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Answer : According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal: char *p = 0; if(p != 0) However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. For instance, the Unix system call "execl" takes a variable-length, null-pointer-terminated list of character pointer arguments. To generate a null pointer in a function call context, an explicit cast is typically required: execl("/bin/sh", "sh", "-c", "ls", (char *)0); If the (c...

Answer : The language definition states that for each pointer type, there is a special value -- the "null pointer" -- which is distinguishable from all other pointer values and which is not the address of any object. That is, the address-of operator & will never yield a null pointer, nor will a successful call to malloc. (malloc returns a null pointer when it fails, and this is a typical use of null pointers: as a "special" pointer value with some other meaning, usually "not allocated" or "not pointing anywhere yet.") A null pointer is conceptually different from an uninitialized pointer. A null pointer is known not to point to any object; an uninitialized pointer might point anywhere. See also questions 49, 55, and 85. As mentioned in the definition above, there is a null pointer for each pointer type, and the internal values of null pointers for different types may be different. Although programmers need not know the internal values, t...

Question: How would you use stacks to implement a queue? Answer: So how could we go about doing this? What if we used two stacks and used one as the incoming stack and the other as the outgoing stack? A queue is a FIFO structure, so when we enqueue something, we need to make sure that it does not get popped off before something that was already there. Similarly, when we dequeue something, we have to make sure that elements that were inserted earlier get ejected first. What’s better than some code!   Stack in; Stack out; void enqueue( int value ) { while( !out.isEmpty() ) { in.push( out.pop() ); } in.push( value ); } int dequeue() { while( !in.isEmpty() ) { out.push( in.pop() ); } return out.pop(); } Isn’t that cool? Help share this on Facebook and Twitter!

      Though compiler automatically provides the default copy constructor, sometime a programmer needs to implement his own copy constructor. We can take up a case here:           In the default constructor some memory allocation has been done for few data members and hence those will be created in the heap. In the destructor corresponding de-allocation code is there. Now if an attempt to copy an object is made, it won’t call the default constructor but it will call the default copy constructor which will copy the data member variable from stack and copy the pointers but won’t allocate any new memory space for the new copied object. So same pointers will exist in both the parent and copied object. This will not only create a great ambiguity but runtime error will occur when attempts will be made to delete both the objects. First object will get deleted properly. When it will try to delete the other object, the common p...

Yes! The default copy constructor is being provided automatically by the compiler if not implemented separately. In this, it puts the code for coping the data members and other variables that stay in stack. If something is being created by malloc or new in the heap, those are not being copied by the default copy constructor provide by the compiler.

Pointers are an extremely powerful programming tool. They can make some things much easier, help improve your program's efficiency, and even allow you to handle unlimited amounts of data. For example, using pointers is one way to have a function modify a variable passed to it. It is also possible to use pointers to dynamically allocate memory, which means that you can write programs that can handle nearly unlimited amounts of data on the fly--you don't need to know, when you write the program, how much memory you need. Wow, that's kind of cool. Actually, it's very cool, as we'll see in some of the next tutorials. For now, let's just get a basic handle on what pointers are and how you use them. What are pointers? Why should you care? Pointers are aptly named: they "point" to locations in memory. Think of a row of safety deposit boxes of various sizes at a local bank. Each safety deposit box will have a number associated with it so t...

What does the error ‘Null PointerAssignment’ mean and what causes this error? Explain one method to process an entire string as one unit? What is the similarity between a Structure, Union and enumeration? Can a Structure contain a Pointer to itself? How can we check whether the contents of two structure variables are same ornot? How are Structure passing and returning implemented by the compiler? How can we read/write Structures from/to data files? What is the difference between an enumeration and a set of pre-processor #defines? what do the ‘c’ and ‘v’ in argc and argv stand for? Are the variables argc and argv are local to main? What is near, far and huge pointers? How many bytes are occupied by them? How would you obtain segment and offset addresses from a far address of amemory location? Are the expressions arr and &arr same for an array of integers? Does mentioning the array name gives the base address in all the contexts? What is the maximum combined length of command line ar...

Note : All the programs are tested under Turbo C/C++ compilers. It is assumed that, Programs run under DOS environment, The underlying machine is an x86 system, Program is compiled using Turbo C/C++ compiler.   The program output may depend on the information based on this assumptions (for example sizeof(int) == 2 may be assumed). Predict the output or error(s) for the following: void main() { int const * p=5; printf("%d",++(*p)); } Answer: Compiler error: Cannot modify a constant value. Explanation: p is a pointer to a "constant integer". But we tried to change the value of the "constant integer". main() { char s[ ]="man"; int i; for(i=0;s[ i ];i++) printf("\n%c%c%c%c",s[ i ],*(s+i),*(i+s),i[s]); } Answer: mmmm aaaa nnnn Explanation: s[i], *(i+s), *(s+i), i[s] are all different ways of expressing the same idea. Generally array name is the base address for that array. Here s is the base address. i is the index number...