Simple question, I dont want to allocate and copy heap allocated memory every time I need to copy an object with heap pointers, its silly, I can just point the pointer to the original heap, and assure the original allocator doesnt delete the pointer.
I guess I need a reference counted pointer....witch one? I dont have experience using smart pointers, I just want one that everytime I assign it it increase the reference count, and just deletes it if reference count is 0..
Is it the case for shared_ptr? can I just close my eyes and use it?
Why not just create my own refcount_ptr that increases the reference count on assignment and check it on destructor?
If you want the pointee to be shared between the original and the copied object, you can use boost::shared_ptr or std::shared_ptr.
Why not just create my own refcount_ptr that increases the reference
count on assignment and check it on destructor?
Well, you can write your own shared_ptr, but usually it's not worth reinventing the wheel unless the existing one doesn't match your needs.
Related
Based on my earlier question, I understand the benefit of using stack allocation. Suppose I have an array of arrays. For example, A is a list of matrices and each element A[i] is a 1x3 matrix. The length of A and the dimension of A[i] are known at run time (given by the user). Each A[i] is a matrix of Float64 and this is also known at run time. However, through out the program, I will be modifying the values of A[i] element by element. What data structure can also allow me to use stack allocation? I tried StaticArrays but it doesn't allow me to modify a static array.
StaticArrays defines MArray (MVector, MMatrix) types that are fixed-size and mutable. If you use these there's a higher chance of the compiler determining that they can be stack-allocated, but it's not guaranteed. Moreover, since the pattern you're using is that you're passing the mutable state vector into a function which presumably modifies it, it's not going to be valid or helpful to stack allocate that anyway. If you're going to allocate state once and modify it throughout the program, it doesn't really matter if it is heap or stack allocated—stack allocation is only a big win for objects that are allocated, used locally and then don't escape the local scope, so they can be “freed” simply by popping the stack.
From the code snippet you showed in the linked question, the state vector is allocated in the outer function, test_for_loop, which shouldn't be a big deal since it's done once at the beginning of execution. Using a variably sized state vector to index into an array with a splat (...) might be an issue, however, and that's done in test_function. Using something with fixed size like MVector might be better for that. It might, however, be better still, to use a state tuple and return a new rather than mutated state tuple at the end. The compiler is very good at turning that kind of thing into very efficient code because of immutability.
Note that by convention test_function should be called test_function! since it modifies its M argument and even more so if it modifies the state vector.
I would also note that this isn't a great question/answer pair since it's not standalone at all and really just a continuation of your other question. StackOverflow isn't very good for this kind of iterative question/discussion interaction, I'm afraid.
What is the right way to use slice in Go. As per Go documentation slice is by default pointer, so is creating slice as *[]Item is the right way?. Since slice are by default pointer isn't this way of creating the slice making it pointer to a pointer.
I feel the right way to create slice is []Item or []*item (slice holding pointers of items)
A bit of theory
Your question has no sense: there's no "right" or "wrong" or "correct" and "incorrect": you can have a pointer to a slice, and you can have a pointer to a pointer to a slice, and you can add levels of such indirection endlessly.
What to do depends on what you need in a particular case.
To help you with the reasoning, I'll try to provide a couple of facts and draw some conclusions.
The first two things to understand about types and values in Go are:
Everything in Go, ever, always, is passed by value.
This means variable assignments (= and :=), passing values to function and method calls, and copying memory which happens internally such as when reallocating backing arrays of slices or rebalancing maps.
Passing by value means that actual bits of the value which is assigned are physically copied into the variable which "receives" the value.
Types in Go—both built-in and user-defined (including those defined in the standard library)—can have value semantics and reference semantics when it comes to assignment.
This one is a bit tricky, and often leads to novices incorrectly assuming that the first rule explained above does not hold.
"The trick" is that if a type contains a pointer (an adderss of a variable) or consists of a single pointer, the value of this pointer is copied when the value of the type is copied.
What does this mean?
Pretty simple: if you assign the value of a variable of type int to another variable of type int, both variables contain identical bits but they are completely independent: change the content of any of them, and another will be unaffected.
If you assign a variable containing a pointer (or consisting of a single pointer) to another one, they both, again, will contain identical bits and are independent in the sense that changing those bits in any of them will not affect the other.
But since the pointer in both these variables contains the address of the same memory location, using those pointers to modify the contents of the memory location they point at will modify the same memory.
In other words, the difference is that an int does not reference anything while a pointer naturally references another memory location—because it contains its address.
Hence, if a type contains at least a single pointer (it may do so by containing a field of another type which itself contains a pointer, and so on—to any nesting level), values of this type will have reference assignment semantics: if you assign a value to another variable, you end up with two values referencing the same memory location.
That is why maps, slices and strings have reference semantics: when you assign variables of these types both variables point to the same underlying memory location.
Let's move on to slices.
