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Answered 2015-01-21 12:47:46

Pass by value and pass by reference apply to functions that accept formal arguments. A formal argument may be a value or a reference. The function's caller uses actual arguments and the calling convention determines how an actual argument is assigned to the corresponding formal argument.

If the formal argument is a value, then the value of the actual argument is assigned to the formal argument.

If the formal argument is a reference, then the address of the actual argument is assigned to the formal argument.

The nett effect is that when you pass by reference, both the formal and actual arguments refer to the same object. Whereas when you pass by value, the formal argument is a copy of the actual argument.

Some languages, like C, do not support the notion of a reference argument and only use pass by value semantics. However, C does support the notion of a pointer data type which is a variable that can store a memory address. The value of a pointer is therefore a reference and passing a pointer by value is the same as passing by reference. This is because when a pointer value is assigned to another pointer, the address (the value of the pointer) is copied, thus the two pointers refer to the same object.

C++ evolved from C and uses the same pass by value semantic by default. However, C++ also has a dedicated reference type that allows native pass by reference semantics. A reference differs from a pointer in that a pointer is a variable and therefore requires memory of its own to store the address it refers to. Thus a pointer has an address and a value, just like any other variable. But a reference does not have its own address because a reference is simply an alias, an alternate name for an object in memory. In other words, the address of a reference is the address of the object it actually refers to, in the same way that Billy could be alternate name for an object named William; they are one and the same object by different names. A pointer is not an alias, it is a completely separate object. Another key difference is that a reference cannot be reassigned, it must be initialised at the point of declaration. In this respect it is similar to the behaviour of constant pointer. Finally, a reference can never refer to a non-existent object, whereas a pointer can. When a pointer is no longer required it must be nullified by assigning it the value zero.

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