Operator new doesn't create structs on the stack #4667

svick · 2018-03-15T16:36:55Z

Contributes to #4561.

svick · 2018-03-15T16:38:52Z

docs/csharp/language-reference/keywords/new-operator.md

 int i = 0;  
 ```  

 For a complete list of default values, see [Default Values Table](../../../csharp/language-reference/keywords/default-values-table.md).  

 Remember that it is an error to declare a default constructor for a [struct](../../../csharp/language-reference/keywords/struct.md) because every value type implicitly has a public default constructor. It is possible to declare parameterized constructors on a struct type to set its initial values, but this is only necessary if values other than the default are required.  

- Value-type objects such as structs are created on the stack, while reference-type objects such as classes are created on the heap. Both types of objects are destroyed automatically, but objects based on value types are destroyed when they go out of scope, whereas objects based on reference types are destroyed at an unspecified time after the last reference to them is removed. For reference types that consume fixed resources such as large amounts of memory, file handles, or network connections, it is sometimes desirable to employ deterministic finalization to ensure that the object is destroyed as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  
+ Both value-type objects such as structs and reference-type objects such as classes are destroyed automatically, but value-type objects are destroyed when they go out of scope, whereas reference-type objects are destroyed at an unspecified time after the last reference to them is removed. For reference types that contain resources such as file handles, or network connections, it is desirable to employ deterministic cleanup to ensure that the resources it contains are released as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  


I'm not completely satisfied with this formulation (when do value types in methods with closures or in coroutines go "out of scope"?), so feel free to suggest how to improve this further.

@svick @BillWagner this paragraph (in both edited and previous versions) triggers some questions:

What does go out of scope mean for value-types?

As for reference-type objects, the sentence that says ...after the last reference to them is removed. That might make one think about how to remove a reference to an object?

Structs also might contain file handles (in general, implement IDisposable). What to do with them?

I'm afraid this paragraph might be still confusing. What was the original reason to write it? To say: while you construct with new, do not care about clean-up as it's automatic in a lot of cases and remember about unmanaged resources?
Is it possible to shorten this paragraph into one sentence and give a link to the page about garbage collection as well?

@pkulikov

Structs also might contain file handles (in general, implement IDisposable). What to do with them?

You're right, even though structs with resources are fairly rare. Still, I have changed it so that it now talks about "types", not just "reference types".

Your remaining points are valid, but I don't know how to clarify the paragraph further. And I'm not sure shortening it to just a couple of links is the right solution either.

@svick that's why I wonder for which problem we look for

the right solution

Imagine that paragraph were removed completely :) What would the docs miss then?

Though I go too far to the philosophy-land. As for now, the paragraph is clearly improved and worth merging.

jzeferino · 2018-03-15T23:01:40Z

docs/csharp/language-reference/keywords/new-operator.md

 int i = 0;  
 ```  

 For a complete list of default values, see [Default Values Table](../../../csharp/language-reference/keywords/default-values-table.md).  

 Remember that it is an error to declare a default constructor for a [struct](../../../csharp/language-reference/keywords/struct.md) because every value type implicitly has a public default constructor. It is possible to declare parameterized constructors on a struct type to set its initial values, but this is only necessary if values other than the default are required.  

- Value-type objects such as structs are created on the stack, while reference-type objects such as classes are created on the heap. Both types of objects are destroyed automatically, but objects based on value types are destroyed when they go out of scope, whereas objects based on reference types are destroyed at an unspecified time after the last reference to them is removed. For reference types that consume fixed resources such as large amounts of memory, file handles, or network connections, it is sometimes desirable to employ deterministic finalization to ensure that the object is destroyed as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  
+ Both value-type objects such as structs and reference-type objects such as classes are destroyed automatically, but value-type objects are destroyed when they go out of scope, whereas reference-type objects are destroyed at an unspecified time after the last reference to them is removed. For types that contain resources such as file handles, or network connections, it is desirable to employ deterministic cleanup to ensure that the resources they contain are released as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  


Maybe should state that reference types references are managed by the GC?

@jzeferino Yeah, good idea, added.

jzeferino

this LGTM.

BillWagner

Thanks for making this update, and sparking a truly enjoyable and complicated discussion 🤔

I like the way you've updated everything, I just have one small comment on wording to avoid making a guarantee that structs are always destroyed when they go out of scope. (they won't be if they are members of a class). Explaining those is out of scope, so I propose a correct, but incomplete re-wording.

BillWagner · 2018-03-16T17:22:57Z

docs/csharp/language-reference/keywords/new-operator.md

 int i = 0;  
 ```  

 For a complete list of default values, see [Default Values Table](../../../csharp/language-reference/keywords/default-values-table.md).  

 Remember that it is an error to declare a default constructor for a [struct](../../../csharp/language-reference/keywords/struct.md) because every value type implicitly has a public default constructor. It is possible to declare parameterized constructors on a struct type to set its initial values, but this is only necessary if values other than the default are required.  

- Value-type objects such as structs are created on the stack, while reference-type objects such as classes are created on the heap. Both types of objects are destroyed automatically, but objects based on value types are destroyed when they go out of scope, whereas objects based on reference types are destroyed at an unspecified time after the last reference to them is removed. For reference types that consume fixed resources such as large amounts of memory, file handles, or network connections, it is sometimes desirable to employ deterministic finalization to ensure that the object is destroyed as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  
+ Both value-type objects such as structs and reference-type objects such as classes are destroyed automatically, but value-type objects are destroyed when they go out of scope, whereas reference-type objects are destroyed by the garbage collector at an unspecified time after the last reference to them is removed. For types that contain resources such as file handles, or network connections, it is desirable to employ deterministic cleanup to ensure that the resources they contain are released as soon as possible. For more information, see [using Statement](../../../csharp/language-reference/keywords/using-statement.md).  


For this topic, I'd be a bit more vague on value types. Where you say

value-type objects are destroyed when they go out of scope,

I'd say

value-type objects may be destroyed when the go out of scope,

Yes, I know that's not 100% accurate, and it does beg the question of "when?"

I don't like the proposed wording, because it's too vague: it doesn't say almost anything.

What do you think about something like the following?

value-type objects are destroyed when their scope is destroyed,

Or maybe use a different term instead of "scope", like "containing context"?

I like "containing context". That worked in all the instances I tried in a thought exercise this morning:

local variables in normal routines

member of struct

member of class

closures for lambda expressions, etc.

local in co-routine

static member of struct or class

Let's go with it.

@BillWagner Done.

BillWagner · 2018-03-17T17:56:29Z

Thanks @svick This is an important start.

I'll when the build finishes.

@BillWagner

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svick added 2 commits March 15, 2018 17:29

Operator new doesn't create structs on the stack

208e2b9

Small improvements to operator new

fd6cdc0

svick requested a review from BillWagner as a code owner March 15, 2018 16:36

svick commented Mar 15, 2018

View reviewed changes

Structs can contain resources too

8671934

jzeferino reviewed Mar 15, 2018

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Mention garbage collector

8d28076

jzeferino approved these changes Mar 16, 2018

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BillWagner requested changes Mar 16, 2018

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Clarified when value types are destroyed

000e767

BillWagner approved these changes Mar 17, 2018

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BillWagner added this to the Sprint 132 (2/24/18 - 3/16/18) milestone Mar 17, 2018

BillWagner added the verify-build-before-merge label Mar 17, 2018

BillWagner merged commit 25bb9ef into dotnet:master Mar 17, 2018

svick deleted the patch-4 branch March 17, 2018 22:07

mairaw removed the verify-build-before-merge label Mar 31, 2018

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Operator new doesn't create structs on the stack #4667

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