Question about passing class as alias parameter in C++?

Question

Possible Duplicate:
Why should the copy constructor accept its parameter by reference in C++?

I have the following code:

class Student {
private:
    int no;
    char name[14];
public:
    void display() const;
    Student(const Student& student); // Line 1
};

I have read that class is a reference type, so why in Line 1 of the above code declared as alias. Is Line 1 equivalent to: Student(const Student student); ?

"I have read that class is a reference type" Why are you reading a C# book instead of a [C++ book](http://tinyurl.com/so-cxxbooks)? — R. Martinho Fernandes, Sep 07 '11 at 14:05

score 5 · Accepted Answer · edited May 23 '17 at 12:34

5

I have read that class is a reference type

In C++, the assertion that “class is a reference type” makes no sense. You may have heard this in connection with C# but this is a completely different matter.

Consequently, the whole discussion is moot. To understand the syntax of copy constructors you first need to understand references (those do exist in C++) and classes in general. Grab a good C++ book for beginners.

edited May 23 '17 at 12:34

Community

1
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answered Sep 07 '11 at 13:59

Konrad Rudolph

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"Reference type" makes perfect sense: that's what `const Student&` is. – Mike Seymour Sep 07 '11 at 15:00
@Mike True. In the context of OP’s text, however, it was used in a way that clearly comes from C#’s usage – to contrast reference (classes) and value (struct) types. I’ll amend my answer. – Konrad Rudolph Sep 07 '11 at 15:25

score 2 · Answer 2 · answered Sep 07 '11 at 13:58

First I would use

std::string name

instead of

char name[14]

and Student(const Student student); is NOT the same as Student(const Student& student);

Student(const Student student) //is a copy
Student(const Student& student) // is a reference to the object

score 2 · Answer 3 · answered Sep 07 '11 at 13:59

2

Classes are not reference types. You probably got that from C# or something. You have to add & to a type name to make it a reference. So Student is a student object, whereas Student& is a reference to a Student object.

answered Sep 07 '11 at 13:59

Seth Carnegie

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Wow, my edit didn't get saved... My answer was way longer than this... – Seth Carnegie Sep 07 '11 at 14:12

score 1 · Answer 4 · answered Sep 07 '11 at 14:04

It is not at all the same code.

Passing a parameter by reference will push the exact object on the function's argument stack. Passing one by value will push a copy.

If you declare your copy constructor without the reference, you'll get an infinite loop, as the program will try to create a copy calling the copy constructor, which in turn will try to create a copy and so on.

score 0 · Answer 5 · answered Sep 07 '11 at 14:02

Class is not a reference type. It is a user defined type in C++. When ever we use & as a part of (member) function argument(s), it means we receive a reference of the parameter(s) passed. Any modification made to the reference will actually modify the passed parameter itself. While in the other case, a copy of the object is made.

score 0 · Answer 6 · answered Sep 07 '11 at 14:02

Class is a reference type in many managed languages (like Java or C#). It is not the case however in C++. If you write Student(const Student student); it means you define a constructor taking one parameter of type Student by value. In essence copying the object into the scope of the function.

Also in C++ we call syntax like this: Student(const Student& student); passing by reference, not alias (in essence the same, but it may be confusing when reading up on some related subject).

Question about passing class as alias parameter in C++?

6 Answers6