Implementation of optional parameters in Objective-c

There are many APIs for optional parameters in Objective-C, such as:

-(instancetype) initWithFormat: (NSString *) format, ... NS_FORMAT_FUNCTION (1,2); <pre name = "code" class = "objc">-(instancetype) initWithTitle: (NSString *) title message: (NSString *) message delegate: (id / * <UIAlertViewDelegate> * /) delegate cancelButtonTitle: (NSString *) cancelButtonTitle otherButtonTitles: (NSString *) otherButtonTitles, ...;
We also sometimes need to write some variable parameters, such as network requests to pass variable parameters, database parameters to perform variable parameter queries, and so on.
One way to define a Man class is to make a big meal.

// initialize and define methods
@interface Man: NSObject
-(NSString *) makeMilk: (NSString *) milk fruit: (NSString *) fruit food: (NSString *) food, ...;
@end
The syntax of a variable parameter is that the first is a fixed parameter, the last is a variable parameter, the type of the variable parameter is the same, and it ends with a comma and an ellipsis.

This method has two parameters (milk and fruit) that are fixed, and the following food can be long or short, depending on the situation.
-(NSString *) makeMilk: (NSString *) milk fruit: (NSString *) fruit food: (NSString *) food, ...;
{
    NSMutableArray * arr = [[NSMutableArray alloc] init];
    va_list params; // Define a pointer to a variable number of parameter lists;
    id argument;
    if (food) {
        // Make the parameter list pointer arg_ptr point to the first optional parameter in the function parameter list. Explanation: argN is a fixed parameter before the first optional parameter, (or, the last fixed parameter; a parameter before ... ), The order of the parameters in the function parameter list in memory is the same as the order of the function declaration. If the declaration of a va function is void va_test (char a, char b, char c,…), then its fixed parameters are a, b, c, and the last fixed parameter argN is c, so it is va_start (arg_ptr, c ).
        va_start (params, food);
        while ((argument = va_arg (params, id))) {// Returns the parameter pointed by the pointer arg_ptr in the parameter list, the return type is type, and the pointer arg_ptr points to the next parameter in the parameter list
            [arr addObject: argument];
        }
        va_end (params); // Release list pointer
    }
    return [NSString stringWithFormat: @ "% @ _% @ _% @", milk, fruit, [arr componentsJoinedByString: @ "_"]];
}
when using it:
Man * man = [[Man alloc] init];
[man makeMilk: @ "马奶" fruit: @ "Apple" food: @ "鱼儿", @ "肉 儿", @ "鸡 儿", @ "鸭 儿", @ "雁儿", nil];
// horse milk _ apple _ meat _ chicken _ duck _ goose


Implementation of optional parameters in Objective-C