构造方法在创建（实例化）对象的时候，自动调用的方法

带参构造方法

-(instancetype)initWith...

#import <Foundation/Foundation.h>

@interface Person : NSObject{
    NSString* _name;
    NSInteger _age;
}
//init方法是系统自带的，实现在NSObject类里
//自定义的构造方法需要显示申明
- (instancetype)initWithName:(NSString*)name;
- (instancetype)initWithName:(NSString*)name age:(NSInteger)age;
-(void)showMessage;
@end

@implementation Person
-(instancetype)init{
//self是一个指针 它表示当前调用该方法的对象本身
//super 用来调用父类方法的关键字
    self = [super init];
    if(self){
        NSLog(@"系统自带的构造方法");
        _name = @"no name";
        _age = 0;
    }
    return self;
}
//自定义的需要显示申明
- (instancetype)initWithName:(NSString*)name{
    self = [super init];
    if(self){
        NSLog(@"调用自定义的构造函数");
        _name = name;
    }
    return self;
}
- (instancetype)initWithName:(NSString *)name age:(NSInteger)age{
    self = [super init];
    if(self){
        _name = name;
        _age = age;
    }
    return  self;
}
- (void)showMessage{
    NSLog(@"name:%@ age:%ld",_name,_age);
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person* p1 = [Person new];
        Person* p2 = [[Person alloc] initWithName:@"dancer"];
        Person* p3 = [[Person alloc] initWithName:@"XiaoMing" age:19];
        [p1 showMessage];
        [p2 showMessage];
        [p3 showMessage];
    }
    return 0;
}

无参构造方法

init->NSObject