Set - 5

Question 1 :

What is the difference between char a[] = "string"; and char *p = "string";? 

Answer :

In the first case 6 bytes are allocated to the variable a which is fixed, where as in the second case if *p is assigned to some other value the allocate memory can change.


Question 2 :

What's the auto keyword good for?

Answer :

Answer1
Not much. It declares an object with automatic storage duration. Which means the object will be destroyed at the end of the objects scope. All variables in functions that are not declared as static and not dynamically allocated have automatic storage duration by default. 

For example

int main(){
	int a; //this is the same as writing "auto int a;"
} 

Answer2
Local variables occur within a scope; they are "local" to a function. They are often called automatic variables because they automatically come into being when the scope is entered and automatically go away when the scope closes. The keyword auto makes this explicit, but local variables default to auto auto auto auto so it is never necessary to declare something as an auto auto auto auto.


Question 3 :

What is the difference between char a[] = "string"; and char *p = "string"; ? 

Answer :

Answer1

a[] = "string";
char *p = "string";

The difference is this:
p is pointing to a constant string, you can never safely say

p[3]='x';

however you can always say a[3]='x';

char a[]="string"; // character array initialization.
char *p="string" ; // non-const pointer to a const-string.( this is permitted only in the case of char pointer in C++ to preserve backward compatibility with C.) 

Answer2

a[] = "string";
char *p = "string";

a[] will have 7 bytes. However, p is only 4 bytes. P is pointing to an adress is either BSS or the data section (depending on which compiler — GNU for the former and CC for the latter).

Answer3

char a[] = "string";
char *p = "string";

for char a[]…….using the array notation 7 bytes of storage in the static memory block are taken up, one for each character and one for the terminating nul character. 

But, in the pointer notation char *p………….the same 7 bytes required, plus N bytes to store the pointer variable "p" (where N depends on the system but is usually a minimum of 2 bytes and can be 4 or more)……


Question 4 :

How do I declare an array of N pointers to functions returning pointers to functions returning pointers to characters? 

Answer :

Answer1
If you want the code to be even slightly readable, you will use typedefs. 

typedef char* (*functiontype_one)(void);
typedef functiontype_one (*functiontype_two)(void);
functiontype_two myarray[N]; //assuming N is a const integral 


Answer2

char* (* (*a[N])())()

Here a is that array. And according to question no function will not take any parameter value.


Question 5 :

What does extern mean in a function declaration?

Answer :

It tells the compiler that a variable or a function exists, even if the compiler hasn't yet seen it in the file currently being compiled. This variable or function may be defined in another file or further down in the current file.


Question 6 :

How do I initialize a pointer to a function?

Answer :

This is the way to initialize a pointer to a function

void fun(int a){
}

void main(){
	void (*fp)(int);
	fp=fun;
	fp(1);
}

 


Question 7 :

How do you link a C++ program to C functions? 

Answer :

By using the extern "C" linkage specification around the C function declarations.


Question 8 :

Explain the scope resolution operator?

Answer :

It permits a program to reference an identifier in the global scope that has been hidden by another identifier with the same name in the local scope.


Question 9 :

What are the differences between a C++ struct and C++ class?

Answer :

The default member and base-class access specifier are different.


Question 10 :

How many ways are there to initialize an int with a constant?

Answer :

Two. 
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the traditional C notation. The second format uses constructor notation. 

int foo = 123;
int bar (123);

 


Question 11 :

How does throwing and catching exceptions differ from using setjmp and longjmp?

Answer :

The throw operation calls the destructors for automatic objects instantiated since entry to the try block.


Question 12 :

What is a default constructor? 

Answer :

Default constructor WITH arguments 

class B { public: B (int m = 0) : n (m) {} int n; }; int main(int argc, char *argv[]) { B b; return 0; }

 


Question 13 :

What is a conversion constructor? 

Answer :

A constructor that accepts one argument of a different type.


Question 14 :

What is the difference between a copy constructor and an overloaded assignment operator? 

Answer :

A copy constructor constructs a new object by using the content of the argument object. An overloaded assignment operator assigns the contents of an existing object to another existing object of the same class.


Question 15 :

When should you use multiple inheritance? 

Answer :

There are three acceptable answers: "Never," "Rarely," and "When the problem domain cannot be accurately modeled any other way."


Question 16 :

What is an Iterator class ?

Answer :

A class that is used to traverse through the objects maintained by a container class. There are five categories of iterators: input iterators, output iterators, forward iterators, bidirectional iterators, random access. An iterator is an entity that gives access to the contents of a container object without violating encapsulation constraints. Access to the contents is granted on a one-at-a-time basis in order. The order can be storage order (as in lists and queues) or some arbitrary order (as in array indices) or according to some ordering relation (as in an ordered binary tree). The iterator is a construct, which provides an interface that, when called, yields either the next element in the container, or some value denoting the fact that there are no more elements to examine. Iterators hide the details of access to and update of the elements of a container class.
The simplest and safest iterators are those that permit read-only access to the contents of a container class.


Question 17 :

What is an incomplete type? 

Answer :

Incomplete types refers to pointers in which there is non availability of the implementation of the referenced location or it points to some location whose value is not available for modification. 

int *i=0x400 // i points to address 400
*i=0; //set the value of memory location pointed by i. 

Incomplete types are otherwise called uninitialized pointers.


Question 18 :

Differentiate between the message and method?

Answer :

Message:
* Objects communicate by sending messages to each other.
* A message is sent to invoke a method.

Method
* Provides response to a message.
* It is an implementation of an operation.


Question 19 :

What is a Null object? 

Answer :

It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object. 


Question 20 :

What is class invariant? 

Answer :

A class invariant is a condition that defines all valid states for an object. It is a logical condition to ensure the correct working of a class. Class invariants must hold when an object is created, and they must be preserved under all operations of the class. In particular all class invariants are both preconditions and post-conditions for all operations or member functions of the class.