS_setregid32 __NR_setregid32
#define SYS_setresgid32 __NR_setresgid32
#define SYS_setresuid32 __NR_setresuid32
#define SYS_setreuid32 __NR_setreuid32
#define SYS_setuid32 __NR_setuid32
#define SYS_sgetmask __NR_sgetmask
#define SYS_sigaction __NR_sigaction
#define SYS_signal __NR_signal
#define SYS_sigpending __NR_sigpending
#define SYS_sigprocmask __NR_sigprocmask
#define SYS_sigreturn __NR_sigreturn
#define SYS_sigsuspend __NR_sigsuspend
#define SYS_socketcall __NR_socketcall
#define SYS_ssetmask __NR_ssetmask
#define SYS_stat64 __NR_stat64
#define SYS_statfs64 __NR_statfs64
#define SYS_stime __NR_stime
#define SYS_stty __NR_stty
#define SYS_truncate64 __NR_truncate64
#define SYS_ugetrlimit __NR_ugetrlimit
#define SYS_ulimit __NR_ulimit
#define SYS_umount __NR_umount
#define SYS_vm86 __NR_vm86
#define SYS_vm86old __NR_vm86old
#define SYS_waitpid __NR_waitpid
#endif
nanosleep()
用户程序中的睡眠:
sleep()
usleep()
nanosleep()
sleep()和nanosleep()都是使进程睡眠一段时间后被唤醒，但是二者的实现完全不同。
Linux中并没有提供系统调用sleep()，sleep()是在库函数中实现的，它是通过调用alarm()来设定报警时间，调用sigsuspend()将进程挂起在信号SIGALARM上，sleep()只能精确到秒级上。
nanosleep()则是Linux中的系统调用，它是使用定时器来实现的，该调用使调用进程睡眠，并往定时器队列上加入一个timer_list型定时器，time_list结构里包括唤醒时间以及唤醒后
执行的函数，通过nanosleep()加入的定时器的执行函数仅仅完成唤醒当前进程的功能。系统通过一定的机制定时检查这些队列（比如通过系统调用陷入核心后，从核心返回用户态前，要检查当前进程的时间片是否已经耗尽，如果是则调用schedule()函数重新调度，该函数中就会检查定时器队列，另外慢中断返回前也会做此检查），如果定时时间已超过，则执行定时器指定的函数唤醒调用进程。当然，由于系统时间片可能丢失，所以nanosleep()精度也不是很高。
alarm()也是通过定时器实现的，但是其精度只精确到秒级，另外，它设置的定时器执行函数是在指定时间向当前进程发送SIGALRM信号。
typeof(var)
是gcc对C语言的一个扩展保留字，用于声明变量类型,var可以是数据类型（int， char*..),也可以是变量表达式。
DEFINE_MY_TYPE(int, one); //It is equivalent to '__thread int my_var_'; which is a thread variable.
int main()
{
__typeof__(int *) x; //It is equivalent to 'int *x';
__typeof__(int) a;//It is equivalent to 'int a';
__typeof__(*x) y;//It is equivalent to 'int y';
__typeof__(&a) b;//It is equivalent to 'int b';
__typeof__(__typeof__(int *)[4]) z; //It is equivalent to 'int *z[4]';
y = *x;
b = &a;
z[0] = x;
z[1] = &a;
return 0;
}.检错返回值错误代码
//不为0为错误代码
#define MCHECK(ret) ({ __typeof__ (ret) errnum = (ret); \
if (__builtin_expect(errnum != 0, 0)) \
__assert_perror_fail (errnum, __FILE__, __LINE__, __func__);})__BEGIN_DECLS 和 __END_DECLS
#if defined(__cplusplus)
#define __BEGIN_DECLS extern "C" {
#define __END_DECLS }
#else
#define __BEGIN_DECLS
#define __END_DECLS
#endif扩充C语言在编译的时候按照C++编译器进行统一处理，使得C++代码能够调用C编译生成的中间代码。
由于c语言的头文件可能被不同类型的编译器读取，因此写C语言的头文件必须慎重。
我们编写代码，经常需要c和c++混合使用，为了使 C 代码和 C++ 代码保持互相兼容的过程调用接口，需要在 C++ 代码里加上 extern “C” 作为符号声明的一部分，为了简化，从而定义了上面的两个宏方面我们使用
__BEGIN_DECLS
……….
__END_DECLS
扩充编译是，这段部分按照BEGIN end 之间的进行编译
__attribute
A few standard library functions, such as abort and exit, cannot return. GCC knows this automatically. Some programs define their own functions that never return. You can declare them noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
;
翻译为：一些库函数，本身没有返回值，例如abort（）和exit（），在编译的时候，GCC自动按照这个规则编译。而对于自己定义的函数，如果你不想让它有返回值，那么可以使用__attribute__ ((noreturn))
for_eac STL
for_each用于逐个遍历容器元素，它对迭代器区间[first，last)所指的每一个元素，执行由单参数函数对象f所定义的操作。
多用assert来确定当前逻辑及程序的正确性，方便调错
C++中const迭代器 和 const_iterator的区别
const_iterator不可改变其指向的值,而iterator可以改变