Are reference variables are NOT available in C?

Answer 1

Yes, reference variables are permitted in C. However a reference variable in C is simply another name for a pointer variable, because pointers can be dereferenced to allow indirect access to the memory they point at.

However, in C++, references are neither pointers nor variables, they serve another purpose altogether, one which is not available in C. So the real answer is that C++ references are not available in C, but reference variables are.

Although it is common practice to avoid using the term reference lest there be any confusion with a C++ reference, it doesn't alter the fact that a C pointer variable is also a C reference variable. They are one and the same thing.

In C++, a reference is entirely different from a C reference. More specifically, it is an alias, an alternate name, for a memory address. Unlike pointer variables, a reference consumes no memory of its own. That is, you cannot obtain the address of a reference since there is none to obtain. Hence the 'address of' operator (&) is used during instantiation to signify that the instance name is actually a reference of the specified type, and not a variable of the type.

The address being referred to must also be assigned to the reference when the reference is instantiated. That is, you cannot declare a reference and then assign a memory address to it elsewhere in your code. By the same token, you cannot subsequently reassign a reference while it remains in scope. Remember, references are not variables. What they refer to may be variable, but not the reference itself. In that respect it is not unlike like a constant pointer to a variable.

References allow you to refer to the same memory address using one or more aliases. Since they consume no memory of their own, there is no practical limit to the number of aliases you can have in your C++ program. When compiled, all references to the same memory are effectively replaced with the actual memory address, or the address of a static location containing the actual memory address if the address was allocated dynamically. So while references are often implemented just as if they were pointers, this is only of concern to developers of C++ compilers. In more general C++ source code, the two concepts must be kept distinct.

C++ references are also much easier to work with than pointers. Whereas pointers to allocated memory must be manually released when no longer required, references do not. When a referenced object falls from scope, the object's destructor is called automatically. More importantly, references can never be NULL so, if you have a reference, a valid object of the specified type is guaranteed to exist at the referred memory location. The only time you will encounter problems with references is when you point at them, and then delete the pointer! If you subsequently access the referenced memory or the reference falls from scope, your program will exhibit undefined behaviour. Obviously this is a programming error, but these type of problems exist with pointers in general, hence pointers are best avoided as much as is possible. If an object is guaranteed to exist, then refer to it, don't point at it. Only use a pointer when an object is not guaranteed to exist, or if you need to change the address being referred to. And if a pointer is non-NULL, refer to the object.