Slices vs pointers to slices
A slice, logically, is a struct of three fields: a pointer to the slice's backing array which actually contains the slice's elements, and two ints: the capacity of the slice and its length.
When you pass around and assign a slice value, these struct values are copied: a pointer and two integers.
As you can see, when you pass a slice value around the backing array is not copied—only a pointer to it.
Now let's consider when you want to use a plain slice or a pointer to a slice.
If you're concerned with performance (memory allocation and/or CPU cycles needed to copy memory), these concerns are unfounded: copying three integers when passing around a slice is dirt-cheap on today's hardware.
Using a pointer to a slice would make copying a tiny bit faster—a single integer rather than three—but these savings will be easily offset by two facts:
The slice's value will almost certainly end up being allocated on the heap so that the compiler can be sure its value will survive crossing boundaries of the function calls—so you will pay for using the memory manager and the garbage collector will have more work.
Using a level of indirection reduces data locality: accessing RAM is slow so CPUs have caches which prefetch data at the addresses following the one at which the data is being read. If the control flow immediately reads memory at another location, the prefetched data is thrown away: cache trashing.
OK, so is there a case when you would want a pointer to a slice?
Yes. For instance, the built-in append function could have been defined as
func append(*[]T, T...)
instead of
func append([]T, T...) []T
(N.B. the T here actually means "any type" because append is not a library fuction and cannot be sensibly defined in plain Go; so it's sort of pseudocode.)
That is, it could accept a pointer to a slice and possibly replace the slice pointed to by the pointer, so you'd call it as append(&slice, element) and not as slice = append(slice, element).
But honestly, in real-world Go projects I have dealt with, the only case of using pointers to slices which I can remember was about pooling slices which are heavily reused—to save on memory reallocations. And that sole case was only due to sync.Pool keeping elements of type interface{} which may be more effective when using pointers¹.
Slices of values vs slices of pointers to values
Exactly the same logic described above applies to the reasoning about this case.
When you put a value in a slice that value is copied. When the slice needs to grow its backing array, the array will be reallocated, and reallocation means physically copying all existing elements into the new memory location.
So, two considerations:
Are elements reasonably small so that copying them is not going to press on memory and CPU resources?
(Note that "small" vs "big" also heavily depens on the frequency of such copying in a working program: copying a couple of megabytes once in a while is not a big deal; copying even tens of kilobytes in a tight time-critical loop can be a big deal.)
Are you program OK with multiple copies of the same data (for instance, values of certain types like sync.Mutex must not be copied after first use)?
If the answer to either question is "no", you should consider keeping pointers in the slice. But when you consider keeping pointers, also think about data locality explained above: if a slice contains data intended for time-critical number-crunching, it's better not have the CPU to chase pointers.
To recap: when you ask about a "correct" or "right" way of doing something, the question has no sense without specifying the set of criteria according to which we could classify all possible solutions to a problem. Still, there are considerations which must be performed when designing the way you're going to store and manipulate data, and I have tried to explain these considerations.
In general, a rule of thumb regarding slices could be:
Slices are designed to be passed around "as is"—as values, not pointers to variables containing their values.
There are legitimate reasons to have pointers to slices, though.
Most of the time you keep values in the slice's elements, not pointers to variables with these values.
Exceptions to this general rule:
Values you intend to store in a slice occupy too much space so that it looks like the envisioned pattern of using slices of them would involve excessive memory pressure.
Types of values you intend to store in a slice require they must not be copied but rather only referenced, existing as a single instance each. A good example are types containing/embedding a field of type sync.Mutex (or, actually, a variable of any other type from the sync package except those which itself have reference semantics such as sync.Pool): if you lock a mutex, copy its value and then unlock the copy, the initially locked copy won't notice, which means you have a grave bug in your code.
A note of caution on correctness vs performance
The text above contains a lot of performance considerations.
I've presented them because Go is a reasonably low-level language: not that low-level as C and C++ and Rust but still providing the programmer with plenty of wiggle-room to use when performance is at stake.
Still, you should very well understand that at this point on your learning curve, correctness must be your top—if not the sole—objective: please take no offence, but if you were after tuning some Go code to shave off some CPU time to execute it, you weren't asking your question in the first place.
In other words, please consider all of the above as a set of facts and considerations to guilde you in your learning and exploration of the subject but do not fall into the trap of trying to think about performance first. Make your programs correct and easy to read and modify.
¹ An interface value is a pair of pointers: to the variable containing the value you have put into the interface value and to a special data structure inside the Go runtime which describes the type of that variable.
So while you can put a slice value into a variable of type interface{} directly—in the sense that it's perfectly fine in the language—if the value's type is not itself a single pointer, the compiler will have to allocate on the heap a variable to contain a copy of your value there, and store a pointer to that new variable into the value of type interface{}.
This is needed to hold that "everything is always passed by value" semantics of the Go assignments.
Consequently, if you put a slice value into a variable of type interface{}, you will end up with a copy of that value on the heap.
