C variadic function: How to specify which type to give to va_arg

问题

In a function like printf, we use stdarg.h to handle the variadic parameters.

void print(int args,...){
    va_list ap;
    va_start(ap, args);
    int i = 0;
    for(i=0; i<args; i++){
        printf("%d\n",va_arg(ap, int));
    }
    va_end(ap);
}

We want to parse the format list (the first argument given to our variadic function) to track the types of the arguments specified in the format list then, call va_arg with the appropriate type.

I make a first loop to parse the format list, store the specifiers letters into an array. So I know which type we expect and how many there is.

ex: ft_like_printf("Watch your %d %s\n", 6, "Spider pig");

specifiers_list = "ds" So d <=> int and s <=> char* (same specifiers as printf)

But how to code it dynamically? What is the syntax to call va_arg with differents types ?

I have read THIS and THAT which I think are what I'm looking for, aren't they ? If yes, what to do with it ? What are the real case scenarios of a struc containing an enum + union or struct containing an union + function pointer ?

To handle the different data types, I had started with this:

typedef struct s_flist
{
    char c;
    (*f)();
}              t_flist;

t_flist flist[] = 
    {
        { 's',  &putstr  },
        { 'i',  &put_number },
        { 'd',  &put_number }
    };

回答1:

Types are not first-class citizens in C.

But there are not that many types you have to care about: you can safely cast from unsigned to signed and the other way around, same goes for char* and void*, so for a basic printf, you have to handle:

char
short
int
long
float
double
void*

union to the rescue !

typedef union
{
    char as_char;
    short as_short;
    int as_int;
    long as_long;
    float as_float;
    double as_double;
    void* as_ptr;
} t_value;

typedef enum {
    CHAR_T,
    INT_T,
    /* It goes on */
    ...
} t_type;

t_value get_value(va_list ap, t_type type) {
    /* You can't avoid this step, there is no way to iterate over types */
    switch (type) {
        case T_CHAR:
            return va_arg(ap, char);
        case T_INT:
            /* ... */
    }
}

Then you just have to create a lookup table, storing both a function pointer and a t_type for each valid format specifier.

来源：https://stackoverflow.com/questions/27483614/c-variadic-function-how-to-specify-which-type-to-give-to-va-arg

标签

variadic-functions