1. What is virtual constructors/destructors?Virtual destructors: If an object (with a non-virtual destructor) is destroyed explicitly by applying the delete operator to a base-class pointer to the object, the base-class destructor function (matching the pointer type) is called on the object.
There is a simple solution to this problem – declare a virtual base-class destructor. This makes all derived-class destructors virtual even though they don’t have the same name as the base-class destructor. Now, if the object in the hierarchy is destroyed explicitly by applying the delete operator to a base-class pointer to a derived-class object, the destructor for the appropriate class is called.
Virtual constructor: Constructors cannot be virtual. Declaring a constructor as a virtual function is a syntax error. Does c++ support multilevel and multiple inheritance?
Yes.
2. Why Garbage collection?
Since C++ does not provide automatic garbage collection like some other languages, smart pointers can be used for that purpose. The simplest garbage collection scheme is reference counting or reference linking, but it is quite possible to implement more sophisticated garbage collection schemes with smart pointers.
3. How to write a swap( ) function which swaps the values of the variables using bitwise operators.?
Ans: Here is the swap( ) function.
swap ( int *x, int *y )
{
*x ^= *y ;
*y ^= *x ;
*x ^= *y ;
}
The swap( ) function uses the bitwise XOR operator and does not require any temporary variable for swapping.
4. What are the advantages of inheritance?• It permits code reusability.
• Reusability saves time in program development.
• It encourages the reuse of proven and debugged high-quality software, thus reducing problem
after a system becomes functional.
5. What is the difference between declaration and definition?The declaration tells the compiler that at some later point we plan to present the definition of this declaration.
E.g.: void stars () //function declaration
The definition contains the actual implementation.
E.g.: void stars () // declarator
{
for(int j=10; j>=0; j--) //function body
cout<<”*”; cout< function_declaration; template
function_declaration;
The only difference between both prototypes is the use of keyword class or typename, its use is
indistinct since both expressions have exactly the same meaning and behave exactly the same way.
6. What do you mean by inline function?The idea behind inline functions is to insert the code of a called function at the point where the
function is called. If done carefully, this can improve the application's
performance in exchange for increased compile time and possibly (but not always) an increase in the size of the generated binary executables.
7. What is virtual class and friend class?Friend classes are used when two or more classes are designed to work together and need access
to each other's implementation in ways that the rest of the world
shouldn't be allowed to have. In other words, they help keep private things private. For instance, it may be desirable for class DatabaseCursor to have more privilege to the internals of class Database than main() has.
8. What is function overloading and operator overloading?Student Resources Function overloading: C++ enables several functions of the same name to be defined, as long as these functions have different sets of parameters (at least as far as their types are concerned). This capability is called function overloading. When an overloaded function is called, the C++ compiler selects the proper function by examining the number, types and order of the arguments in the call.
Function overloading is commonly used to create several functions of the same name that perform similar tasks but on different data types.
Operator overloading allows existing C++ operators to be redefined so that they work on objects
of user-defined classes. Overloaded operators are syntactic sugar for
equivalent function calls. They form a pleasant facade that doesn't add anything fundamental to the language (but they can improve understandability and reduce
maintenance costs).
9. Difference between realloc() and free()?The free subroutine frees a block of memory previously allocated by the malloc subroutine.
Undefined results occur if the Pointer parameter is not a valid pointer. If the Pointer parameter is a null value, no action will occur. The realloc subroutine changes the size of the block of memory pointed to by the Pointer parameter to the number of bytes specified by the Size parameter and returns a new pointer to the block. The pointer specified by the Pointer parameter must have been created with the malloc, calloc, or realloc subroutines and not been deallocated with the free or realloc subroutines. Undefined results occur if the Pointer parameter is not a valid pointer
10. What do you mean by binding of data and functions?Encapsulation.
11. What is abstraction?Abstraction is of the process of hiding unwanted details from the user.12. What is encapsulation?Packaging an object’s variables within its methods is called encapsulation.13. What is the difference between an object and a class?
Classes and objects are separate but related concepts. Every object belongs to a class and every
class contains one or more related objects.
Ø A Class is static. All of the attributes of a class are fixed before, during, and after the execution of a program. The attributes of a class don't change.
Ø The class to which an object belongs is also (usually) static. If a particular object belongs to a
certain class at the time that it is created then it almost certainly will still belong to that class right up until the time that it is destroyed.
Ø An Object on the other hand has a limited lifespan. Objects are created and eventually destroyed. Also during that lifetime, the attributes of the object may undergo significant change.
14. What is polymorphism? Explain with an example?"Poly" means "many" and "morph" means "form". Polymorphism is the ability of an object (or
reference) to assume (be replaced by) or become many different forms of
object.
