__thread是GCC内置的线程局部存储设施,__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，但是各线程独立不干扰的变量；
只能修饰POD类型(类似整型指针的标量)，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。
#include <pthread.h>#include <cstdio>#include <cstdlib>#include <assert.h>#include <stdint.h>__thread uint64_t pkey = 0;void run2( ){
FILE* fp = NULL; if( !pkey )
{ char fName[128] = ""; sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
fp = fopen( fName, "w" );
pkey = reinterpret_cast<uint64_t>( fp );
}else fp = reinterpret_cast<FILE*>( pkey ); fprintf( fp, "hello __thread 2\n" ); return ;
}void* run1( void* arg ){
FILE* fp = NULL; if( !pkey )
{ char fName[128] = ""; sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
fp = fopen( fName, "w" );
pkey = reinterpret_cast<uint64_t>( fp );
}else fp = reinterpret_cast<FILE*>( pkey ); fprintf( fp, "hello __thread 1\n" );
run2(); return NULL;
}int main(int argc, char const *argv[]){ char fName[128] = ""; sprintf( fName, "thread%lu.log", static_cast<unsigned long>( pthread_self() ) );
FILE* fp = fopen( fName, "w" );
pkey = reinterpret_cast<uint64_t>( fp ); fprintf( fp, "hello __thread\n" ); pthread_t threads[2];
pthread_create( &threads[0], NULL, run1, NULL );
pthread_create( &threads[1], NULL, run1, NULL );
pthread_join( threads[0], NULL );
pthread_join( threads[1], NULL ); return 0;
}pthread_key_t
参考：关键字：__thread & pthread_key_t
pthread_key_t 优于 __thread 从下面几个方面来说：
依赖 linux 环境的 libpthread， 而非 gcc 编译器可移植性增强
如上所示，可以认为对每个 pthread_key, 库内部提供了一个 __thread void* 接受 pthread_setspecific 设置的指针，从而可以指向 class 类型
pthread_key_t 可以作为函数的局部变量，也可以作为局部变量。
#include <pthread.h>
// pthread_key_t, pthread_setspecific, pthread_getspecific, pthread_self
// pthread_key_create, pthread_key_delete, pthread_create, pthread_join#include <iostream>#include <cstdio>#include <cstdlib>using namespace std;
static pthread_key_t pkt;// 1, callback function to destroy resource associated with key// 2, the in_param is pthread_getspecific()// 3, gettid()是内核给线程（轻量级进程）分配的进程id，全局（所有进程中）唯一// 4, pthread_self()是在用户态实现的，获取的id实际上是主线程分配给子线程的线程描述符的地址而已，只是在当前进程空间中是唯一的。void destroy( void *arg ){ printf("exit at thread %d, fclose file \n", static_cast<int>( pthread_self() ) ); if( arg ) fclose( reinterpret_cast<FILE*>(arg) );
}// 5, pthread_getspecific() Return current value of the thread-specific data slot identified by KEY.void writeLog( const char* log ){
FILE* logHandle = reinterpret_cast<FILE*>( pthread_getspecific( pkt) ); fprintf( logHandle, "%s\n", log );
}// 6, pthread_setspecific Store POINTER in the thread-specific data slot identified by KEY void* work( void* arg){
FILE* logHandle = NULL; char fileName[128] = ""; sprintf( fileName, "Thread%d.log", static_cast<int>(pthread_self()) );
logHandle = fopen( fileName, "w");
pthread_setspecific( pkt, reinterpret_cast<void*>( logHandle ) );
writeLog( "Thread starting." );
}// 7, pthread_key_create( &pkt, destroy ) Create a key value identifying a location in the thread-specific //identifying 识别// data area. Each thread maintains a distinct thread-specific data area.// the destroy callback function will called with the key is dectroyed// 8, pthread_key_delete( ) detroy the key use callback function clear the resourceint main(int argc, char const *argv[]){
