About default constructor

Reply to About default constructor on Mon, 08 Aug 2011 10:51:12 GMT

goetz — Mon, 08 Aug 2011 10:51:12 GMT

The C++ standard is very clear about what a default constructor is (section 12.1, number 5):

[quote]
A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a default constructor is implicitly declared. [...]
[/quote]

Reply to About default constructor on Mon, 08 Aug 2011 10:28:49 GMT

AlterX — Mon, 08 Aug 2011 10:28:49 GMT

it's just a defination...the default constructor is always one without parameters (included optional parameters)

Reply to About default constructor on Thu, 28 Jul 2011 07:11:27 GMT

Eddy — Thu, 28 Jul 2011 07:11:27 GMT

bq. You can test it yourself. Put a debug in the constructor. Create an instance of A and see if your constructor is called or not.

The practical meaning of a default constructor is that if you don't define one yourself the compiler is making one implicitly for you.

Have a look at this example :

@class A { public:
//A(int x = 0) { v = x; cout << " test my constructor";} //just for later use
private:
int v;
}; @

So for instance when you use class A in another class B, the default constructor, implicitly created by the compiler, will be called.

@class B { public:
private:
A a;
}; @

If you uncomment line 2 of class A, you will see the cout message, meaning your constructor is called with no parameter. Thus it's a default constructor. And you as a programmer took control over it.

In this example there is no practical need to define one since the result of both will be te same. Of course in real life things aren't that simple ;)

Reply to About default constructor on Thu, 28 Jul 2011 02:16:11 GMT

Chuck Gao — Thu, 28 Jul 2011 02:16:11 GMT

@A(int x = 0) @
is a default constructor with the default argument.

if @A(int x)@ without default argument, it's not a default constructor

Reply to About default constructor on Wed, 27 Jul 2011 20:40:47 GMT

dangelog — Wed, 27 Jul 2011 20:40:47 GMT

[quote author="Edico" date="1311779819"]The definition for default constructor: "The constructor that takes no arguments is known as the default constructor."[/quote]

The definition is wrong.

http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.4

Reply to About default constructor on Wed, 27 Jul 2011 18:51:40 GMT

giesbert — Wed, 27 Jul 2011 18:51:40 GMT

It is a default constuictor as the argument is optional.

Reply to About default constructor on Wed, 27 Jul 2011 18:18:53 GMT

lgeyer — Wed, 27 Jul 2011 18:18:53 GMT

bq.
"ISO/IEC 14882:2003, 12.1.5":http://cs.nyu.edu/courses/summer11/G22.2110-001/documents/c++2003std.pdf A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a default constructor is implicitly declared.

[quote author="Edico" date="1311779819"]Is A(int x = 0); a default constructor?[/quote]

Yes.

Reply to About default constructor on Wed, 27 Jul 2011 15:46:50 GMT

Eddy — Wed, 27 Jul 2011 15:46:50 GMT

You can test it yourself. Put a debug in the constructor. Create an instance of A and see if your constructor is called or not.

Reply to About default constructor on Wed, 27 Jul 2011 15:38:07 GMT

HuXiKa — Wed, 27 Jul 2011 15:38:07 GMT

As you just said, “The constructor that takes no arguments is known as the default constructor.”
The A(int x = 0) constructor takes int x as an argument, so no, it isn't a default constructor. The default constructor would be
@class A {
public:
A();
...
@