Error in simple C++ inheritance

Question

I get

error: no matching function for call to 'Base::Base()'

in code

class Base {
private:
     char *field;
public:
    Base(char *c){
        field = c;
    }
};

class Derived : public Base {
public:
    Derived(char *c){}
};

Error disappears as soon as I add

Base() {}

constructor. Why does the C++ compiler strictly require a Base() constructor with no arguments? What if creating a Base object with no arguments makes no sense?

P.S. For example, I do not have the same error in similar Java code because I would have to add

super("")

as a first statement of body of Derived constructor. And that is really reasonable.

"What if creating Base object with no arguments makes no sense?" then your Derived class makes no sense. — R. Martinho Fernandes
– R. Martinho Fernandes, Commented May 10, 2013 at 16:44
"What if creating Base object with no arguments makes no sense?" Well, then don't do that? — Nik Bougalis
– Nik Bougalis, Commented May 10, 2013 at 16:47
What where you thinking when posting this HTML cluttered mess? Stackoverflow handles the syntax highlighting very well if you give it a chance ... — filmor
– filmor, Commented May 10, 2013 at 16:48

Cameron · Accepted Answer · 2013-05-10 16:44:52Z

5

C++ doesn't require a default constructor in the base class -- but as in Java, you need to call the equivalent of Super, otherwise the compiler will attempt to call the default constructor (which you don't have, hence the error):

Derived(char *c) : Base(*c) { }

The bit after the : is called the initializer-list, and is most useful -- not only can you call base class constructors there, but you can initialize member variables (: var(val), var2(val2) syntax) that have non-default constructors.

answered May 10, 2013 at 16:44

Cameron

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2 Comments

Jay Foreman Over a year ago

Java doesn't require default constructor, too.

Cameron Over a year ago

@user: Right, ambiguous wording on my part, sorry. I meant, in situations like this, you need to manually call the non-default base constructor, as you would in Java.

juanchopanza · Accepted Answer · 2013-05-10 16:50:03Z

5

Why does C++ compiler strongly require Base() constructor with no arguments?

Because Derived(char*) attempts to call Base(), since you don't tell it to call any other constructor, which you can do like this:

 Derived(char *c) : Base('x') {}

or, using the char pointed at by the Derived constructor argument, and making sure not to de-reference a null pointer:

 Derived(char *c) : Base( c ? *c : 'x') {}

edited May 10, 2013 at 16:50

answered May 10, 2013 at 16:44

juanchopanza

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1 Comment

Kerrek SB Over a year ago

Good choice of character constant :-)

Kerrek SB · Accepted Answer · 2013-05-10 16:44:14Z

3

You have to specify the base initializer:

Derived(char * c)  :  Base('x') { }

answered May 10, 2013 at 16:44

Kerrek SB

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1 Comment

juanchopanza Over a year ago

Good choice of character constant!

stardust · Accepted Answer · 2013-05-10 16:54:16Z

2

Thsi is because

Derived(char *c){}

implicitly calls

Base()

unless you provide which constructor to call explicitly It is equivalent to

Derived(char *c) : Base() {}
                   ^^^^^^

And Base::Base() is not available since you have implemented your own constructor. Otherwise it would have been enabled by default. When you provide it explicitly the code complies. but not doing what you want probably.

edited May 10, 2013 at 16:54

answered May 10, 2013 at 16:44

stardust

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Comments

Mats Petersson · Accepted Answer · 2013-05-10 16:44:29Z

1

Your class Base doesn't have a constructor that takes no arguments. You can either make one that does, or add an initalizer for the existing one:

 Derived(char *c) : Base('a') { ... }

answered May 10, 2013 at 16:44

Mats Petersson

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Comments

dchhetri · Accepted Answer · 2013-05-10 16:44:38Z

1

As soon as you explictly create a constructor for Base, the compiler no longer generates a default ctor for you base. That is the reason why you are getting that error message. For example

class Base{}; //here compiler auto-generates 'Base(){}' for you

class Base2{ 
 Base2(char c){}
}; //here compiler does NOT auto-generate `Base2(){}` for you. 
   //Hence that constructor does not exist and you have to create it yourself

answered May 10, 2013 at 16:44

dchhetri

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Comments

Mike Seymour · Accepted Answer · 2013-05-10 16:58:46Z

1

Why does C++ compiler strongly require Base() constructor with no arguments?

It doesn't. It requires derived classes to use one of its constructors; if they don't provide any arguments, then it will try to use the default constructor.

What if creating Base object with no arguments makes no sense?

Then the derived class will have to provide the argument(s) for the constructor it uses:

Derived(char *c) : Base(c) {}

edited May 10, 2013 at 16:58

answered May 10, 2013 at 16:48

Mike Seymour

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