copy-on-write

var absences = [0, 2, 0, 4, 0, 3, 1, 0] let midpoint = absences.count / 2 var firstHalf = absences.prefix(upTo: midpoint) let secondHalf = absences.suffix(from: midpoint) Quotation from Apple: Neither the firstHalf nor secondHalf slices allocate any new storage of their own. Instead, each presents a view onto the storage of the absences array. When I try to mutate firstHalf as the following: firstHalf[1] = 19 the values of firstHalf changes but the original array absences remains the same ( firstHalf[1] is equal to 19 while absences[1] equals to 2) So what happens in the background. Did I

This question already has an answer here: Operator “[<-” in RStudio and R 1 answer I there any way to avoid copy-on-modify for in-place modifications of matrices in R ? I am trying to copy a smaller matrix to a slice of larger matrix as follows. library(data.table) y <- matrix(c(11,21,31,12,22,32),nrow=3,ncol=2) address(y) [1] "08429190" y[2:3,] <- matrix(c(1,1,8,12),nrow=2) address(y) [1] "0E033D28" Nick Kennedy I get the same behaviour as the OP using R 3.2.0 running within RStudio 0.99.441 on Windows 8.1 and using pryr::address . The issue is RStudio has a reference to y for its Environment

I have a very large structure, which I want to ensure is not copied needlessly. How can I make a copy-on-write container for it? A copy-on-write is usually a struct wrapper over some backing object. public final class MutableHeapStore<T>: NonObjectiveCBase { public typealias Storage = T public private(set) var storage: Storage public init(storage: Storage) { self.storage = storage } } public struct COW<T> { public typealias Storage = MutableHeapStore<T> public typealias Value = T public var storage: Storage public init(storage: Storage) { self.storage = storage } public var value: Value { get

I have a large buffer: char *buf = malloc(1000000000); // 1GB If I forked a new process, it would have a buf which shared memory with the parent's buf until one or the other wrote to it. Even then, only one new 4KiB block would need to be allocated by the kernel, the rest would continue to be shared. I'd like to make a copy of buf, but I'm only going to change a little of the copy. I'd like copy-on-write behaviour without forking. (Like you get for free when forking.) Is this possible? You'll want to create a file on disk or a POSIX shared memory segment ( shm_open ) for the block. The first

STL standard do not require from std::string to be refcounted. But in fact most of C++ implementations provide refcounted, copy-on-write strings, allowing you passing string by value as a primitive type. Also these implementations (at least g++) use atomic operations making these string lock-free and thread safe. Easy test shows copy-on-write semantics: #include <iostream> #include <string> using namespace std; void foo(string s) { cout<<(void*)s.c_str()<<endl; string ss=s; cout<<(void*)ss.c_str()<<endl; char p=ss[0]; cout<<(void*)ss.c_str()<<endl; } int main() { string s="coocko"; cout<<(void

I am building a game engine library in C++. A little while back I was using Qt to build an application and was rather fascinated with its use of Implicit Sharing . I am wondering if anybody could explain this technique in greater detail or could offer a simple example of this in action. The key idea behind implicit sharing seems to go around using the more common term copy-on-write . The idea behind copy-on-write is to have each object serve as a wrapper around a pointer to the actual implementation. Each implementation object keeps track of the number of pointers into it. Whenever an