练习作品：
// 最多可以存放 56 个数值的容器
template< typename T >
class SafeValuePool
{
public:
SafeValuePool() : _mark(0) {}
uint8 lock()
{
for( ;; ){
for( int i=0; i<56; ++i ){
assert( (_mark & 0xFFFFFFFFFFFFFF00LL)== 0xFFFFFFFFFFFFFF00LL ); // _mark 的最后8位用来防止 ABA
uint64 v=_mark;
if(( v & (1LL << (i+8) )==0 ){
uint64 n=(v | (1LL << (i+8))) + 1) & 0xFFFFFFFFFFFFFF7FLL;
if( CAS2( &_mark, v, n ) )
return i;
}
}
}
}
T& operator[]( uint8 index )
{
return _pool[index];
}
bool test( uint8 index )
{
return (_mark & (1LL << (index+8))!=0);
}
void unlock( uint8 index )
{
do{
uint64 v=_mark;
uint64 n=(( v & ~( 1LL << index ) ) + 1 ) & 0xFFFFFFFFFFFFFF7FLL;
}while( !CAS2( &_mark, v, n ) );
}
private:
volatile uint64 _mark;
T _pool[56];
};
