CODEWITHSUNDEEP
c++default-constructor
SoraMarch
SoraMarch

Reputation: 1

Calling an object from a class with Default Constructor

I'm wondering what a call to an object is supposed to display. I have a class called big_number that has a few different constructors. In another method, I'm declaring an object 'a' using class big_number as follows:

big_number a;

cout << "Default constructor gives " << a << endl;

And my constructor is:

big_number::big_number()
{
    head_ptr = 0;
    tail_ptr = 0;
    positive = false;
    digits = 0;
    base = 10;
}

(Although I'm sure that this constructor is wrong).

The full code of the testing file:

    int main()
{
    int n1, n2;
    unsigned int base;
    string s;

    char choice;
    do
    {
        cout << "Type 'd' to test default constructor" << endl;
        cout << "Type 'i' to test int constructor" << endl;
        cout << "Type 's' to test string constructor" << endl;
        cout << "Type 'a' to test assignment" << endl;
        cout << "Type '>' to test input operator" << endl;
        cout << "Type '=' to test comparison operators" << endl;
        cout << "Type 'q' to quit" << endl;
        cin >> choice;

        if (toupper(choice) == 'D')
        {
            big_number a;
            cout << "Default constructor gives " << a << endl;
        }
//More Code

Upvotes: 0

Views: 66

Answers (1)

Oguk
Oguk

Reputation: 1137

If by "call to an object" you mean your call of the operator<< on the object a with cout as the stream argument: It displays whatever you define it to display (in a member function of big_number or a free function). There is no "default" operator<< for user-defined classes. So, if you define it like

#include <iostream>

struct big_number
{
    template<typename T>
    friend T& operator<< (T& stream, const big_number& bn);
};

template<typename T>
T& operator<<(T& stream, const big_number& bn) {
    return (stream << "Hello World");
}

int main()
{
    big_number a;
    std::cout << a << std::endl;
}

... it will just display "Hello world".

The purpose of making it a friend function is so it can access the private data members of big_number (since you usually want it to display something that depends on the data stored in the object, and not a constant "Hello world"). So, within the operator<< definition, you would probably iterate through the digits in your linked list and push them to the stream (if I understand correctly what you are trying to do).

Upvotes: 1

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