Why is string a reference type, even though it's normally primitive data type such as int, float, or double.
In addition to the reasons posted by Dan:
Value types are, by definition those types which store their values in themselves, rather than referring to a value somewhere else. That's why value types are called "value types" and reference types are called "reference types". So your question is really "why does a string refer to its contents rather than simply containing its contents?"
It's because value types have the nice property that every instance of a given value type is of the same size in memory.
So what? Why is this a nice property? Well, suppose strings were value types that could be of any size and consider the following:
string[] mystrings = new string[3];
What are the initial contents of that array of three strings? There is no "null" for value types, so the only sensible thing to do is to create an array of three empty strings. How would that be laid out in memory? Think about that for a bit. How would you do it?
Now suppose you say
string[] mystrings = new string[3];
mystrings[1] = "hello";
Now we have "", "hello" and "" in the array. Where in memory does the "hello" go? How large is the slot that was allocated for mystrings[1] anyway? The memory for the array and its elements has to go somewhere.
This leaves the CLR with the following choices:
resize the array every time you change one of its elements, copying the entire thing, which could be megabytes in size
disallow creating arrays of value types of unknown size
disallow creating value types of unknown size
The CLR team chose the latter one. Making strings into reference types means that you can create arrays of them efficiently.
Yikes, this answer got accepted and then I changed it. I should probably include the original answer at the bottom since that's what was accepted by the OP.
New Answer
Update: Here's the thing. string absolutely needs to behave like a reference type. The reasons for this have been touched on by all answers so far: the string type does not have a constant size, it makes no sense to copy the entire contents of a string from one method to another, string[] arrays would otherwise have to resize themelves -- just to name a few.
But you could still define string as a struct that internally points to a char[] array or even a char* pointer and an int for its length, make it immutable, and voila!, you'd have a type that behaves like a reference type but is technically a value type.
This would seem quite silly, honestly. As Eric Lippert has pointed out in a few of the comments to other answers, defining a value type like this is basically the same as defining a reference type. In nearly every sense, it would be indistinguishable from a reference type defined the same way.
So the answer to the question "Why is string a reference type?" is, basically: "To make it a value type would just be silly." But if that's the only reason, then really, the logical conclusion is that string could actually have been defined as a struct as described above and there would be no particularly good argument against that choice.
However, there are reasons that it's better to make string a class than a struct that are more than purely intellectual. Here are a couple I was able to think of:
To prevent boxing
If string were a value type, then every time you passed it to some method expecting an object it would have to be boxed, which would create a new object, which would bloat the heap and cause pointless GC pressure. Since strings are basically everywhere, having them cause boxing all the time would be a big problem.
For intuitive equality comparison
Yes, string could override Equals regardless of whether it's a reference type or value type. But if it were a value type, then ReferenceEquals("a", "a") would return false! This is because both arguments would get boxed, and boxed arguments never have equal references (as far as I know).
So, even though it's true that you could define a value type to act just like a reference type by having it consist of a single reference type field, it would still not be exactly the same. So I maintain this as the more complete reason why string is a reference type: you could make it a value type, but this would only burden it with unnecessary weaknesses.
Original Answer
It's a reference type because only references to it are passed around.
If it were a value type then every time you passed a string from one method to another the entire string would be copied*.
Since it is a reference type, instead of string values like "Hello world!" being passed around -- "Hello world!" is 12 characters, by the way, which means it requires (at least) 24 bytes of storage -- only references to those strings are passed around. Passing around a reference is much cheaper than passing every single character in a string.
Also, it's really not a normal primitive data type. Who told you that?
*Actually, this isn't stricly true. If the string internally held a char[] array, then as long as the array type is a reference type, the contents of the string would actually not be passed by value -- only the reference to the array would be. I still think this is basically right answer, though.
String is a reference type, not a value type. In many cases, you know the length of the string and the content of the string, in such cases, it is easy to allocate the memory for the string. but consider something like this.
string s = Console.ReadLine();
is it not possible to know the allocation details for "s" in compilation time. User enters the values and all the entered string/line is stored in the s. So, strings are stored on heap so that memory is reallocated to fit the content for the string s. And reference to this string is stored on stack.
To learn more please read: .net zero by petzold
Read: Garbage collection from CLR Via C# for allocation details on stack.
Edit: Console.WriteLine(); to Console.ReadLine();
Related
There are many examples, let's take array copy method as an example. The signature of the Array.Copy is method is as below
public static void Copy (Array sourceArray, long sourceIndex, Array destinationArray, long destinationIndex, long length);
Judging only from signature, one can not tell that the sourceArray will not be changed while the destinationArray will be altered, even if it is some thing as simple as an array of Int. The guarantee coming from the keyword "ref" for programmers have lost here.
