Using the const keyword when declaring a pointer indicates that the value pointed to must not be changed. Let’s start closer to the beginning. Pointer to the destination array where the content is to be copied, type-casted to a pointer of type void*. Templates let you quickly answer FAQs or store snippets for re-use. So, you can use the address of a pointer to navigate and iterate through an array. So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. Declaration of a pointer to constant is given below: The above code runs successfully, and it shows the value of 'ptr' in the output. This error means that we cannot change the value of the variable to which the pointer is pointing. Note the use of const, because from the function I’m returning a string literal, a string defined in double quotes, which is a constant.. Passing by value, passing by reference in C, // pvalue now points to a new address in memory, but is allowed because the pointer itself was not used to make the change, // the asterisk in front of the const keyword ensures that the address cannot be changed, // the below results in an error for trying to change the address of the pointer, //the * placed before the pointer name means the value it points to cannot change using the pointer, // the * placed before the const keyword means the address cannot change, // initializing pointers to null is good practice to prevent errors, // however, the below only works on a pointer variable, Basics of the C programming language (20 Part Series). Now, we write the code in which we are changing the value of the variable to which the pointer points. num Number of bytes to copy. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. void exchange(int& a, int& b) {int t = a a = b b = t} // To help clients detect any reliance on undefined iteration behavior, // whenever we can, we invalidate access through the m_current pointer // by setting it to NULL whenever the state of the iteration is not defined. I've been tinkering with computers since I was a teen. We declare two variables, i.e., a and b with the values 100 and 200 respectively. Then, we try to modify the value of the variable 'b' through the pointer 'ptr'. struct null_deleter{ void operator()(void const *) const { }};static X x;shared_ptr createX(){ shared_ptr px(&x, null_deleter()); return px;} The same technique works for any object known to outlive the pointer. JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. © Copyright 2011-2018 www.javatpoint.com. The const keyword specifies that the pointer cannot be modified after initialization; the pointer is protected from modification thereafter. It can neither change the address of the variable to which it is pointing nor it can change the value placed at this address. Therefore, we conclude that the constant pointer to a constant can change neither address nor value, which is pointing by this pointer. strings). While we are talking about void pointer we got a doubt size for memory allocation. The void pointer, also known as the generic pointer, is a special type of pointer that can be pointed at objects of any data type! const int *ptr = &x; int const … In the example below, the same void pointer is reassigned to 3 different data types and then cast to that data type and dereferenced to print out the value. To declare a pointer to a const value, use the const keyword before the data type: 1. In other words, constant pointer is a pointer that can only point to single object throughout the program. DEV Community © 2016 - 2021. str − This is the pointer to the block of memory where the search is performed. The type name void means "absence of any type." A char pointer pointer can also be looked at as a pointer to a string. The pointer address can be reassigned as well, just not the value of the pointer's reference: There is a way to ensure that the address stored in a pointer cannot be changed, using a different syntax: The above ensures that the pointer will always point to the same thing. Declaration for a constant pointer to a constant is given below: The above code shows the error "assignment of read-only location '*ptr'" and "assignment of read-only variable 'ptr'". const void * const myptr means both the thing pointed by pointer and the pointer itself cannot be changed. The value can be changed using the value variable name (because const was not applied to the initial variable declaration), but it cannot be changed using the pointer. Syntax to declare constant pointer A constant pointer to a constant is a pointer, which is a combination of the above two pointers. void *memcpy(void *dest, const void * src, size_t n) Parameters. If the list is accessed through a pointer to a const CObList, then GetHead returns a CObject pointer. The compiler will check that you do not inadvertently try to change the pointer's reference elsewhere in the code: It is also possible to create a pointer where both the value and the reference are constant in the same declaration... ...or, if the varible the pointer references is declared as a constant as well, then it will be completely immutable: the compiler will not allow changes to variable itself, the pointer address, or the pointer reference value. 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