Example: function overloading, function overriding, virtual functions. Another example can be a plus ‘+’ sign, used for adding two integers or for using it to concatenate two strings.
15. What do you mean by inheritance?Inheritance is the process of creating new classes, called derived classes, from existing classes or
base classes. The derived class inherits all the capabilities of the base class, but can add
embellishments and refinements of its own.16. What is a scope resolution operator?A scope resolution operator (::), can be used to define the member functions of a class outside the class.17. What are virtual functions?A virtual function allows derived classes to replace the implementation provided by the base class. The compiler makes sure the replacement is always called whenever
the object in question is actually of the derived class, even if the object is accessed by a base pointer rather than a derived pointer. This allows algorithms in the base class to be replaced in the derived class, even if users don't know about the derived class.
18. What is friend function?As the name suggests, the function acts as a friend to a class. As a friend of a class, it can access its private and protected members. A friend function is not a member of
the class. But it must be listed in the class definition.
19. What is the difference between class and structure?Structure: Initially (in C) a structure was used to bundle different type of data types together to
perform a particular functionality. But C++ extended the structure to contain functions also. The
major difference is that all declarations inside a structure are by default public.
Class: Class is a successor of Structure. By default all the members inside the class are private.
20. What is public, protected, private?Ø Public, protected and private are three access specifiers in C++.
Ø Public data members and member functions are accessible outside the class.
Ø Protected data members and member functions are only available to derived classes.
Ø Private data members and member functions can’t be accessed outside the class.
However there is an exception can be using friend classes.
21. What is an object?Object is a software bundle of variables and related methods. Objects have state and behavior.22. What is a class?Class is a user-defined data type in C++. It can be created to solve a particular kind of problem.
After creation the user need not know the specifics of the working of a class.
Q. When linking C or Assembly language modules with C++ modules I get undefined symbol errors at link time. It appears that none of the C or Assembly public symbols can be found.
A. C++ is a strongly typed language. In order to support the language to its fullest, Turbo C++ must attach information to the symbols generated for function names and variables. When this is done, the symbol will no longer match the standard C style function name. In order to link correctly, the compiler must be notified that the symbol is declared in an external module without type information tacked on to the symbol. This is done by prototyping the function as type extern "C". Here is a quick example: extern "C" int normal_c_func( float, int, char ); // name not altered void cplusplus_function( int ); // name altered See related comments under Linker Errors and in the Paradox Engine question in this section.
Q. Classes with static data members are getting linker errors ("undefined").
A. This code is built into Turbo C++ 1.0 but not in version 3.0. In the 1.0 compiler, static members without definitions were given a default value of 0. This default definition will no longer be made in the compiler. The programmer must now give an explicit definition for each static member. Here is a quick example:
class A
{
static int i;
};
A linker error saying that A::i is not defined will result unless the source also contains a line such as:
int A::i = 1;
Q. What potential problems can arise from typecasting a base class pointer into a derived class pointer so that the derived class's member functions can be called?
A. Syntactically this is allowable. There is always the possibility of a base pointer actually pointing to a base class. If this is typecast to a derived type, the method being called may not exist in the base class. Therefore, you would be grabbing the address of a function that does not exist.
Q: What's the difference between the keywords STRUCT and CLASS?
A: The members of a STRUCT are PUBLIC by default, while in CLASS, they default to PRIVATE. They are otherwise functionally equivalent.
Q: I have declared a derived class from a base class, but I can't access any of the base class members with the derived class function.
A: Derived classes DO NOT get access to private members of a base class. In order to access members of a base class, the base class members must be declared as either public or protected. If they are public, then any portion of the program can access them. If they are protected, they are accessible by the class members, friends, and any derived classes.
Q: How can I use the Paradox Engine 1.0 with C++?,
A: Because the Paradox Engine functions are all compiled as C functions, you will have to assure that the names of the functions do not get "mangled" by the C++ compiler. To do this you need to prototype the Engine functions as extern "C". In the pxengine.h header file insert the following code at the lines indicated.
/* inserted at line # 268 */
#ifdef __cplusplus
extern "C" {
#endif
/* inserted at line # 732, just before the final #endif */
#ifdef __cplusplus
}
#endif
Paradox Engine version 2.0 is "aware" of C++ and thus does not require any modifications to its header file.
Q: I have a class that is derived from three base classes. Can I insure that one base class constructor will be called before all other constructors?
A: If you declare the base class as a virtual base class, its constructor will be called before any non-virtual base class constructors. Otherwise the constructors are called in left-to-right order on the declaration line for the class.
Q: Are the standard library I/O functions still available for use with the C++ iostreams library?
