Why method is not getting called - objective-c

Question

I'm trying to learn how to make a class and object and how to call methods in Objective-C. My small program creates an object of the City class, allows to name that object, set an age, population, and get these values to print. But when I call a method to set these values, I get a (null) and zeros in result. Here's my code:
City.h
#import <Foundation/Foundation.h>
#interface City : NSObject
-(void) setName:(NSString *)theName Age:(int)theAge Population:(int)thePopulation;
-(void) getName;
-(void) getAge;
-(void) getPopulation;
-(void) nextDay;
#end
City.m
#import "City.h"
#implementation City
{
NSString *name;
int age;
int population;
}
-(void) setName:(NSString *)theName Age:(int)theAge Population:(int)thePopulation
{
theName = name;
theAge = age;
thePopulation = population;
}
-(void) getName
{
NSLog(#"Name is %#", name);
}
-(void) getAge
{
NSLog(#"Age is %d", age);
}
-(void) getPopulation
{
NSLog(#"Population today is %d", population);
}
main.m
int main()
{
City *moscow = [[City alloc] init];
[moscow setName:#"Msk" Age:120 Population:1000];
[moscow getName];
[moscow getAge];
[moscow getPopulation];
}
the result of the running is:
Name is (null)
Age is 0
Population today is 0
Program ended with exit code: 0
what am I doing wrong?

Answer 1

The problem is that the instance variables of City is never set. The code in setName:Age:Population: assigns the values of the instance variables (name, age, and population) to the arguments variables (theName, theAge, and thePopulation). Swapping these will cause the setter to assign the arguments to the instance variables:
name = theName;
age = theAge;
population = thePopulation;
That said, it's more idiomatic Objective-C to use properties—instead of instance variables and manual getters and setters—and to use an initializer to set the initial values. With those changes the City class would look something like this:
City.h
NS_ASSUME_NONNULL_BEGIN
#interface City : NSObject
#property (copy) NSString *name;
#property (assign) NSInteger age;
#property (assign) NSInteger population;
- (instancetype)initWithName:(NSString *)name
age:(NSInteger)age
population:(NSInteger)population;
#end
NS_ASSUME_NONNULL_END
City.m
#import "City.h"
#implementation City
- (instancetype)initWithName:(NSString *)name
age:(NSInteger)age
population:(NSInteger)population
{
self = [super init];
if (self) {
_name = [name copy];
_age = age;
_population = population;
}
return self;
}
#end
Two things to note about this code:
The string is copied—both in the initializer and in the property—to protect against having a NSMutableString is passed and later be mutated (which would mutate the value of name as well. For the common case where an immutable NSString is passed, the copy is equivalent to a "retain".
The synthesized instance variables are used when assigning values in the initializer. This is to protect against having a subclass override any of these properties and have a custom setter method run before the object is fully initialized (have all its variables set to their initial values). This only applies to initializers, custom setters, and dealloc. Everything else should use the properties to access and modify these values.