It seems to me that the the destinationArray parameter should better be marked as "ref Array". If it had been done this way, the syntax would be more consistent with the usage of the keyword "ref", indicating that the passed in object might be modified by the callee and the change is visible for the caller. The only benefit I can think of concerning mitting the keyword "ref", is that saves a few key strokes. or it is just mimicking the C/C++ style without much thinking.
My question is: what are some seasonings behind this design decision?
Update: For the record, I am advocating that an array be of the same value/reference category as its elements, thus making a clear extinction between Fun(array) and Fun(ref array), that is the same guarantee programmers get with Fun(int) and Fun(ref int). Optimization for efficiency can be left to the implementation level.
Array is a reference type. You can pass references by value and the instances they reference will still be the same ones that get modified. The callee is modifying the same instance using its own reference to it and has no reason to change it into a completely different instance entirely (which is where ref would actually come into use).
There isn't any convention that states to use ref when passing reference types — you generally don't need to most of the time, except as mentioned if your method actually intends to change the instance entirely like so:
class Foo { public int Value; }
public static void ReplaceFoo(ref Foo foo)
{
foo = new Foo { Value = 2 };
}
var foo = new Foo { Value = 1 };
Console.WriteLine(foo.Value);
ReplaceFoo(ref foo);
Console.WriteLine(foo.Value);
Judging only from signature, one can not tell that the sourceArray will not be changed while the destinationArray will be altered
Why is this a problem? No one reads APIs only paying attention to method signatures and ignoring parameter names. Signatures are there for the compiler to distinguish overloads. Anyone reading the API for Array.Copy() would understand that sourceArray is going to be unchanged, being where the method is getting the values from, and destinationArray is going to be modified, being the one receiving the values — unless they don't speak English (which is fine, but most APIs are written in English).
The only other scenario I can think of where a reader would be confused is if they didn't have the prior knowledge that arrays are reference types in .NET. But misusing ref in a situation where it's not needed at best and inappropriate at worst doesn't solve that problem.
C# (and .NET) include both reference types and value types.
Normally (absent ref or out keywords), parameters are passed to methods by value. So, if you pass an integer to a function, the value of the integer is passed. If you put a a variable referring to an array in a function call (remembering that all arrays are instances of the reference type System.Array), the value of that variable, i.e., the reference to to the array, is passed to the function.
So, within the function, the code gets to play on that array. When the function returns, that variable (in the scope of the caller) still refers to that same object. However, the function may have mutated that array, so the variable (in the caller scope) may be referring to a changed object.
If you pass a value type by reference (with the ref keyword), the function can change the value of the parameter, and when the function returns, the variable (in the caller scope) will receive the new value.
But, if you use ref (or out) on a parameter of reference type, you are passing a reference by reference. So, for example, you could pass in an array of five integers and the function could assign that parameter and array of ten integers (they are of the same type, but definitely differentobjects). In the caller, when the function returns, the variable associated with that parameter will see what it refers to completely change during the call.
In your example, the caller will instantiate two arrays of the same type and compatible lengths (usually the same length if the source and destination indexes are 0 and the length is sourceArray.Length). The function does not change what object the destination array parameter refers to, it just fills the destination from the source.
In fact, if the destination was by ref, it wouldn't be as flexible. Consider a case where the destination is 30 entries long, and your intention is to fill the middle ten array entries with the source. It just works. It wouldn't with a ref destination parameter (without a lot more work).
The reason for omitting the ref keyword is that in most cases, it won't make any difference to include it, so it's superfluous. However, it does actually make a difference in some cases. An array is a reference type, and that means a value representing that reference gets passed. Normally, updating the passed in value will trigger updates to the original object. BUT if you create a NEW array and assign the passed in parameter to the new item, the reference gets lost - whereas the ref keyword preserves it.
A String is a reference type even though it has most of the characteristics of a value type such as being immutable and having == overloaded to compare the text rather than making sure they reference the same object.
Why isn't string just a value type then?
Strings aren't value types since they can be huge, and need to be stored on the heap. Value types are (in all implementations of the CLR as of yet) stored on the stack. Stack allocating strings would break all sorts of things: the stack is only 1MB for 32-bit and 4MB for 64-bit, you'd have to box each string, incurring a copy penalty, you couldn't intern strings, and memory usage would balloon, etc...
(Edit: Added clarification about value type storage being an implementation detail, which leads to this situation where we have a type with value sematics not inheriting from System.ValueType. Thanks Ben.)
