Objective-C : Create and using Objects [duplicate] - objective-c

Question

This question already has answers here:
What is the difference between class and instance methods?
(18 answers)
Closed 9 years ago.
I'm a c++ & c programmer , and i'm new to the world of objective-C , so i have some problem
understanding how it works , here a short code , that confused me,
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
#autoreleasepool {
NSDate *now = [NSDate date];
NSLog(#"The date is %#", now);
double seconds = [now timeIntervalSince1970];
NSLog(#"It has been %f seconds since the start of 1970.", seconds);
}
return 0; }
now is pointer to an object type NSdate my question is why we can not do this :
double seconds = [NSDate timeIntervalSince1970];
normally the first part is the type of the object and the second part is the method
i'm sorry if this is a bad question but i want to understand Objective-C very well from the begining.
Thanks

Answer 1

You can do something similar with
[NSDate timeIntervalSinceReferenceDate];
Although the reference date in this case is 1 January 2001
But, this is a class method. You can call it on the class.
The other methods, such as timeIntervalSince1970 are instance methods, which need to be called on actual objects of the class. In NSDate's case there is no class method for the time interval since 1970.
If you really want to, you can add a Category on to NSDate and add a class method that does this.

Answer 2

This is a class method:
NSDate *now = [NSDate date];
You don't need an instance of the object.
This is an instance method:
[now timeIntervalSince1970];
And you need and instance of the object.
Same in C++ as: Class::classMethod() and myClass::instanceMethod()
More info here:
https://softwareengineering.stackexchange.com/questions/191856/what-is-a-static-method-compared-to-instance-class-private-public-methods