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What is const pointer?

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โˆ™ 2006-07-11 16:18:49

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In computer science, const-correctness is the form of program correctness that deals with the proper declaration of objects as mutable or immutable. The term is mostly used in a C or C++ context, and takes its name from the const keyword in those languages. The idea of const-ness does not imply that the variable as it is stored in the computer's memory is unwriteable. Rather, const-ness is a compile-time construct that indicates what a programmer may do, not necessarily what he or she can do. In addition, a class method can be declared as const, indicating that calling that method does not change the object. Such const methods can only call other const methods but cannot assign member variables. (In C++, a member variable can be declared as mutable, indicating that a const method can change its value. Mutable member variables can be used for caching and reference counting, where the logical meaning of the object is unchanged, but the object is not physically constant since its bitwise representation may change.) In C++, all data types, including those defined by the user, can be declared const, and all objects should be unless they need to be modified. Such proactive use of const makes values "easier to understand, track, and reason about," and thus, it increases the readability and comprehensibility of code and makes working in teams and maintaining code simpler because it communicates something about a value's intended use. For simple data types, applying the const qualifier is straightforward. It can go on either side of the type for historical reasons (that is, const char foo = 'a'; is equivalent to char const foo = 'a';). On some implementations, using const on both sides of the type (for instance, const char const) generates a warning but not an error. For pointer and reference types, the syntax is slightly more subtle. A pointer object can be declared as a const pointer or a pointer to a const object (or both). A const pointer cannot be reassigned to point to a different object from the one it is initially assigned, but it can be used to modify the object that it points to (called the "pointee"). (Reference variables are thus an alternate syntax for const pointers.) A pointer to a const object, on the other hand, can be reassigned to point to another object of the same type or of a convertible type, but it cannot be used to modify any object. A const pointer to a const object can also be declared and can neither be used to modify the pointee nor be reassigned to point to another object. The following code illustrates these subtleties: void Foo( int * ptr, int const * ptrToConst, int * const constPtr, int const * const constPtrToConst ) { *ptr = 0; // OK: modifies the pointee ptr = 0; // OK: modifies the pointer *ptrToConst = 0; // Error! Cannot modify the pointee ptrToConst = 0; // OK: modifies the pointer *constPtr = 0; // OK: modifies the pointee constPtr = 0; // Error! Cannot modify the pointer *constPtrToConst = 0; // Error! Cannot modify the pointee constPtrToConst = 0; // Error! Cannot modify the pointer To render the syntax for pointers more comprehensible, a rule of thumb is to read the declaration from right to left. Thus, everything before the star can be identified as the pointee type and everything to after are the pointer properties. (For instance, in our example above, constPtrToConst can be read as a const pointer that refers to a const int.) References follow similar rules. A declaration of a const reference is redundant since references can never be made to refer to another object: int i = 42; int const & refToConst = i; // OK int & const constRef = i; // Error the "const" is redundant Even more complicated declarations can result when using multidimensional arrays and references (or pointers) to pointers. Generally speaking, these should be avoided or replaced with higher level structures because they are confusing and prone to error.

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What is a const pointer?

There are four ways to declare a pointer: <type> * <name>; - a variable pointer to a variable type. const <type> * <name>; - a variable pointer to a constant type. <type> * const <name> {address}; - a constant pointer to a variable type. const <type> * const <name> {address}; - a constant pointer to a constant type. The last two declarations are constant pointers. Being constants, you must assign an address to them at the point of instantiation. Note that the 2nd declaration is a pointer to a constant, not a constant pointer. That is, the object being referred to is treated as if it were constant but the pointer itself is variable so we can still change the address, but not the value at that address.


Can a function return a pointer?

Yes. A function can return a pointer... const char* GetHelloString() { return "Hello!"; } ... returns a pointer to the string "Hello!".


How would you read int const p?

int const *p declares a 'p' pointer, that points to a constant integer


What is the difference between a reference and a pointer in c?

The difference that i learnt very recently and the one i remember :-)Reference cannot be changed whereas pointer value can be changed.Actually, const pointer = reference.