Because of this, keeping pointers to slices in data structures such as sync.Map makes code uglier but results in lesser memory churn.
From: A Tour of C++ (Second edition)
13.2.1 unique_ptr and shared_ptr
Using make_shared() is not just more convenient than separately
making an object using new and then passing it to a shared_ptr, it is
also notably more efficient because it does not need a separate
allocation for the use count that is essential in the implementation
of a shared_ptr.
My question: How come shared_ptr does need to allocate memory for reference counting and make_shared() not? (Will it allocate it only once there are at least two references to the data?)
Edit: I haven't noticed the word "seperate" in the text, so my question is irrelevant ,Tough - I would still like to ask why make_shared() is more efficient
A shared pointer contains two parts: The pointer to the "object" you have created, and a pointer to a special control block that contains the reference counter and possibly some other meta-data needed.
If you create your own std::shared_ptr object, these two memory block will be allocated separately. If you use std::make_shared then the function will only make a single allocation for both blocks of memory.
When we move std::vector we just steal its content. So this code:
std::vector<MyClass> v{ std::move(tmpVec) };
will not allocate new memory, will not call any of constructors of MyClass.
But what if I want to split a temporary vector? In theory, I could steal the content as we did before and distribute it among new vectors. In practice I can't do this. The best so far solution I found is to use std::move() from <algorithm> header. But here the operator new will be called for every new vector. Additionally, move constructor (if available) will be called for every element we move.
What else can I do (c++17 counts)?
In theory, I could steal the content as we did before and distribute it among new vectors.
No, you cannot.
A memory allocation cannot be broken up into multiple memory allocations. At least, not without doing multiple memory allocations, then copying/moving the elements from the original into those separate pieces.
You cannot create separate vectors that have different storage without actually copying/moving the elements to those different memory buffers. You can of course take separate ranges of that vector and do whatever you can with such ranges (iterator/pointer pairs, gsl::span, etc). But each range would always be referencing elements ultimately owned by the source vector; they cannot independently own subranges of a vector.
You can write a span class that stores two pointers, and does not own the data between them. It can have many vector-like operations on it.
It should also support slicing itself (without allocation) into sub components.
You can write an shared_span class that has both those two pointers, and a shared_ptr which represents (possibly shared) ownership of the underlying buffer. It should support the operations of span, except functions returning span (like without_front(std::size_t count=1)) should instead return shared_span (with shared ownership).
You can write a move constructor from vector to shared_span easily. You may even be able to write a function from shared_span to vector with a special allocator that doesn't allocate until it grows. Making that fully portable would be very difficult.
If it is possible (I am uncertain), you could take a std::vector, move its storage into a shared_ptr<std::vector>, feed that to an allocator, build two std::vector<T, special_allocator>s that use that memory, and do what you want.
But you could just replace your request for vector doing this with code that consume a shared_span. shared_span could even have a concept of extra "dead" memory before/after the buffer it is using, giving it performance approaching std::vector.
There is a span in the gsl library you could possibly use. I am unaware of a publicly available shared_span.
Please don't mark this as duplicate question, as this is more specific to golang and requesting advise on some best practice when declaring variables to store large byte arrays when reading from a channel.
Forgive me for this dumb question, but the reason for this question is just my curiosity to determine what could be the best practice on writing a high-performance stream consumer reading large size byte array from multiple channels. (although premature optimization is the root of all evil, this is more of a curiosity). I have read answers about similar senario specific to C here, but I am requesting answer specific to go, as it is a garbage collected language, and their documentation here says "From a correctness standpoint, you don't need to know where the variable is allocated".
If I have the following code to read from a channel,
for {
select {
case msg := <-stream.Messages():
...snip...
Variable msg is within the scope of the case statement.
What happens once it goes out of scope of case statement? Since this is declared in the same native function, and the size of stream could be a large byte slice, is the variable going to be stored in heap or stack, and if heap, will it be garbage collected, or does stack pointer comes into picture?
Since this is inside an infinite for loop, and the size of stream is a large byte slice, is creating the variable and allocating memory every time an overhead,or should I declare the variable ahead, and keeps on over-writing it in every iteration, so that if there is a garbage collection involved, which I am not sure, I could possibly reduce the garbage?
Shouldn't I be bothered about it at all?
Thank you.
Shouldn't I be bothered about it at all?
No.
(And once it bothers you: profile.)
If the channel value type is a slice, the value of the variable msg is just the slice descriptor, which is small (see https://blog.golang.org/go-slices-usage-and-internals). The array that contains the data the slice refers to will have been allocated elsewhere before the slice was placed on the channel. Assuming the value must survive after the function that allocated it returns, it will be on the heap. Note that the contents of the slice are not actually being moved or copied by the channel receive operation.
Once the value of msg becomes unreachable (by the variable going out of scope or being assigned a different value), assuming there are no other references to the array underlying the slice, it will be subject to garbage collection.
It's hard to say whether some amount of optimization would be helpful without knowing more about how the program works.