It is not a value type because performance (space and time!) would be terrible if it were a value type and its value had to be copied every time it were passed to and returned from methods, etc.
It has value semantics to keep the world sane. Can you imagine how difficult it would be to code if
string s = "hello";
string t = "hello";
bool b = (s == t);
set b to be false? Imagine how difficult coding just about any application would be.
A string is a reference type with value semantics. This design is a tradeoff which allows certain performance optimizations.
The distinction between reference types and value types are basically a performance tradeoff in the design of the language. Reference types have some overhead on construction and destruction and garbage collection, because they are created on the heap. Value types on the other hand have overhead on assignments and method calls (if the data size is larger than a pointer), because the whole object is copied in memory rather than just a pointer. Because strings can be (and typically are) much larger than the size of a pointer, they are designed as reference types. Furthermore the size of a value type must be known at compile time, which is not always the case for strings.
But strings have value semantics which means they are immutable and compared by value (i.e. character by character for a string), not by comparing references. This allows certain optimizations:
Interning means that if multiple strings are known to be equal, the compiler can just use a single string, thereby saving memory. This optimization only works if strings are immutable, otherwise changing one string would have unpredictable results on other strings.
String literals (which are known at compile time) can be interned and stored in a special static area of memory by the compiler. This saves time at runtime since they don't need to be allocated and garbage collected.
Immutable strings does increase the cost for certain operations. For example you can't replace a single character in-place, you have to allocate a new string for any change. But this is a small cost compared to the benefit of the optimizations.
Value semantics effectively hides the distinction between reference type and value types for the user. If a type has value semantics, it doesn't matter for the user if the type is a value type or reference type - it can be considered an implementation detail.
This is a late answer to an old question, but all other answers are missing the point, which is that .NET did not have generics until .NET 2.0 in 2005.
String is a reference type instead of a value type because it was of crucial importance for Microsoft to ensure that strings could be stored in the most efficient way in non-generic collections, such as System.Collections.ArrayList.
Storing a value-type in a non-generic collection requires a special conversion to the type object which is called boxing. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap.
Reading the value from the collection requires the inverse operation which is called unboxing.
Both boxing and unboxing have non-negligible cost: boxing requires an additional allocation, unboxing requires type checking.
Some answers claim incorrectly that string could never have been implemented as a value type because its size is variable. Actually it is easy to implement string as a fixed-length data structure containing two fields: an integer for the length of the string, and a pointer to a char array. You can also use a Small String Optimization strategy on top of that.
If generics had existed from day one I guess having string as a value type would probably have been a better solution, with simpler semantics, better memory usage and better cache locality. A List<string> containing only small strings could have been a single contiguous block of memory.
Not only strings are immutable reference types.
Multi-cast delegates too.
That is why it is safe to write
protected void OnMyEventHandler()
{
delegate handler = this.MyEventHandler;
if (null != handler)
{
handler(this, new EventArgs());
}
}
I suppose that strings are immutable because this is the most safe method to work with them and allocate memory.
Why they are not Value types? Previous authors are right about stack size etc. I would also add that making strings a reference types allow to save on assembly size when you use the same constant string in the program. If you define
string s1 = "my string";
//some code here
string s2 = "my string";
Chances are that both instances of "my string" constant will be allocated in your assembly only once.
If you would like to manage strings like usual reference type, put the string inside a new StringBuilder(string s). Or use MemoryStreams.
If you are to create a library, where you expect a huge strings to be passed in your functions, either define a parameter as a StringBuilder or as a Stream.
In a very simple words any value which has a definite size can be treated as a value type.
Also, the way strings are implemented (different for each platform) and when you start stitching them together. Like using a StringBuilder. It allocats a buffer for you to copy into, once you reach the end, it allocates even more memory for you, in the hopes that if you do a large concatenation performance won't be hindered.
Maybe Jon Skeet can help up out here?
It is mainly a performance issue.
Having strings behave LIKE value type helps when writing code, but having it BE a value type would make a huge performance hit.
For an in-depth look, take a peek at a nice article on strings in the .net framework.
How can you tell string is a reference type? I'm not sure that it matters how it is implemented. Strings in C# are immutable precisely so that you don't have to worry about this issue.
Actually strings have very few resemblances to value types. For starters, not all value types are immutable, you can change the value of an Int32 all you want and it it would still be the same address on the stack.
Strings are immutable for a very good reason, it has nothing to do with it being a reference type, but has a lot to do with memory management. It's just more efficient to create a new object when string size changes than to shift things around on the managed heap. I think you're mixing together value/reference types and immutable objects concepts.
As far as "==": Like you said "==" is an operator overload, and again it was implemented for a very good reason to make framework more useful when working with strings.
