Constructor being called with less arguments in C++

Question

I have class Foo with a constructor as given:

class Foo {
 public:
   Foo(int w, char x, int y, int z);
   ...
 };

int main()
{
   Foo abc (10, 'a');
}

Can I use that constructor like this? When constructor signature do not match?

So How do I give default value?

What have you tried? What is your stumbling point? Which diagnostic did you find hard to understand? — Kerrek SB, Dec 20 '11 at 13:54

score 5 · Accepted Answer · answered Dec 20 '11 at 13:54

Not unless the parameters at the tail of the signature have defaults, for example:

class Foo {
public:
    Foo(int w, char x, int y=5, int z=0);
    ...
};

If there are defaults, then you can supply only the non-defaulted parameters, and optionally some defaulted ones, i.e. any of the following invocations would be valid:

Foo abc (10, 'a');
Foo abc (10, 'a', 3);
Foo abc (10, 'a', 42, 11);

score 2 · Answer 2 · answered Dec 20 '11 at 13:53

2

You cannot, unless the missing constructor arguments have default values.

For example, you can do this:

class Foo {
 public:
   Foo(int w, char x, int y = 0, int z = 1);
   ...
 };

int main()
{
   Foo abc (10, 'a'); /* y is implicitly 0 and z is 1 */
}

answered Dec 20 '11 at 13:53

Jon

428,835
81
738
806

1

or unless constructor overload. – DonCallisto Dec 20 '11 at 13:54

score 1 · Answer 3 · answered Dec 20 '11 at 13:58

To provide default parameters, equal them to zero or else with some default value.

class Foo {
 public:
   Foo(int w, char x, int y = 0, int z = 0) { }
   // ...
 };

Or,

class Foo {
 public:
   Foo(int w, char x, int = 0, int = 0);
   // ...
 };

// Define your constructor here, note 'no default parameters'
Foo::Foo(int w, char x, int y, int z) { }

score 0 · Answer 4 · answered Dec 20 '11 at 13:53

0

No - if there is no constructor which accepts two arguments with matching types, you can't.

answered Dec 20 '11 at 13:53

user1071136

15,636
4
42
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So how do I give default value? – AJ. Dec 20 '11 at 13:54
Change your _declaration_ of the ctor to `Foo(int w, char x, int y = 10, int z = 10);`. Do not change the _definition_ (the place where you write the code itself). – user1071136 Dec 20 '11 at 14:03

score 0 · Answer 5 · edited May 23 '17 at 12:27

I'd suggest to overload the constructor rather than providing default values.

class Foo {
public:
    Foo(int w, char x); // y = 0, z = 1
    Foo(int w, char x, int y); // z = 1
    Foo(int w, char x, int y, int z);
};

It's a matter of style in the end: cons you need to duplicate the initializer list, because constructors cannot be chained, pros readability IMHO. Make sure to read this thread.

Constructor being called with less arguments in C++

5 Answers5

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