A: Yes, using
#include
functions such as printf() and scanf() will continue to be available. However, using them in conjunction with stream oriented functions can lead to unpredictable behaviour.
Q. In C++, given two variables of the same name, one local and one global, how do I access the global instance within the local scope?
A. Use the scope (::) operator.
int x = 10;
for(int x=0; x < ::x; x++)
{
cout << "Loop # " << x << "\n"; // This will loop 10 times
}
Q. Will the following two functions be overloaded by the compiler, or will the compiler flag it as an error? Why?
void test( int x, double y); & int test( int a, double b);
A. The compiler will flag this as a redeclaration error because neither return types nor argument names are considered when determining unique signatures for overloading functions. Only number and type of arguments are considered.
Q. If I pass a character to a function which only accepts an int, what will the compiler do? Will it flag it as an error?
A. No. The compiler will promote the char to an int and use the integer representation in the function instead of the character itself.
Q. I was trying to allocate an array of function pointers using the new operator but I keep getting declaration syntax errors using the following
syntax: new int(*[10])(); What's wrong?
A. The new operator is a unary operator and binds first to the int keyword producing the following: (new int) (*[10])();
You need to put parentheses around the expression to produce the expected results: new (int (*[10]());
Q. What are inline functions? What are their advantages? How are they declared?
A. An inline function is a function which gets textually inserted by the compiler, much like macros. The advantage is that execution time is shortened because linker overhead is minimized. They are declared by using the inline keyword when the function is declared:
inline void func(void) { cout << "printing inline function \n"; }
or by including the function declaration and code body within a class:
class test
{
tv public:
void func(void) { cout << "inline function within a class.\n"}
};
Q. If I don't specify either public or private sections in a class, what is the default?
A. In a class, all members are private by default if neither public nor private sections are declared.
Q. What does the _seg modifier do?
A. Using _seg causes a pointer to become a storage place for a segment value, rather than an offset ( or a segment/offset ). For instance, if "int _seg *x" contains the value 0x40, then when you use "*x", the value pointed to will be at segment 0x40, offset 0. If you add a value to the pointer, the value is multiplied by the size of the pointer type. That new value is used as an offset, and is combined with the segment value contained in the pointer. For instance,
int _seg *x;
int value;
x = (int _seg *)0x40;
value = *(x + 20);
value is assigned the value of the integer at 0x40:0x28
(Remember, 20 * sizeof(int) = 40 = 0x28).
Q. Can I statically allocate more than 64K of data in a single module?
A. Yes. Far data items are now supported:
...
char far array1[60000L];
char far array2[60000L];
...
For arrays larger than 64k use:
char huge array3[100000L];
Q. What is a friend member function?
A. Declaring a friend gives non-members of a class access to the non-public members of a class.
Q. Why do I get a "Type name expected" error on my definition of a friend class in my new class?
A You need to let the compiler know that the label you use for your friend class is another class. If you do not want to define your entire class, you can simply have "class xxx", where xxx is your label.
Q: How can I output hex values in upper case using the iostream libraries?
A: You need to set the state of the stream using setf(). For example,
#include
int main(void)
{
cout << hex;
cout << "\nNot upper-case : " << 255;
cout.setf(ios::upper-case);
cout << "\nUppercase : " << 255;
return 0;
}
Q. What is the "this" pointer?
A. "this" is a local variable in the body of a non-static member function. It is a pointer to the object for which the function was invoked. It cannot be used outside of a class member function body.
Q. Why does a binary member function only accept a single argument?
A. The first argument is defined implicitly.
Q. Looking through the class libraries there are definitions in classes which look like:
class test {
int funct( void ) const;
};
What is the const keyword doing here?
A. There is a pointer to the object for which a function is called known as the 'this' pointer. By default the type of 'this' is X *const ( a constant pointer). The const keyword changes the type to const X *const ( a constant pointer to constant data ).
Q: I want to use _new_handler and set_new_handler.
A: Turbo C++ supports _new_handler and set_new_handler. The type of _new_handler is as follows.
typedef void (*vfp)(void);
vfp _new_handler;
vfp set_new_handler( vfp );
Q: I would like to use C++ fstreams on a file opened in binary mode, how is this done?
A: Use ios::binary as the open mode for the file:
#include
ifstream binfile;
binfile.open("myfile.bin", ios::binary);
Q: How can I get at the DOS file handle associated with my iostream?
A: Using a combination of member functions fd() and rdbuf() you can get at the file handle.
#include
#define fstrno(s) (((s).rdbuf())->fd())
ifstream test("test.txt");
cout << "handle is " << fstrno(test) << '\n';
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