I know that std::vector<T>::push_back() has move semantics support. So, when I add a named temporary instance to a vector, I can use std::move().
What are the other common places in the STL that I should grow the habit to add std::move()
I know that std::vector<T>::push_back() has move semantics support.
The support that push_back has is simply an additional overload that takes an rvalue reference, so that the new value T inside the vector can be constructed by invoking T(T&&) instead of T(const T&). The advantage is that the former can be implemented way more efficiently because it assumes that the passed rvalue reference is never going to be used afterwards.
Most Standard Library containers have added similar overloads to their push/enqueue/insert member functions. Additionally, the concept of emplacement has been added (e.g. std::vector<T>::emplace_back), where the values are constructed in place inside the container in order to avoid unnecessary temporaries. Emplacement should be preferred to insertion/pushing.
So, when I add a named temporary instance to a vector, I can use std::move().
"Named temporary" doesn't really make much sense. The idea is that you have an lvalue you don't care about anymore, and you want to turn it into a temporary by using std::move. Example:
Foo foo;
some_vector.emplace_back(std::move(foo));
// I'm sure `foo` won't be used from now on
Just remember that std::move is not special: it literally means static_cast<T&&>.
What are the other common places in the STL that I should grow the habit to add std::move?
This is a really broad question - you should add std::move everywhere it makes sense, not just in the context of the Standard Library. If you have a lvalue you know you're not going to use anymore in a particular code path, and you want to pass it/store it somewhere, then std::move it.
Obviously, move semantics/r-value references were a much needed addition in C++11. One thing that has always bugged me though, is std::move. The purpose of std::move is to transform an l-value into an r-value. And yet, the compiler is perfectly happy to let you continue using that value as an l-value and you get to find out at runtime that you screwed up.
It seems like there is a missed opportunity to define move (or some other name) as a keyword (similar to *_cast) and actually have the compiler understand that the referenced value can no longer be used as an l-value here. I'm sure there is some implementation work to do this, but is there some fundamental reason why this wasn't done?
In C++, moved-from objects in are still objects. They can be used. They are usually in a defined state.
There are some optimizations you can do when you are willing to 'rip the guts' out of an object and use it elsewhere. The C++ committee decided these optimizations should be done implicitly and automatically in a few cases; usually where elision was already permitted, but where it wouldn't work for whatever reason.
Then, the ability to explicitly do this was added. Making this operation end the lifetime of its right hand side would complicate the lifetime rules of C++ to an extreme degree; rather than doing that, they noted they could be highly efficient without complicating the lifetime rules of C++ and leaving them exactly as-is.
It turns out there are a handful of flaws in this; to this extent, C++20 may be adding some "move and destroy the source" operations. In particular, a number of move-construction like operations are easier to write as nothrow if you can both move and destroy the source in one fell swoop.
Actually having it change the lifetime of automatic storage variables is not in the cards. Even describing how such a change would work, let alone making sure it doesn't break anything horribly, would be a challenge.
A simple example of why having it always happen wouldn't be good might be:
Foo foo;
if (some_condition) {
bar = std::move(foo);
}
the lifetime of foo is now a function of some_condition? You'd either have to ban the above with that kind of construct, or go down a pit of madness you may never get out of.
This SO post seems to suggest that language indeed supports accepting temporaries only for arguments.
Why is then unique_ptr designed to accepted aliased pointers, like this:
auto ptr = new Widget{};
auto uptr = std::unique_ptr<Widget>(ptr);
Why not just restrict to:
auto uptr = std::unique_ptr<Widget>(new Widget{});
Because this doesn't help.
It's required that the pointer is created by a suitable allocation function for the given deleter, which is different to it being temporary.
You can't distinguish pointers created by a new expression, function call and by & operator by a value category - they are all rvalues.
There is no good reason as far as I know, I think it this an underexploitation of the C++ language from the library.
I asked the same question Why is raw pointer to shared_ptr construction allowed in all cases?
I still don't find a good reason for not making the constructor of std::unique_ptr accepting only r-value references, either from new or from a factory. Here I put it as answer and not as a comment to put it in at the same level of the other answer with which I don't agree.
If you really need to accept an existing pointer, you can always cast, that is use std::move. So it is not an excuse, because that is not the norm.
Matrix<T, Dynamic, Dynamic>::InnerIterator doesn't seem to use the standard way (operator *) to dereference it.
It has a member function value().
I am rather surprised, as this would not interface well with third party algorithms, like STL algorithms.
Why it doesn't declare operator *? Even worse is that value() does not return by reference, so you can't change the underlying value of the pointed element. What is the proper way to use it?
This is because most of the times, the value alone in useless without the respective inner/row/column indices, as returned by index()/row()/col(). In other cases, you only care about the indices. Anyway, you can easily write a little wrapper to make it compatible with STL if needed.
Finally, if you want to modify the value, there is a valueRef() method returning by non const reference.
This from Bjarne Stroustrup's The C++ Programming Language, Fourth Edition 3.3.2.
We didn’t really want a copy; we just wanted to get the result out of
a function: we wanted to move a Vector rather than to copy it.
Fortunately, we can state that intent:
class Vector {
// ...
Vector(const Vector& a); // copy constructor
Vector& operator=(const Vector& a); // copy assignment
Vector(Vector&& a); // move constructor
Vector& operator=(Vector&& a); // move assignment
};
Given that definition, the compiler will choose the move constructor
to implement the transfer of the return value out of the function.
This means that r=x+y+z will involve no copying of Vectors. Instead,
Vectors are just moved.As is typical, Vector’s move constructor is
trivial to define...
I know Golang supports traditional passing by value and passing by reference using Go style pointers.
