Question

Bjarne wrote:-
For a type T, T() is the notation for the default value , as defined by the default constructor .
What happen when we don’t declare default constructor ? For example

using namespace std;

class date{
    int n,m;
    public:
   int day(){return n;}
   int month(){return m;}
          };//no default constructor

int main()
{
     date any =date();
     cout<<any.month()<<endl;   
     cout<<any.day()<<endl;
return 0;

}

Output of this program is 0 and 0 every time i run my program. I haven’t declare any default constructor then why there exits a default value i.e. 0?

EDIT-

    class date{
        int n,m;
        public:
        date (){
        m=1;}
       int day(){return n;}
       int month(){return m;}
     };

 int main()
  {
     date any =date();
     cout<<any.month()<<endl;   
     cout<<any.day()<<endl;
return 0;

}

After reading answers i provide a default constructor but now n is getting garbage value but according to answers it should be 0 as m is out of reach of any other constructor and it is value initialisation as mentioned in answer

Answer 1

The behavior you see is Well-Defined for your class.

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How & Why is the behavior Well-Defined?

The rule is:
If you do not provide a no argument constructor the compiler generates one for your program in case your program needs one.
Caveat:
The compiler does not generate the no argument constructor if your program defines any constructor for the class.

As per the C++ Standard an object can be initialized in 3 ways:

• Zero Initialization
• Default Initialization &
• Value Initialization

When, a type name or constructor initializer is followed by () the initialization is through value initialization.

Thus,

date any =date();
              ^^^

Value Initializes an nameless object and then copies it in to the local object any,
while:

date any;

would be a Default Initialization.

Value Initialization gives an initial value of zero to members that are out of reach of any constructor.
In your program, n and m are beyond the reach of any constructor and hence get initialized to 0.

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Answer to Edited Question:
In your edited case, your class provides a no argument constructor, date(), which is capable(& should) initialize members n and m, but this constructor doesn’t initialize both the members, So In this case no zero initialization takes place, and the uninitialized members in the object have an Indeterminate(any random) value, further this temporary object is copied to any object which displays the shows indeterminate member values.

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For Standerdese Fans:
The rules for object Initialization are aptly defined in:

C++03 Standard 8.5/5:

To zero-initialize an object of type T means:
— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject is zero-initialized;
— if T is a union type, the object’s first named data member is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.

To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.

To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized