no default constructor error

This is a discussion on no default constructor error within the C++ Programming forums, part of the General Programming Boards category; I get the error: error C2512: 'Mouse' : no appropriate default constructor available Error executing cl.exe. in the following code, ...

  1. #1
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    no default constructor error

    I get the error:

    error C2512: 'Mouse' : no appropriate default constructor available
    Error executing cl.exe.

    in the following code, and I can't figure out why the class Mouse default constructor is called:
    Code:
    #include <iostream>
    using namespace std;
    
    class Mouse
    {
    public:
         Mouse(int n)
    	 {
    		 length = n;
    	 }	
    	 
    	 void Display()
    	 {
    		 cout<<length<<endl;
    	 }
    
    	 int GetLength()
    	 {
    		 return length;
    	 }
    	 
    private:
         int length;
    };
    
    class Cat
    {
    public:
    	Cat(Mouse m)//********ERROR
    	{
    		member1 = m;
    	}
    	
    
    	void Display()
    	{
    		cout<<member1.GetLength()<<endl;
    	}
    
    private: 
    	Mouse member1; 
    };
    
    int main()
    {
    	Mouse my_Mouse(3);
    
    	Cat my_Cat(my_Mouse);
    
    	my_Cat.Display();
    
    	
        return 0;
    }
    If I add a default constructor to class Mouse, then it works fine. When is the default constructor called?
    Last edited by 7stud; 08-22-2003 at 11:54 PM.

  2. #2
    C++ Developer XSquared's Avatar
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    It would be called if you didn't specify any parameters. Like this:
    Mouse my_mouse;
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  3. #3
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    Thanks for the reply.

    Anyone else?

  4. #4
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    It's because in your Cat class you have as a private member the statement

    Code:
    Mouse member1;
    What do you think this does? Well that would call the default constructor at some point.
    "...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers

  5. #5
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    Code:
    Cat(Mouse m):member1(m){}

  6. #6
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    Mouse member1;

    What do you think this does? Well that would call the default constructor at some point.


    That's not obvious. This doesn't call the default constructor:

    Mouse a_Mouse(3);
    Mouse m = a_Mouse;

    So, what's the step- by-step process of what's happening(--not at the assembly level). The code doesn't work even if main is empty, and I never try to instantiate an object.
    Last edited by 7stud; 08-23-2003 at 12:37 AM.

  7. #7
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    Originally posted by 7stud
    Mouse member1;

    What do you think this does? Well that would call the default constructor at some point.


    That's not obvious. This doesn't call the default constructor:

    Mouse a_Mouse(3);
    Mouse m = a_Mouse;
    You're right that doesn't call the default constructor. That calls the copy constructor. Even though you didn't write your own copy constructor your class is provided with one.

    I wasn't trying to sound like an $$$ in my post. Maybe that's the way it came off, sorry bout that.
    "...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers

  8. #8
    Cat
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    This is a great reason to always use initializer lists for construction. For example, this:

    Code:
    Cat(Mouse m)
    	{
    		member1 = m;
    	}
    does the following:

    1) Constructs its private data member, "member1", via default constructor
    2) Receives a copy (via the copy constructor) of the Mouse passed as the parameter
    3) Uses the assignment operator to equate the member to the copy.

    So you use the default constructor (which is an error), the copy constructor, and the assignment operator.

    In contrast, this:

    Code:
    Cat(Mouse &m) : member1(m)
    	{
    	}
    Does the following:

    1) Constructs its member variable using the copy constructor from the reference (which is passed by address, not by copy).

    So this doesn't call the default constructor OR the assignment operator, which makes it more efficient, and legal (there IS no default constructor).

  9. #9
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    Thanks, Cat.

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