Copy on Write

SourcePro Core : Essential Tools Module User's Guide : Advanced Topics : Copy on Write

Copy on Write

Classes RWCString, RWWString, and RWTValVirtualArray<T> use a technique called copy on write to minimize copying. This technique offers the advantage of easy-to-understand value semantics with the speed of reference counted pointer implementation.

Here is how the technique works. When an RWCString is initialized with another RWCString via the copy constructor:

RWCString(const RWCString&);

the two strings share the same data until one of them tries to write to it. At that point, a copy of the data is made and the two strings go their separate ways. Copying only at “write” time makes copies of strings, particularly read-only copies, very inexpensive. In the following example, you can see how four objects share one copy of a string until one of the objects attempts to change the string:

#include <rw/cstring.h>

RWCString g; // Global object

void setGlobal(RWCString x) { g = x; }

int main(){

RWCString a("kernel"); // 1

RWCString b(a);

RWCString c(a); // 2

setGlobal(a); // Still only one copy of "kernel"! // 3

b += "s"; // Now b has its own data: "kernels" // 4

return 0;

}

//1 The actual allocation and initialization of the memory to hold the string kernel occurs at the RWCString object a. When objects b and c are created from a, they merely increment a reference count in the original data and return. At this point, there are three objects looking at the same piece of data.

//2 When objects b and c are created from a, they merely increment a reference count in the original data and return. At this point, there are three objects looking at the same piece of data.

//3 The function setGlobal() sets the value of g, the global RWCString, to the same value. Now the reference count is up to four, and there is still only one copy of the string kernel.

//4 Finally, object b tries to change the value of the string. It looks at the reference count and sees that it is greater than one, implying that the string is being shared by more than one object. At this point, a clone of the string is made and modified. The reference count of the original string drops back down to three, while the reference count of the newly cloned string is one.