What is difference between String str and String s equals new String?

Answer 1

Btw, this is a C++ question.

in 'String str', str is an object and most probabily (based on most String implementations) we can use

str = "my name";

int len = str.length();

String s = new String; <== this is something wrong.

it should be

String *s = new String;

here 's' is a pointer and the object is allocated/instanciated by 'new' operator

you can use it

*s = "my name"

int len = s->length()

Answer 2

The first statement, "String str", simply declares a variable named str of type String. It does not allocate memory or assign any value to the variable.

The second statement, "String s = new String", declares a variable named s of type String and allocates memory for it. It also creates a new instance of the String class and assigns it to the variable s.