C++ Interview Questions And Answers Set - 5

What problems might the following macro bring to the application?
#define sq(x) x*x

Anything wrong with this code?
T *p = new T[10];
delete p;

Everything is correct, Only the first element of the array will be deleted?, The entire array will be deleted, but only the first element destructor will be called.

Anything wrong with this code?
T *p = 0;
delete p;

Yes, the program will crash in an attempt to delete a null pointer.

How do you decide which integer type to use?
It depends on our requirement. When we are required an integer to be stored in 1 byte (means less than or equal to 255) we use short int, for 2 bytes we use int, for 8 bytes we use long int.

A char is for 1-byte integers, a short is for 2-byte integers, an int is generally a 2-byte or 4-byte integer (though not necessarily), a long is a 4-byte integer, and a long long is a 8-byte integer.

What does extern mean in a function declaration?
Using extern in a function declaration we can make a function such that it can used outside the file in which it is defined.

An extern variable, function definition, or declaration also makes the described variable or function usable by the succeeding part of the current source file. This declaration does not replace the definition. The declaration is used to describe the variable that is externally defined.

If a declaration for an identifier already exists at file scope, any extern declaration of the same identifier found within a block refers to that same object. If no other declaration for the identifier exists at file scope, the identifier has external linkage.

What can I safely assume about the initial values of variables which are not explicitly initialized?
It depends on complier which may assign any garbage value to a variable if it is not initialized.

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

What?s the auto keyword good for?
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.

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

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

What does extern mean in a function declaration?
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.

How do I initialize a pointer to a function?
This is the way to initialize a pointer to a function
void fun(int a)
{

}

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

}

How do you link a C++ program to C functions?
By using the extern "C" linkage specification around the C function declarations.

Explain the scope resolution operator.
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.

What are the differences between a C++ struct and C++ class?
The default member and base-class access specifier are different.

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