Does Go support "move semantics" the way C++11 does, as described by Stroustrup above, to avoid the useless copying back and forth? If so, is this automatic, or does it require us to do something in our code to make it happen.
Note: A few answers have been posted - I have to digest them a bit, so I haven't accepted one yet - thanks.
The breakdown is like here:
Everything in Go is passed by value.
But there are five built-in "reference types" which are passed by value as well but internally they hold references to separately maintained data structure: maps, slices, channels, strings and function values (there is no way to mutate the data the latter two reference).
Your own answer, #Vector, is incorrect is that nothing in Go is passed by reference. Rather, there are types with reference semantics. Values of them are still passed by value (sic!).
Your confusion suppsedly stems from the fact your mind is supposedly currently burdened by C++, Java etc while these things in Go are done mostly "as in C".
Take arrays and slices for instance. An array is passed by value in Go, but a slice is a packed struct containing a pointer (to an underlying array) and two platform-sized integers (the length and the capacity of the slice), and it's the value of this structure which is copied — a pointer and two integers — when it's assigned or returned etc. Should you copy a "bare" array, it would be copied literally — with all its elements.
The same applies to channels and maps. You can think of types defining channels and maps as declared something like this:
type Map struct {
impl *mapImplementation
}
type Slice struct {
impl *sliceImplementation
}
(By the way, if you know C++, you should be aware that some C++ code uses this trick to lower exposure of the details into header files.)
So when you later have
m := make(map[int]string)
you could think of it as m having the type Map and so when you later do
x := m
the value of m gets copied, but it contains just a single pointer, and so both x and m now reference the same underlying data structure. Was m copied by reference ("move semantics")? Surely not! Do values of type map and slice and channel have reference semantincs? Yes!
Note that these three types of this kind are not at all special: implementing your custom type by embedding in it a pointer to some complicated data structure is a rather common pattern.
In other words, Go allows the programmer to decide what semantics they want for their types. And Go happens to have five built-in types which have reference semantics already (while all the other built-in types have value semantics). Picking one semantics over the other does not affect the rule of copying everything by value in any way. For instance, it's fine to have pointers to values of any kind of type in Go, and assign them (so long they have compatible types) — these pointers will be copied by value.
Another angle to look at this is that many Go packages (standard and 3rd-party) prefer to work with pointers to (complex) values. One example is os.Open() (which opens a file on a filesystem) returning a value of the type *os.File. That is, it returns a pointer and expects the calling code to pass this pointer around. Surely, the Go authors might have declared os.File to be a struct containing a single pointer, essentially making this value have reference semantics but they did not do that. I think the reason for this is that there's no special syntax to work with the values of this type so there's no reason to make them work as maps, channels and slices. KISS, in other words.
Recommended reading:
"Go Data Structures"
"Go Slices: Usage and Internals"
Arrays, slices (and strings): The mechanics of 'append'"
A thead on golang-nuts — pay close attention to the reply by Rob Pike.
The Go Programming Language Specification
Calls
In a function call, the function value and arguments are evaluated in
the usual order. After they are evaluated, the parameters of the call
are passed by value to the function and the called function begins
execution. The return parameters of the function are passed by value
back to the calling function when the function returns.
In Go, everything is passed by value.
Rob Pike
In Go, everything is passed by value. Everything.
There are some types (pointers, channels, maps, slices) that have
reference-like properties, but in those cases the relevant data
structure (pointer, channel pointer, map header, slice header) holds a
pointer to an underlying, shared object (pointed-to thing, channel
descriptor, hash table, array); the data structure itself is passed by
value. Always.
Always.
-rob
It is my understanding that Go, as well as Java and C# never had the excessive copying costs of C++, but do not solve ownership transference to containers. Therefore there is still copying involved. As C++ becomes more of a value-semantics language, with references/pointers being relegated to i) smart-pointer managed objects inside classes and ii) dependence references, move semantics solves the problem of excessive copying. Note that this has nothing to do with "pass by value", nowadays everyone passes objects by Reference (&) or Const Reference (const &) in C++.
Let's look at this (1) :
BigObject BO(big,stuff,inside);
vector<BigObject> vo;
vo.reserve(1000000);
vo.push_back(BO);
Or (2)
vector<BigObject> vo;
vo.reserve(1000000);
vo.push_back(BigObject(big,stuff,inside));
Although you're passing by reference to the vector vo, in C++03 there was a copy inside the vector code.
In the second case, there is a temporary object that has to be constructed and then is copied inside the vector. Since it can only be accessed by the vector, that is a wasteful copy.
However, in the first case, our intent could be just to give control of BO to the vector itself. C++17 allows this:
(1, C++17)
vector<BigObject> vo;
vo.reserve(1000000);
vo.emplace_back(big,stuff,inside);
Or (2, C++17)
vector<BigObject> vo;
vo.reserve(1000000);
vo.push_back(BigObject(big,stuff,inside));
From what I've read, it is not clear that Java, C# or Go are exempt from the same copy duplication that C++03 suffered from in the case of containers.
The old-fashioned COW (copy-on-write) technique, also had the same problems, since the resources will be copied as soon as the object inside the vector is duplicated.
Stroustrup is talking about C++, which allows you to pass containers, etc by value - so the excessive copying becomes an issue.
In Go, (like in Delphi, Java, etc) when you pass a container type, etc they are always references, so it's a non-issue. Regardless, you don't have to deal with it or worry about in GoLang - the compiler just does what it needs to do, and from what I've seen thus far, it's doing it right.
Tnx to #KerrekSB for putting me on the right track.
#KerrekSB - I hope this is the right answer. If it's wrong, you bear no responsibility.:)