What is a reference? What is a pointer? What does RVallue and LValue reference mean in C++? What are the differences between a pointer and a reference variable in C++? Let’s find out the answers to these questions. Also, by learning C++ Pointers and References, you can build C++ applications easily with C++ IDE.
Table of Contents
What is a Reference in C++?
A reference in the C++ programming language, is a new name for an existing object that stored in an address of memory. In other words, the definition of a reference in C++ is such that it needs to exist.
What is the syntax for a Reference in C++?
Here is the syntax for the reference in C++,
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<Type>& <ReferenceName>; |
What is a Pointer?
A Pointer is a variable that hold addresses. The asterisk character ‘*’ is used to define pointers; they are used before the variable name. Pointers are some of the strongest aspects of the C & C++ programming languages. They allow you to reach any kind of type (including very long size bitmaps or videos or other data etc.) without copying the whole data. Please read this post for more about pointers,
What are the differences between a pointer and a reference?
Generally, references are often confused with pointers. There are 3 main differences between references and pointers. These are;
- A reference should have an existing object, an object that stored in an address of memory. It cannot have NULL references. It must always be able to assume that a reference is connected to an object in the memory. Pointers can be NULL that means it can be not pointing any object address.
- A reference must be initialized when it is created. Pointers can be initialized at any time.
- When a reference is initialized to an object, it cannot be changed to refer to another object while pointers can be pointed to another object at any time.
1. A reference should have an existing object
As shown example above a reference needs an existing object to define.
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#include <iostream> int main() { float gravity = 9.81; float &g = gravity; std::cout << g; getchar(); return 0; } |
pointers don’t need an existing object, we can initialize when it is needed.
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#include <iostream> int main() { float *g; // No error here, we define a pointer but address of the value is unknown return 0; } |
2. A reference must be initialized when it is created
For example this example below give error at the reference declaration,
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#include <iostream> int main() { float &g; // Error in this line return 0; } |
pointers can be defined null at the beginning and they can be defined at anytime. For example we can define a bitmap pointer and we can initialize this bitmap pointer when use imports a bitmap to our application. When use uploads a new bitmap we can delete this bitmap from the memory and we can initialize a new bitmap to the same pointer. Here is a simple example,
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#include <iostream> int main() { float *g; float gravity = 9.81; // some operations here g = &gravity; // pointers can be initialized again and again after definition return 0; } |
3. When a reference is initialized to an object, it cannot be changed
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#include <iostream> int main() { float gravity = 9.81; float density = 1.05; float &g = gravity; &g = density; //Error here std::cout << g; getchar(); return 0; } |
we can change pointers.
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#include <iostream> int main() { float gravity = 9.81; float density = 1.05; float *x; x = &gravity; x = &density; std::cout << x; getchar(); return 0; } |
What do RVallue and LValue references mean in C++?
Rvalue References are a compound type like standard C++ references, which are referred to as lvalue references. An lvalue reference is formed by appending the ampersand character (&) to a type
In the C programming language, you can pass arguments only by value. In C++, you can pass arguments by value or by LValue Reference. C++ reference types, closely related to pointer types, create aliases for objects.
A Brief Introduction to the Rvalue References in C++ are explained here and to the LValue References in C++ are explained here in the official DocWiki of Embarcadero. Also RValue and LValue Refereces are explained well here and here in open-std.org.
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