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Q&A

Print value from constructor

+2
−1
#include<iostream>
using namespace std;

class Book{
	public:
		string title;
		string author;
		int pages;
		Book(string aTitle, string aAuthor, int aPages){
			aTitle = title;
			aAuthor = author;
			aPages = pages;
			
		}
};

int main(){
	Book book1("Harry","JK",500);
	
	Book book2("Lord","Tolkein",450);
	cout << "Title : " << book1.title << "; Author : " << book1.author << "; Pages : " << book1.pages;
	return 0;
}

This outputs

Title : ; Author : ; Pages : 404155672%

The output I expected :

Title : Harry; Author : JK; Pages : 500;

I had tried to get the same output using printf also.

printf("Title : %s",book1.title);

This only outputs,

Title : ;

String from constructor doesn't show properly. Even, in first code Integer had converted to hexadecimal.

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1 comment thread

Your assignments in the constructor are "upside down". (1 comment)

1 answer

+4
−0

That's because of this part of the code:

Book(string aTitle, string aAuthor, int aPages){
	aTitle = title;
	aAuthor = author;
	aPages = pages;
}

You're attibuting the properties from the class to the function parameter values. As the properties aren't initialized yet, they're getting just random values. It should have been the oposite. Like this:

Book(string aTitle, string aAuthor, int aPages) {
	title = aTitle;
	author = aAuthor;
	pages = aPages;
}

you can also use member initializer syntax:

Book(string aTitle, string aAuthor, int aPages) : title(aTitle), author(aAuthor), pages(aPages) {

}

or like this:

Book(string aTitle, string aAuthor, int aPages) : title{aTitle}, author{aAuthor}, pages{aPages} {

}
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