pthread_key_create( &pkt, destroy ); pthread_t pids[2] = {0};
pthread_create( &pids[0], NULL, work, NULL );
pthread_create( &pids[1], NULL, work, NULL );
pthread_join( pids[0], NULL );
pthread_join( pids[1], NULL );
pthread_key_delete( pkt ); printf("stop\n"); return 0;
}ThreadLocal
参考：关键字：__thread & pthread_key_t
对 pthread_key_t 进行了 RAII 的封装，使用更加安全。
#include <pthread.h>#include <boost/noncopyable.hpp> // noncopyable#include <boost/checked_delete.hpp> // check_delete#include <cstdio>#include <cstdlib>#include <string>#include <stdexcept>template<typename T>class ThreadLocal : public boost::noncopyable
{ public: typedef ThreadLocal<T>* pThreadLocal;
ThreadLocal()
{ pthread_key_create( &pkey_, &ThreadLocal::destroy ); }
~ThreadLocal()
{ pthread_key_delete( pkey_ ); } T& value()
{
T* pvalue = reinterpret_cast<T*>( pthread_getspecific( pkey_ ) ); if( !pvalue )
{
T* obj = new T();
pthread_setspecific( pkey_, reinterpret_cast<void*>( obj ) );
pvalue = obj;
} return *pvalue;
} private: static void destroy( void* arg )
{
T* obj = reinterpret_cast<T*>( arg );
boost::checked_delete( obj );
} pthread_key_t pkey_;
};class Logger{
public:
Logger()
{ char fName[128] = ""; sprintf( fName, "log_%lu.log", static_cast<unsigned long>( pthread_self() ) );
fp = fopen( fName, "w" ); if( !fp ) throw std::runtime_error( std::string("can not create ") + fName );
}
~Logger() { fclose( fp ); } void log( const std::string& s ) { fprintf( fp, "%s\n", s.c_str() ); } private:
FILE* fp;
};void* run( void* arg ){ auto ptllogger = reinterpret_cast< ThreadLocal<Logger>::pThreadLocal>( arg);
Logger& plogger = ptllogger->value();
plogger.log( "Hello thread local" );
}int main(){
ThreadLocal<Logger>::pThreadLocal p = new ThreadLocal<Logger>;
Logger& plogger = p->value();
plogger.log( "Hello thread local" ); pthread_t threads[2] = {0};
pthread_create( &threads[0], NULL, run, reinterpret_cast<void*>( p ) );
pthread_create( &threads[1], NULL, run, reinterpret_cast<void*>( p ) );
pthread_join( threads[0], NULL );
pthread_join( threads[1], NULL ); delete p;
}附录
C++ 获取类中成员函数的函数指针
class A {public: static void staticmember(){cout<<"static"<<endl;} //static member
void nonstatic(){cout<<"nonstatic"<<endl;} //nonstatic member
virtual void virtualmember(){cout<<"virtual"<<endl;};//virtual member};int main(){
A a; //static成员函数,取得的是该函数在内存中的实际地址，而且因为static成员是全局的，所以不能用A::限定符
void (*ptrstatic)() = &A::staticmember;
//nonstatic成员函数 取得的是该函数在内存中的实际地址
void (A::*ptrnonstatic)() = &A::nonstatic; //虚函数取得的是虚函数表中的偏移值，这样可以保证能过指针调用时同样的多态效果
void (A::*ptrvirtual)() = &A::virtualmember; //函数指针的使用方式
ptrstatic();
(a.*ptrnonstatic)();
(a.*ptrvirtual)();
}static_cast, dynamic_cast, reinterpret_cast， const_cast
参见：c++ 数据类型转换： static_cast dynamic_cast reinterpret_cast const_cast
上行转换（把子类的指针或引用转换成基类表示）, 下行转换（把基类指针或引用转换成子类表示）
类指针或引用的上行转换static_cast 和 dynamic_cast 都可以
类指针或引用的下行转换用dynamic_cast并且判断转换后是否为空
基本数据类型之间的转换用static_cast, 但是由于数值范围的不同，需要用户保证转换的安全性
不同类型之间的指针或引用的转换用reinterpret_cast，它的本质是对指向内存的比特位的重解释
消除数据的const、volatile、__unaligned属性，用const_cast
作者：呆呆的张先生
链接：https://www.jianshu.com/p/495ea7ce649b