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Closed 10 years ago.
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Pass by Reference / Value in C++
I was wondering what the difference is between a call by value/reference/name. And why would it be beneficial to use one over another?
call by value: a copy of the parameters is passed to the function
call be reference: no extra copy is made, the caller's variable is passed directly.
Major difference is that one extra unnecessary copy is made in call by value paradigm... You should always use call be reference (or const reference) unless a callee needs to modify the variable and you don't want the changes to your caller's variable...
Call by value creates a copy of the argument which gets passed to the function - so for a large object that could create a large overhead. It also stops you making any changes to the argument inside the function as they will be reflected in the copy only. Call by reference passes a reference to the object and so changes can be made to that object - unless of course you pass by const reference.
Related
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What are the differences between a pointer variable and a reference variable?
(44 answers)
Closed 6 years ago.
I'm wondering if there is any difference between passing an object reference as a parameter or as an argument. Is the code below equivalent? Is there situations where I should use one or the other?
void foo(Object &object){
object.update()
}
Object object
foo(object)
VS
void bar(Object *object){
object->update()
}
Object object
bar(&object)
You seem a bit confused. None of your examples pass by value. First example passes by reference, the second passes by pointer.
The main differences are:
Pass by value foo(Object object): the function gets a copy of the argument and cannot change the original.
Pass by reference foo(Object& object): the function gets a reference (not copy) to the original object and can modify it. References cannot be "un-bound", they always refer to a valid object (with certain exceptions which are usually bugs).
Pass by pointer foo(Object* object): as passing by reference, except it is valid for the pointer to not refer to anything (it can be nullptr which is useful if you need to signal that).
When you take by reference and the function can be inlined, the compiler is - theoretically - not required to generate the address of the instance. Depending on your type, unary prefix operator&() might do more than just returning this. Also, it's valid for a pointer to be nullptr, you should check for that (or use gsl's not_null).
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Is it better in C++ to pass by value or pass by reference-to-const?
(11 answers)
Closed 7 years ago.
I studied that objects can be passed by referece (I mean that the parameters of the function are references). But why one prefers them over simply passing objects. My motive is to just use that object and in no sense modify it, so I do not mean the trivial advantages. So, what are the advantages of passing a reference to an object?
Here are some advantages of passing by reference:
No new copy of variable is made, so overhead of copying is saved. This
Makes program execute faster specially when passing object of large structs or classes.
Array or Object can be pass
Sometimes function need to change the original value(eg. Sorting an array, swapping) and sometimes changing value inside function is useful.
Can return multiple values from a function.
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Why pass by const reference instead of by value?
(8 answers)
Closed 8 years ago.
The point of passing by reference, I thought, was to pass in the actual variable either to save time or to modify the variable. Thus -- and this is going to make me sound stupid -- I used to think that passing a constant reference somehow temporarily changed the passed-in variable to a constant (if it wasn't already) and then changed it back to a non-constant (unless it was constant to begin with) when the function was done executing. I just learned today when I was reading C++ Primer that passing in a constant reference creates a temporary constant variable. That is,
foo(const type& obj)
is the same as
foo(const type obj)
because they both create a constant object obj and set it equal to whatever is passed in.
So what's the point of passing by constant reference??? What's the point of references anyways? They're the same thing as pointers, except that they need to be initialized and are thus less useful.
when you pass constant reference no temporary copy is created.....while if you pass constant argument new copy is created....
you can check this by printing the addresses of variable in caller function and callee function.
The right way of doing it is using a constant reference:
foo(const type& obj)
because that way the object is not copied, and that is more efficient especially if the object is big or the function is called multiple times.
No. A constant variable creates a temporary variable but a constant reference may not.
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References in VB.Net
I want to pass a medium large Customer db object, but I don't want to pass it by value, because I think it would be unnecessary.
In c++ when you had a large object it was inefficient to pass it by value, because a copy was created from it, so you passed it by reference so that there was no copy (of the object passed) created. I used to pass the parameter as a constant because that way if I tried to change the object inside the function the compiler wouldn't let me so that I wouldn't harm the passed object (because it was passed by reference). Is it possible to mimick this in vb.net or is it not needed?
My strong suspicion is that you're getting confused about how values are passed in VB.
If your CustomerDb type is a class, then every expression of that type will have a value which is already a reference. By default, that reference will be passed by value - but it's still only the reference which is passed, not a whole object.
If your CustomerDb type is a structure, then you really will be passing the whole value each time - and you should strongly consider changing it to a class anyway...
Of course, when you pass a reference by value, that doesn't stop the object from being modified within the method, but it does mean that changes to the parameter variable itself aren't reflected in the calling code.
Read my article on C# parameter passing - it's much the same in VB.
In .NET, classes (which I assume Customer is) are reference types. Passing a reference type as an argument will pass a reference (pointer) to the object. So passing a class is perfectly efficient and no copy is created.
If you pass a class by reference, then the method could change the reference (pointer) and it would be reflected in the calling method.
I don't know off the top of my head if there's an easy way to make the argument read only. Since reference types do pass a reference, any changes to the object will be reflected in the original. You might need a copy if you don't want the original to be modified.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
What's the difference between passing by reference vs. passing by value?
If I have a function that takes in some parameters and then does something with the parameters without needing to change their inherent value, is there any benefit from using pass by reference versus pass by value?
Yes. Passing by value copies the argument, which might be very expensive (or not even possible). If you want to pass by reference, but not modify the object, pass by const-reference.
As an example of an object that cannot be passed by value:
class Foo {
public:
Foo() {}
private:
Foo(const Foo&); // copy-constructor is not accessible
}
Here is the general guide:
Pass by reference when
You need to cause a side effect on the object that will be visible to the caller
or you cannot pass by value because of the lack of an accessible copy constructor or something and you need side effects
Pass by const reference when you have
A large object
and/or you cannot pass by value
and/or no need for side effects
Pass by value when
The object is small
and/or You do not need side effects
and/or You need a copy of the value to work with anyway