I was reading Copy and Swap.
I tried reading some links on Copy Elision but could not figure out properly what it meant. Can somebody please explain what this optimization is, and especially what is mean by the following text
This is not just a matter of convenience but in fact an optimization. If the parameter (s) binds to a lvalue (another non-const object), a copy of the object is made automatically while creating the parameter (s). However, when s binds to a rvalue (temporary object, literal), the copy is typically elided, which saves a call to a copy constructor and a destructor. In the earlier version of the assignment operator where the parameter is accepted as const reference, copy elision does not happen when the reference binds to a rvalue. This results into an additional object being created and destroyed.
Copy-and-Swap Idiom in C++ But when overloading the assignment operator, it can become quite difficult to implement. The copy and swap idiom is a solution for the same. This idiom uses the copy-constructor to build a local copy of the data. It then swaps the old data with the new data using the swap function.
In a return statement or a throw-expression, if the compiler cannot perform copy elision but the conditions for copy elision are met or would be met, except that the source is a function parameter, the compiler will attempt to use the move constructor even if the object is designated by an lvalue; see return statement ...
You can see this behavior in compiler versions Visual Studio 2017 15.6, Clang 4, GCC 7, and above. Despite the name of the paper and what you might read on the Internet, the new rules do not guarantee copy elision. Instead, the new value category rules are defined such that no copy exists in the first place.
Copy elision is an optimization implemented by most compilers to prevent extra (potentially expensive) copies in certain situations. It makes returning by value or pass-by-value feasible in practice (restrictions apply).
The copy constructor exists to make copies. In theory when you write a line like:
CLASS c(foo());
The compiler would have to call the copy constructor to copy the return of foo()
into c
.
Copy elision is a technique to skip calling the copy constructor so as not to pay for the overhead.
For example, the compiler can arrange that foo()
will directly construct its return value into c
.
Here's another example. Let's say you have a function:
void doit(CLASS c);
If you call it with an actual argument, the compiler has to invoke the copy constructor so that the original parameter cannot be modified:
CLASS c1; doit(c1);
But now consider a different example, let's say you call your function like this:
doit(c1 + c1);
operator+
is going to have to create a temporary object (an rvalue). Instead of invoking the copy constructor before calling doit()
, the compiler can pass the temporary that was created by operator+
and pass that to doit()
instead.
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