std::nothrow

std::nothrow

1、在内存不足时，new (std::nothrow)并不抛出异常，而是将指针置NULL。

若不使用std::nothrow,则分配失败时程序直接抛出异常。

2、使用方式：

 #include <new>
 #include <iostream> // for std::cerr
 #include <cstdlib> // for std::exit()
 Task * ptask = new (std::nothrow) Task;
 if (!ptask)
 {
 std::cerr<<"allocation failure!";
 std::exit();
 }
 //... allocation succeeded; continue normally   

 #include <new>
 std::nothrow_t nt;
 Task * ptask = new (nt) Task; //user-defined argument
 if (!ptask)
 //...   

3、分配失败是非常普通的，它们通常在植入性和不支持异常的可移动的器件中发生更频繁。因此，应用程序开发者在这个环境中使用nothrow new来替代普通的new是非常安全的。

4、一种宏实现的方式，取自ACE：

 #   define ACE_nothrow   ::std::nothrow
 #   define ENOMEM
 #   define errno (* (ACE_CE_Errno::instance ()))

 #if defined (ACE_NEW_THROWS_EXCEPTIONS)
 #  if defined (ACE_HAS_NEW_NOTHROW)
 #    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) \
    do { POINTER = new (ACE_nothrow) CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; return RET_VAL; } \
    } while ()
 #    define ACE_NEW(POINTER,CONSTRUCTOR) \
    do { POINTER = new(ACE_nothrow) CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; return; } \
    } while ()
 #    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
    do { POINTER = new(ACE_nothrow) CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; } \
    } while ()

 #  else

 #    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) \
    do { try { POINTER = new CONSTRUCTOR; } \
      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = ; return RET_VAL; } \
    } while ()

 #    define ACE_NEW(POINTER,CONSTRUCTOR) \
    do { try { POINTER = new CONSTRUCTOR; } \
      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = ; return; } \
    } while ()

 #    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
    do { try { POINTER = new CONSTRUCTOR; } \
      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = ; } \
    } while ()
 #  endif /* ACE_HAS_NEW_NOTHROW */

 #else /* ACE_NEW_THROWS_EXCEPTIONS */

 # define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) \
    do { POINTER = new CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; return RET_VAL; } \
    } while ()
 # define ACE_NEW(POINTER,CONSTRUCTOR) \
    do { POINTER = new CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; return; } \
    } while ()
 # define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
    do { POINTER = new CONSTRUCTOR; \
      if (POINTER == ) { errno = ENOMEM; } \
    } while ()
 #endif

使用时：

 ACE_Thread_Descriptor *thr_desc = ;
 ACE_NEW_RETURN (thr_desc,
                       ACE_Thread_Descriptor,
                       -);