std::nothrow - 走看看

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：

 1 #include <new>  
 2 #include <iostream> // for std::cerr  
 3 #include <cstdlib> // for std::exit()  
 4 Task * ptask = new (std::nothrow) Task;  
 5 if (!ptask)  
 6 {  
 7 std::cerr<<"allocation failure!";  
 8 std::exit(1);  
 9 }  
10 //... allocation succeeded; continue normally

1 #include <new>  
2 std::nothrow_t nt;  
3 Task * ptask = new (nt) Task; //user-defined argument  
4 if (!ptask)  
5 //...

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：

 1 #   define ACE_nothrow   ::std::nothrow
 2 #   define ENOMEM          12
 3 #   define errno (* (ACE_CE_Errno::instance ()))
 4 
 5 #if defined (ACE_NEW_THROWS_EXCEPTIONS)
 6 #  if defined (ACE_HAS_NEW_NOTHROW)
 7 #    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) 
 8    do { POINTER = new (ACE_nothrow) CONSTRUCTOR; 
 9      if (POINTER == 0) { errno = ENOMEM; return RET_VAL; } 
10    } while (0)
11 #    define ACE_NEW(POINTER,CONSTRUCTOR) 
12    do { POINTER = new(ACE_nothrow) CONSTRUCTOR; 
13      if (POINTER == 0) { errno = ENOMEM; return; } 
14    } while (0)
15 #    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) 
16    do { POINTER = new(ACE_nothrow) CONSTRUCTOR; 
17      if (POINTER == 0) { errno = ENOMEM; } 
18    } while (0)
19 
20 #  else
21 
22 #    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) 
23    do { try { POINTER = new CONSTRUCTOR; } 
24      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; return RET_VAL; } 
25    } while (0)
26 
27 #    define ACE_NEW(POINTER,CONSTRUCTOR) 
28    do { try { POINTER = new CONSTRUCTOR; } 
29      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; return; } 
30    } while (0)
31 
32 #    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) 
33    do { try { POINTER = new CONSTRUCTOR; } 
34      catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; } 
35    } while (0)
36 #  endif /* ACE_HAS_NEW_NOTHROW */
37 
38 #else /* ACE_NEW_THROWS_EXCEPTIONS */
39 
40 # define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) 
41    do { POINTER = new CONSTRUCTOR; 
42      if (POINTER == 0) { errno = ENOMEM; return RET_VAL; } 
43    } while (0)
44 # define ACE_NEW(POINTER,CONSTRUCTOR) 
45    do { POINTER = new CONSTRUCTOR; 
46      if (POINTER == 0) { errno = ENOMEM; return; } 
47    } while (0)
48 # define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) 
49    do { POINTER = new CONSTRUCTOR; 
50      if (POINTER == 0) { errno = ENOMEM; } 
51    } while (0)
52 #endif

使用时：

1 ACE_Thread_Descriptor *thr_desc = 0;
2 ACE_NEW_RETURN (thr_desc,
3                       ACE_Thread_Descriptor,
4                       -1);