Suppose I need to write a function which might take either a constant reference to an lvalue or a temporary value, is there any advantage in terms of performance in writing both overloads, the one taking a const T& and one taking T&&, if we do not want to move from the rvalue?
I was assuming having two overloads (or just writing the function once with universal references) would be beneficial but I can't pin down the exact reason. I even tried a small example: https://godbolt.org/z/53r34x4Mj but I can't really make sense of the generated code.
If you're never going to move from the argument under any circumstance, nor modify it, there is no benefit to writing the version that takes an r-value reference; the const type& version will accept r-values just fine (keeping them alive until the function returns) with no additional overhead.
The only reason to write both versions is if you plan to potentially modify or move-from the argument only when receiving an r-value reference, where the const reference case never does so, and doesn't copy either, because if it would need to copy anyway, you may as well just accept by value and get the best of all worlds (construct directly into argument for prvalues, move construct for xvalues, and copy for everything else, but without hand-writing the code to handle that).
Related
I have been using c++11 for some time but I always avoided using std::move because I was scared that, while reading a library where the user does not have the access to the code, it would try to use the variable after I move it.
So basically something like
void loadData(std::string&& path);
Would not be enough to make the user understand that it will be moved.
Is it expected that the use of && would imply that the data will be moved. I know that comments can be used to explain the use case, but a lot of people dont pay attention to that.
Is it safe to assume that when you see a && the data will be moved, or when should I use std::move and how to make it explicit from the signature.
Is it expected that the use of && would imply that the data will be moved.
Generally speaking yes. A user cannot call loadData with an lvalue. They must provide a prvalue or an xvalue. So if you have a variable to pass, your code would generally look like loadData(std::move(variable)), which is a pretty good indicator of what you're doing from your side. forwarding could also be employed, but you'd still see it at the call site.
Indeed, generally speaking it is extremely rude to move from a parameter which is not an rvalue reference.
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.
I'm a C++ senior programmer. I'm currently doing some Go programming. The only feature I really miss is the const qualifier. In go, if you want to modify an object, you pass its pointer. If you don't want to modify it, you pass it by value. But if the struct is big, you should pass it by pointer, which overrides the no-modification feature. Worse, you can pass an object by value, but if it contains a pointer, you can actually modify its contents, with terrible race condition dangers. Some language types like maps and slices have this feature. This happens in a language that's supposed to be built for concurrency. So the issue of avoiding modification is really non-existent in Go, and you should pass small objects that do not contain pointers (you must be aware that the object does not contain a pointer) by value, if they aren't gonna be modified.
With const, you can pass objects by const pointer and don't worrying about modification. Type-safety is about having a contract that allows speed and prevents type-related bugs. Another feature that does this too is the const qualifier.
The const type qualifier in C/C++ has various meanings. When applied to a variable, it means that the variable is immutable. That's a useful feature, and one that is missing from Go, but it's not the one you seem to be talking about.
You are talking about the way that const can be used as a partially enforced contract for a function. A function can give a pointer parameter the const qualifier to mean that the function won't change any values using that pointer. (Unless, of course, the function uses a cast (a const_cast in C++). Or, in C++, the pointer points to a field that is declared mutable.)
Go has a very simple type system. Many languages have a complex type system in which you enforce the correctness of your program by writing types. In many cases this means that a good deal of programming involves writing type declarations. Go takes a different approach: most of your programming involves writing code, not types. You write correct code by writing correct code, not by writing types that catch cases where you write incorrect code. If you want to catch incorrect code, you write analyzers, like go vet that look for cases that are invalid in your code. These kinds of analyzers are much much easier to write for Go than for C/C++, because the language is simpler.
There are advantages and disadvantages to this kind of approach. Go is making a clear choice here: write code, not types. It's not the right choice for everyone.
Please treat it as an expanded comment. I'm not any programming language designer, so can't go deep inside the details here, but will present my opinion as a long-term developer in C++ and short-term developer in Go.
Const is a non-trivial feature for the compiler, so one would have to make sure whether it's providing enough advantage for the user to implement it as well as won't sacrifice the simplicity of syntax. You might think it's just a const qualifier we're talking about, but looking at C++ itself, it's not so easy – there're a lot of caveats.
You say const is a contract and you shouldn't be able to modify it at any circumstances. One of your arguments against using read only interfaces is that you can cast it to original type and do whatever you want. Sure you can. The same way you can show a middle finger to the contract in C++ by using const_cast. For some reason it was added to the language and, not sure I should be proud of it, I've used it once or twice.
There's another modifier in C++ allowing you to relax the contract – mutable. Someone realised that const structures might actually need to have some fields modified, usually mutexes protecting internal variables. I guess you would need something similar in Go in order to be able to implement thread-safe structures.
When it comes simple const int x people can easily follow. But then pointers jump in and people really get consfused. const int * x, int * const x, const int * const x – these are all valid declarations of x, each with different contract. I know it's not a rocket science to choose the right one, but does your experience as a senior C++ programmer tell you people widely understand these and are always using the right one? And I haven't even mentioned things like const int * const * * * const * const x. It blows my mind.
Before I move to point 4, I would like to cite the following:
Worse, you can pass an object by value, but if it contains a pointer,
you can actually modify its contents
Now this is interesting accusation. There's the same issue in C++; worse – it exists even if you declare object as const, which means you can't solve the problem with a simple const qualifier. See the next point:
Per 3, and pointers, it's not so easy to express the very right contract and things sometimes get unexpected. This piece of code surprised a few people:
struct S {
int *x;
};
int main() {
int n = 7;
const S s = {&n}; // don't touch s, it's read only!
*s.x = 666; // wait, what? s is const! is satan involved?
}
I'm sure it's natural for you why the code above compiles. It's the pointer value you can't modify (the address it points to), not the value behind it. You must admit there're people around that would raise their eyebrow.
I don't know if it makes any point, but I've been using const in C++ all the time. Very accurate. Going mental about it. Not sure whether is has ever saved my ass, but after moving to Go I must admit I've never missed it. And having in mind all these edge cases and exceptions I can really believe creators of a minimalistic language like Go would decide to skip on this one.
Type-safety is about having a contract that allows speed and prevents
type-related bugs.
Agreed. For example, in Go, I love there're no implicit conversions between types. This is really preventing me from type-related bugs.
Another feature that does this too is the const qualifier.
Per my whole answer – I don't agree. Where a general const contract would do this for sure, a simple const qualifier is not enough. You then need a mutable one, maybe kind of a const_cast feature and still – it can leave you with misleading believes of protection, because it's hard to understand what exactly is constant.
Hopefully some language creators will design a perfect way of defining constants all over in our code and then we'll see it in Go. Or move over to the new language. But personally, I don't think C++'s way is a particularly good one.
(Alternative would be to follow functional programming paradigms, which would love to see all their "variables" immutable.)
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.:)