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Xcode – Calling object without creating it, in CPP Oops

In the code below unfortunately I didn’t create/name any object and when compiling I didn’t get any error and also was able to access member function. How I need to this behaviour is this something temporary object created in stack if so if i want to access somewhere later in the code can I access same.

#include <iostream>
#include <string>

class Person {
public:
    std::string name;
    int age;

    Person(std::string n, int a) : name(n), age(a) {}
    void getInfo() {
        std::cout << "Name : " << name << std::endl;
        std::cout << "Age : " << age << std::endl;
    }
};

class Student : public Person {
public:
    int rollNo;
    Student(std::string n, int a, int r) : Person(n, a), rollNo(r) {}

    void getInfo() {
        std::cout << "Name : " << name << std::endl;
        std::cout << "Age : " << age << std::endl;
        std::cout << "Roll No : " << rollNo << std::endl;
    }
};

int main(int argc, const char* argv[]) {
    Student("Goofupp", 27, 1001).getInfo();  // does any memory also created here ?? how to understand this behaviour

    Student rahul("Rahul", 23, 1002);  // there is block of memory allocated for rahul

    rahul.getInfo();

    return 0;
}

I was expecting error saying "declaration does not define anything" thing like we get when we define like below

int main()
{
  int;     
  cout<<"hellon";     
  return 0; 
}
Tags:

2

Answers


  1. 0

    This line

    Student("Goofupp", 27, 1001).getInfo();

    calls the constructor to create an instance of Student. Its a temporary object that ceases to exist after the ; and it has no name. Because it stays alive until the ; you can call a method.

    In the code below unfortunately I didn’t create[…] any object […]

    You do create a temporary object.

    In the code below unfortunately I didn’t […]name any object […]

    Temporary objects don’t need a name. There is no need for a name because anyhow you cannot refer to the object after its lifetime ended.

    I was expecting error saying "declaration does not define anything"

    There is no such error, because the line is not a declaration.

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  2. 0

    Its because we can explicitly call constructors of a class. That’s why the line :

    Student("Goofupp",27,1001).getInfo();

    worked correctly.
    The logic is very simple. It just first calls the relevent constructor, in your case, it was :

    Student(string n, int a, int r);

    Memory for object was allocated due to call to its constructor. Then on that object, you called the method getInfo(). getInfo() gives some prints some output and then the function ends. The object wasn’t stored anywhere so it goes out of scope and got destroyed by destructor after that line.

    The anwser to your 2nd question is that this was temporary, so you can’t use it later in the code. To use this, you can name it, assign to another Student object or maybe pass to another function:

    class Student { 
    public:
        //Provide a constructor
        Student (string _name, int _age, int _rollNo) : name(_name), age(_age), rollNo(_rollNo) {}

        void getInfo() {
            /* Give your implementation */
        }

    private:
        string name;
        int age;
        int rollNo;
};

void doSomething(Student s) {
    //Assuming this calls getInfo method
    s.getInfo();
}

int main() {

    Student s1 = Student("Example1", 20, 1000); //Assign it to other Student object
    Student s2("Example2", 20, 1001); //Give it a name
    
    doSomething(Student("Example3", 20, 1002)); //Or pass it to other function

    return 0;
}
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