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(1);  
}  
//... 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          12
#   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 == 0) { errno = ENOMEM; return RET_VAL; } \
   } while (0)
#    define ACE_NEW(POINTER,CONSTRUCTOR) \
   do { POINTER = new(ACE_nothrow) CONSTRUCTOR; \
     if (POINTER == 0) { errno = ENOMEM; return; } \
   } while (0)
#    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
   do { POINTER = new(ACE_nothrow) CONSTRUCTOR; \
     if (POINTER == 0) { errno = ENOMEM; } \
   } while (0)

#  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 = 0; return RET_VAL; } \
   } while (0)

#    define ACE_NEW(POINTER,CONSTRUCTOR) \
   do { try { POINTER = new CONSTRUCTOR; } \
     catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; return; } \
   } while (0)

#    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
   do { try { POINTER = new CONSTRUCTOR; } \
     catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; } \
   } while (0)
#  endif /* ACE_HAS_NEW_NOTHROW */

#else /* ACE_NEW_THROWS_EXCEPTIONS */

# define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) \
   do { POINTER = new CONSTRUCTOR; \
     if (POINTER == 0) { errno = ENOMEM; return RET_VAL; } \
   } while (0)
# define ACE_NEW(POINTER,CONSTRUCTOR) \
   do { POINTER = new CONSTRUCTOR; \
     if (POINTER == 0) { errno = ENOMEM; return; } \
   } while (0)
# define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) \
   do { POINTER = new CONSTRUCTOR; \
     if (POINTER == 0) { errno = ENOMEM; } \
   } while (0)
#endif

使用时：

ACE_Thread_Descriptor *thr_desc = 0;
ACE_NEW_RETURN (thr_desc,
                      ACE_Thread_Descriptor,
                      -1);