If I define structs at the module level, I can reference not-yet defined structs.
struct S {
ComesLater c;
}
struct ComesLater {}
But If I do the same inside an unittest or a function block, it doesn't work:
unittest {
struct S {
ComesLater c;
}
struct ComesLater {}
}
Error: undefined identifier 'ComesLater'
Why is that? How can I get order-independent declarations inside functions? Is there some kind of forward-declaration in d? I need this because I generate structs using mixin and ordering the declarations in the order of their inner-dependencies would be quite some effort, sometimes impossible, if there are circularly referencing structs. (using pointers.)
Declarations inside functions, unittests, or anywhere else that statements can actually be executed are indeed order-dependent because their values may depend on the code before them running. Think of a local variable:
int a;
writeln(a);
a = b;
int b = get_user_line();
If order wasn't important there, when would the two functions get called? Would the user be asked for a line before the writeln as the declarations are rewritten?
The current behavior of making b an undefined variable error keeps it simple and straightforward.
It works independent of order in other contexts because there is no executable code that it can depend on, so there's no behavior that can change if the compiler needs to internally think about it differently.
So:
How can I get order-independent declarations inside functions?
Change the context such that there is no executable code... put it all inside another struct!
void main() { // or unittest { }
struct Holder {
static struct S {
C c;
}
static struct C {}
}
}
Since execution happens around the holder and doesn't happen inside it, the order of declaration inside doesn't matter again. Since you can define almost anything inside a struct, you can use this for variables, functions, other structs, and so on. Basically all you have to do is wrap your existing code inside the struct Holder {} brackets.
By making everything static inside, you can just use it like a container and reference the stuff with Holder.S, etc., on the outside.
Related
I've been using a bunch of modules that have a build() function which returns a struct. However, when I try to create my own "super" struct to bundle them together, I run into the error module `xxx` is private rustc(E0603). If there is a trait I can pass the individual variable as a parameter but cannot figure out how to define/box it up for a struct.
The current example of this I'm hitting is when creating a hyper client.
// Error due to privacy and cannot use the trait to define the member type
// Both the "hyper_rustls::connector" and "hyper::client::connect::http" modules are private.
struct SecureClient {
client: hyper::client::Client<
hyper_rustls::connector::HttpsConnector<hyper::client::connect::http::HttpConnector>>
}
// Works, but passing the client everywhere as an individual variable is not realistic.
fn use_client(client: hyper::client::Client<impl hyper::client::connect::Connect>) -> () {
()
}
let https_conn = hyper_rustls::HttpsConnector::new(4);
let client: hyper::client::Client<_, hyper::Body> = hyper::Client::builder().build(https_conn);
Being newish to Rust, I'm struggling to figure out what the proper jargon is for what I'm trying to do, let alone make it work. Links to any docs or code examples about this would be appreciated.
Thanks
I'm not sure what you want to do, but you can use the public re-export hyper_rustls::HttpsConnector instead of the private hyper_rustls::connector::HttpsConnector and the public re-export hyper::client::HttpConnector instead of the private hyper::client::connect::http::HttpConnector.
You can read about re-exports here: https://doc.rust-lang.org/book/ch07-04-bringing-paths-into-scope-with-the-use-keyword.html#re-exporting-names-with-pub-use
Consider an operation with a standard asynchronous interface:
std::future<void> op();
Internally, op needs to perform a (variable) number of asynchronous operations to complete; the number of these operations is finite but unbounded, and depends on the results of the previous asynchronous operations.
Here's a (bad) attempt:
/* An object of this class will store the shared execution state in the members;
* the asynchronous op is its member. */
class shared
{
private:
// shared state
private:
// Actually does some operation (asynchronously).
void do_op()
{
...
// Might need to launch more ops.
if(...)
launch_next_ops();
}
public:
// Launches next ops
void launch_next_ops()
{
...
std::async(&shared::do_op, this);
}
}
std::future<void> op()
{
shared s;
s.launch_next_ops();
// Return some future of s used for the entire operation.
...
// s destructed - delayed BOOM!
};
The problem, of course, is that s goes out of scope, so later methods will not work.
To amend this, here are the changes:
class shared : public std::enable_shared_from_this<shared>
{
private:
/* The member now takes a shared pointer to itself; hopefully
* this will keep it alive. */
void do_op(std::shared_ptr<shared> p); // [*]
void launch_next_ops()
{
...
std::async(&shared::do_op, this, shared_from_this());
}
}
std::future<void> op()
{
std::shared_ptr<shared> s{new shared{}};
s->launch_next_ops();
...
};
(Asides from the weirdness of an object calling its method with a shared pointer to itself, )the problem is with the line marked [*]. The compiler (correctly) warns that it's an unused variable.
Of course, it's possible to fool it somehow, but is this an indication of a fundamental problem? Is there any chance the compiler will optimize away the argument and leave the method with a dead object? Is there a better alternative to this entire scheme? I don't find the resulting code the most intuitive.
No, the compiler will not optimize away the argument. Indeed, that's irrelevant as the lifetime extension comes from shared_from_this() being bound by decay-copy ([thread.decaycopy]) into the result of the call to std::async ([futures.async]/3).
If you want to avoid the warning of an unused argument, just leave it unnamed; compilers that warn on unused arguments will not warn on unused unnamed arguments.
An alternative is to make do_op static, meaning that you have to use its shared_ptr argument; this also addresses the duplication between this and shared_from_this. Since this is fairly cumbersome, you might want to use a lambda to convert shared_from_this to a this pointer:
std::async([](std::shared_ptr<shared> const& self){ self->do_op(); }, shared_from_this());
If you can use C++14 init-captures this becomes even simpler:
std::async([self = shared_from_this()]{ self->do_op(); });
I'm trying to declare a map in a separate file, and then access it from my main function.