The fact that many mention the stack and memory with respect to value types and primitive types is because they must fit into a register in the microprocessor. You cannot push or pop something to/from the stack if it takes more bits than a register has....the instructions are, for example "pop eax" -- because eax is 32 bits wide on a 32-bit system.
Floating-point primitive types are handled by the FPU, which is 80 bits wide.
This was all decided long before there was an OOP language to obfuscate the definition of primitive type and I assume that value type is a term that has been created specifically for OOP languages.
Isn't just as simple as Strings are made up of characters arrays. I look at strings as character arrays[]. Therefore they are on the heap because the reference memory location is stored on the stack and points to the beginning of the array's memory location on the heap. The string size is not known before it is allocated ...perfect for the heap.
That is why a string is really immutable because when you change it even if it is of the same size the compiler doesn't know that and has to allocate a new array and assign characters to the positions in the array. It makes sense if you think of strings as a way that languages protect you from having to allocate memory on the fly (read C like programming)
After reading the MSDN article on the ref keyword, I am confused as to what C# does when you pass a value type using the ref keyword. The documentation states that the ValueTypes are not boxed. My question is how does C# handle passing a value type as a reference? Is it passing some copy to the data that is allocated on the Stack? Thanks.
Is it passing some copy to the data that is allocated on the Stack?
No, it does not make a copy. ref and out keyword can be compared to passing by pointer in C or passing by reference in C++, when the memory location (i.e. an address) of the variable is passed to the target method. The method that takes a reference would then modify the value directly in place using the memory location passed in.
Knowing that the variable is passed by reference, compiler inserts instructions that treat the ref variable as an address, allowing in-place modifications.
tl;dr: Boxing isn't "how you create a reference"; it's "how you package a primitive value type for consumers who don't expect that exact type".
In .NET, reference types are class instances on the heap. Value types like int or double are just the bytes: A 32-bit int is just four bytes worth of zeroes and ones. When you put it in, say a System.List (the old-timey pre-generic kind, that Granpaw whittled out down at the General Store), then take it back out, how will the compiler know what to do if you call GetType() on it? It would just have four bytes of... what? Who knows? If it stored a pointer in the List, it would have a pointer to four bytes of... who knows?
In your own method, the generated code knows what your variable is. Regular strong type-checking. But that doesn't work when you send your variable's value it to somebody else who only knows he's expecting Object.
So when you add an int to a List, or pass it to a function that takes Object as an argument, the compiler has to add some information to it so everybody else knows what he's getting.
So "Boxing" means packaging a non-reference value into an object that can be treated as an instance of Object. For ordinary ref parameters, that's not necessary, because the type is known the whole way: The code generated for the guts of the function doesn't have to be prepared to deal with any arbitrary reference type. It knows it's getting (for example) a pointer to an integer, and that's all it's going to get. Boxing provides capability that's not required in this case, and so the compiler doesn't waste your users' cycles on it.
Boxing isn't the only way to have a reference (in the broadest sense of the term) to, for example, a double. Rather, boxing is the only way to treat a double as an object that can be stored in a System.List: It has to be on the heap, it has to be castable to Object, has to have run-time type information, etc. etc.
For the following, all all the caller or the callee need is the address of 64 zeroes and ones somewhere:
void f(ref double d) { d *= 2; }
Is it better to transfer small or large strings by reference in C#? I assumed transferring by value would force the runtime to create a clone of the input string, and thus be slower. Is it recommended for all string functions to transfer values by reference therefore?
I assumed transferring by value would force the runtime to create a clone of the input string, and thus be slower.
Your assumption is incorrect. String is a reference type - calling a method with a string argument just copies that reference, by value. There's no cloning involved. It's a fixed size - 4 or 8 bytes depending on which CLR you're using.
(Even if it were a value type, it would have to basically contain a reference to something else - it wouldn't make sense to have a variable-size value type allocated directly on the stack. How much space would be allocated for the variable? What would happen if you changed the value of the variable to a shorter or longer string?)
Seeing as new instances of value types are created every time they are passed as arguments, I started thinking about scenarios where using the ref or out keywords can show a substantial performance improvement.
After a while it hit me that while I see the deficit of using value types I didn't know of any advantages.
So my question is rather straight forward - what is the purpose of having value types? what do we gain by copying a structure instead of just creating a new reference to it?
It seems to me that it would be a lot easier to only have reference types like in Java.
Edit: Just to clear this up, I am not referring to value types smaller than 8 bytes (max size of a reference), but rather value types that are 8 bytes or more.
For example - the Rectangle struct that contains four int values.