What is the implicit name of the parameter that gets passed into the set method of class in c?

Every non-static member function has a hidden pointer parameter named this which refers to the instance of the class the function was invoked against. For a given class, C, the type of the hidden this pointer is const C* but if the function is declared const, the pointer is const C* const. When referring to any class member, m, from within any non-static member function, this->m is implied.


What is the difference between the Constant pointer and pointer constant and explain it with an example?

Pointer to constant *ptr=10 statement is invalid in pointer to constant i.e assigning value is illegal ptr++ statement is valid in pointer to constant. pointer can be incremented and decremented, Pointer is pointing to constant data object. Declaration: const int *ptr; Constant pointers: *ptr= 10 is absolutely valid in constant pointers i.e assigning value is perfectly legal ptr+++ statement is invalid in constant pointers. pointer can not be incremented or decremented. Declaration; int *const ptr;


Is it possible to delete a const object in C plus plus?

Yes. However, making a const pointer seems rather pointless, as you will be able to allocate and deallocate memory for it, but you will be unable to change the contents.


How will you declare null pointer in C?

#define NULL ((void *)0) /* defined in <stddef.h> */ const char *mynullvar = NULL;


What is a pointer in the array?

A pointer into an array of elements of type E is a pointer to a single element of type E:typedef ..... E;E array[123];E* const pointer = &array[18]; // points to the 19th element inside 'array'An array of pointers is an array whose elements are pointers:typedef .... E;E* array[123];E** const pointer = &array[18]; // points to the 19th pointer within 'array'Referencing the name of the array variable without use of the index operator itself is a constant pointer to its first element. Therefore, the following if-clause is always true:typedef .... E;E array[123];if (array &array[N]) { // ALWAYS true ...}


What is array passer c plus plus?

//Array Passer //Demonstrates relationship between pointers and arrays #include <iostream> using namespace std; void increase(int* const array, const int NUM_ELEMENTS); void display(const int* const array, const int NUM_ELEMENTS); int main() { cout << "Creating an array of high scores.\n\n"; const int NUM_SCORES = 3; int highScores[NUM_SCORES] = {5000, 3500, 2700}; cout << "Displaying scores using array name as a constant pointer.\n"; cout << *highScores << endl; cout << *(highScores + 1) << endl; cout << *(highScores + 2) << "\n\n"; cout << "Increasing scores by passing array as a constant pointer.\n\n"; increase(highScores, NUM_SCORES); cout << "Displaying scores by passing array as a constant pointer to a constant.\n"; display(highScores, NUM_SCORES); return 0; } void increase(int* const array, const int NUM_ELEMENTS) { for (int i = 0; i < NUM_ELEMENTS; ++i) array [i] += 500; } void display(const int* const array, const int NUM_ELEMENTS) { for (int i = 0; i < NUM_ELEMENTS; ++i) cout << array[i] << endl; }


What operator takes an object a reference or a pointer and returns a reference to global const object of type typeinfo?

The C++ typeid operator returns a const-qualified lvalue object of type std::type_info, as defined in the standard library header.


Can a pointer be indexed like array?

A pointer is normally considered an atomic data type. Therefore, a pointer cannot "be indexed like an array" in that it is not possible to access portions of a pointer through an index. However, a pointer can point to data which can be accessed through pointer operations as well as through index operations. Further, multiple pointers can be arranged in an array of pointers, and each individual pointer within that array of pointers can be reached through its array index. Example 1: an array of pointers const char* const array_of_pointers[] = { "Hello, World", "Hello, Venus", "Hello, Mars" }; In this example, each of array_of_pointers[0], array_of_pointers[1] or array_of_pointers[2] evaluates to one pointer, selected with the array index 0..2. Example 2: access to data through pointer using an array-style index void example(char* const text) { *text = 'a'; // set first character pointed to to 'a' *(text+1) = 'b'; // second char becomes 'b' // the following are equivalent to the above, but use index notation: text[0] = 'a'; text[1] = 'b'; } Key to example 2 is the declaration of the pointer as a constant pointer.

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