C + + variable parameter function __jquery

Va_list is a group of macros that solve the problem of variable parameter in C language.
He has so many members:

1) va_list type variable:

#ifdef  _m_alpha
typedef struct {
        char *a0;       /* Pointer to the homed integer argument
        /int offset;     /* byte offset of next parameter *
/} va_list;
#else
typedef char *  va_list;
#endif

2 _intsizeof macros, gets the space length occupied by the type, and takes a minimum length of integral times int:

#define _INTSIZEOF (n)   (sizeof (n) + sizeof (int)-1) & ~ (sizeof (int)-1))

3 Va_start macros, gets the address of the first parameter of the variable argument list (the AP is a pointer of type va_list, and V is the leftmost parameter of the variable parameter):

#define VA_START (AP,V)  (AP = (va_list) &v + _intsizeof (v))

4 VA_ARG the macro, gets the current parameter of the variable parameter, returns the specified type and points the pointer to the next parameter (the T parameter describes the type of the current parameter):

#define VA_ARG (ap,t) (    * (t *) (AP + _intsizeof (t))-_intsizeof (t))

5 va_end macros, empty va_list variable parameter list:

#define VA_END (AP)      (AP = (va_list) 0)

Second, the use of va_list:

(1) First in the function defines a va_list type of variable, which is the pointer to the parameter;
(2) then using the Va_start macro to initialize the variable just defined by the va_list variable;
(3) Then return the variable parameter with Va_arg, and the second parameter of the va_arg is the type of the parameter you want to return (if the function has more than one variable parameter, call Va_arg to fetch each parameter sequentially);
(4) Finally using the Va_end macro to end the acquisition of variable parameters.

Third, the use of va_list should pay attention to the problem:
(1) The type and number of variable parameters are completely controlled by program code, which can not recognize the number and type of different parameters intelligently;
(2) If we do not need one by one detailed each parameter, just copy the variable list to a buffer, available vsprintf function;
(3) because the compiler for the variable parameters of the function of the prototype check is not strict enough for error-checking programming. Bad for us to write high quality code;

Summary: The function principle of variable parameters is actually very simple, and the VA series is defined as a macro definition, and the implementation is related to the stack. We write a variable parameter of the C function, there are advantages and disadvantages, so in unnecessary situations, we do not need to use variable parameters, if in C + +, we should use C + + polymorphism to achieve variable parameter function, try to avoid using the C language method to achieve.