An instance of a one-byte value type takes up one byte. A reference type takes up the space for the reference plus the sync block and the virtual function table and ...
To copy a reference, you copy a four (or eight) byte reference. To copy a four-byte integer, you copy a four byte integer. Copying small value types is no more expensive than copying references.
Value types that contain no references need not be examined by the garbage collector at all. Every reference must be tracked by the garbage collector.
Value types are usually more performant than reference types:
A reference type costs extra memory for the reference and performance when dereferencing
A value type does not need extra garbage collection. It gets garbage collected together with the instance it lives in. Local variables in methods get cleaned up upon method leave.
Value type arrays are efficient in combination with caches. (Think of an array of ints compared with an array of instances of type Integer)
"Creating a reference" is not the problem. This is just a copy of 32/64 bits. Creating the object is what is costly. Actually creating the object is cheap but collecting it isn't.
Value types are good for performance when they are small and discarded often. They can be used in huge arrays very efficiently. A struct has no object header. There are a lot of other performance differences.
Edit: Eric Lippert posed a great example in the comments: "How many bytes does an array of one million bytes take up if they are value types? How many does it take up if they are reference types?"
I will answer: If struct packing is set to 1 such an array will take 1 million and 16 bytes (on 32 bit system). Using reference types it will take:
array, object header: 12
array, length: 4
array, data: 4*(1 million) = 4m
1 million objects, headers = 12 * (1 million)
1 million objects, data padded to 4 bytes: 4 * (1 million)
And that is why using value types in large arrays can be a good idea.
The gain is visible if your data is small (<16 bytes), you have lots of instances and/or you manipulate them a lot, especially passing to functions. This is because creating an object is relatively expensive compared to creating a small value type instance. And as someone else pointed out, objects need to be collected and that is even more expensive. Plus, very small value types take less memory than their reference type equivalents.
Example of non-primitive value type in .NET is Point structure (System.Drawing).
Every variable has a lifecycle. but not every variable need the flexibility for your variable to perform high but not managed in heap.
Value types (Struct) contain their data allocate in stack or allocated in-line in a structure. Reference types (Class) store a reference to the value's memory address, and are allocated on the heap.
what is the purpose of having value types?
Value types are quite efficient to handle simple data, (It should be use to represent immutable types to represent value)
Value type objects cannot be allocated on the garbage-collected heap, and the variable representing the object does not contain a pointer to an object; the variable contains the object itself.
what do we gain by copying a structure instead of just creating a new reference to it?
If you copy a struct, C# creates a new copy of the object and assigns the copy of the object to a separate struct instance. However, if you copy a class, C# creates a new copy of the reference to the object and assigns the copy of the reference to the separate class instance. Structs can't have destructors, but classes can have destructors.
One major advantage of value types like Rectangle is that if one has n storage locations of type Rectangle, one can be certain that one has n distinct instances of type Rectangle. If one has an array MyArray of type Rectangle, of length at least two, a statement like MyArray[0] = MyArray[1] will copy the fields of MyArray[1] into those of MyArray[0], but they will continue to refer to distinct Rectangle instances. If one then performs a statement line MyArray[0].X += 4 that will modify field X of one instance, without modifying the X value of any other array slot or Rectangle instance. Note, by the way, that creating the array instantly populates it with writable Rectangle instances.
Imagine if Rectangle were a mutable class type. Creating an array of mutable Rectangle instances would require that one first dimension the array, and then assign to each element in the array a new Rectangle instance. If one wanted to copy the value of one rectangle instance to another, one would have to say something like MyArray[0].CopyValuesFrom(MyArray[1]) [which would, of course, fail if MyArray[0] had not been populated with a reference to a new instance). If one were to accidentally say MyArray[0] = MyArray[1], then writing to MyArray[0].X would also affect MyArray[1].X. Nasty stuff.
It's important to note that there are a few places in C# and vb.net where the compiler will implicitly copy a value type and then act upon a copy as though it was the original. This is a really unfortunate language design, and has prompted some people to put forth the proposition that value types should be immutable (since most situations involving implicit copying only cause problems with mutable value types). Back when compilers were very bad at warning of cases where semantically-dubious copies would yield broken behavior, such a notion might have been reasonable. It should be considered obsolete today, though, given that any decent modern compiler will flag errors in most scenarios where implicit copying would yield broken semantics, including all scenarios where structs are only mutated via constructors, property setters, or external assignments to public mutable fields. A statement like MyArray[0].X += 5 is far more readable than MyArray[0] = new Rectangle(MyArray[0].X + 5, MyArray[0].Y, MyArray[0].Width, MyArray[0].Height).