I want Rust's equivalent (or whatever comes closest) to this C++ map:
static const std::map<std::string, std::vector<std::string>> table = {
{ "a", { "foo" } },
{ "e", { "bar", "baz" } }
};
This is my attempt in Rust.
table.rs
use std::container::Map;
pub static table: &'static Map<~str, ~[~str]> = (~[
(~"a", ~[~"foo"]),
(~"e", ~[~"bar", ~"baz"])
]).move_iter().collect();
main.rs
mod table;
fn main() {
println(fmt!("%?", table::table));
}
The above gives two compiler errors in table.rs, saying "constant contains unimplemented expression type".
I also have the feeling that the map declaration is less than optimal for the purpose.
Finally, I'm using Rust 0.8.
As Chris Morgan noted, rust doesn't allow you to run user code in order to initialize global variables before main is entered, unlike C++. So you are mostly limited to primitive types that you can initialize with literal expressions. This is, afaik, part of the design and unlikely to change, even though the particular error message is probably not final.
Depending on your use case, you might want to change your code so you're manually passing your map as an argument to all the functions that will want to use it (ugh!), use task-local storage to initialize a tls slot with your map early on and then refer to it later in the same task (ugh?), or use unsafe code and a static mut variable to do much the same with your map wrapped in an Option maybe so it can start its life as None (ugh!).
I used the below to see how dart calls methods passed in to other methods to see what context the passed in method would/can be called under.
void main() {
var one = new IDable(1);
var two = new IDable(2);
print('one ${caller(one.getMyId)}'); //one 1
print('two ${caller(two.getMyId)}'); //two 2
print('one ${callerJustForThree(one.getMyId)}'); //NoSuchMethod Exception
}
class IDable{
int id;
IDable(this.id);
int getMyId(){
return id;
}
}
caller(fn){
return fn();
}
callerJustForThree(fn){
var three = new IDable(3);
three.fn();
}
So how does caller manager to call its argument fn without a context i.e. one.fn(), and why does callerJustForThree fail to call a passed in fn on an object which has that function defined for it?
In Dart there is a difference between an instance-method, declared as part of a class, and other functions (like closures and static functions).
Instance methods are the only ones (except for constructors) that can access this. Conceptually they are part of the class description and not the object. That is, when you do a method call o.foo() Dart first extracts the class-type of o. Then it searches for foo in the class description (recursively going through the super classes, if necessary). Finally it applies the found method with this set to o.
In addition to being able to invoke methods on objects (o.foo()) it is also possible to get a bound closure: o.foo (without the parenthesis for the invocation). However, and this is crucial, this form is just syntactic sugar for (<args>) => o.foo(<args>). That is, this just creates a fresh closure that captures o and redirects calls to it to the instance method.
This whole setup has several important consequences:
You can tear off instance methods and get a bound closure. The result of o.foo is automatically bound to o. No need to bind it yourself (but also no way to bind it to a different instance). This is way, in your example, one.getMyId works. You are actually getting the following closure: () => one.getMyId() instead.
It is not possible to add or remove methods to objects. You would need to change the class description and this is something that is (intentionally) not supported.
var f = o.foo; implies that you get a fresh closure all the time. This means that you cannot use this bound closure as a key in a hashtable. For example, register(o.foo) followed by unregister(o.foo) will most likely not work, because each o.foo will be different. You can easily see this by trying print(o.foo == o.foo).
You cannot transfer methods from one object to another. However you try to access instance methods, they will always be bound.
Looking at your examples:
print('one ${caller(one.getMyId)}'); //one 1
print('two ${caller(two.getMyId)}'); //two 2
print('one ${callerJustForThree(one.getMyId)}'); //NoSuchMethod Exception
These lines are equivalent to:
print('one ${caller(() => one.getMyId())}');
print('two ${caller(() => two.getMyId())}');
print('one ${callerJustForThree(() => one.getMyId())}';
Inside callerJustForThree:
callerJustForThree(fn){
var three = new IDable(3);
three.fn();
}
The given argument fn is completely ignored. When doing three.fn() in the last line Dart will find the class description of three (which is IDable) and then search for fn in it. Since it doesn't find one it will call the noSuchMethod fallback. The fn argument is ignored.
If you want to call an instance member depending on some argument you could rewrite the last example as follows:
main() {
...
callerJustForThree((o) => o.getMyId());
}
callerJustForThree(invokeIDableMember){
var three = new IDable(3);
invokeIDableMember(three);
}
I'll try to explain, which is not necessarily a strength of mine. If something I wrote isn't understandable, feel free to give me a shout.
Think of methods as normal objects, like every other variable, too.
When you call caller(one.getMyId), you aren't really passing a reference to the method of the class definition - you pass the method "object" specific for instance one.
In callerJustForThree, you pass the same method "object" of instance one. But you don't call it. Instead of calling the object fn in the scope if your method, you are calling the object fn of the instance three, which doesn't exist, because you didn't define it in the class.
Consider this code, using normal variables:
void main() {
var one = new IDable(1);
var two = new IDable(2);
caller(one.id);
caller(two.id);
callerJustForThree(one.id);
}
class IDable{
int id;
IDable(this.id);
}
caller(param){
print(param);
}
callerJustForThree(param){
var three = new IDable(3);
print(three.id); // This works
print(param); // This works, too
print(three.param); // But why should this work?
}
It's exactly the same concept. Think of your callbacks as normal variables, and everything makes sense. At least I hope so, if I explained it good enough.
i have a structure defined, which contains a public field and a public property named _one and One respectively, now i instantiate the struct in the main function (not creating new object), and called the Property from the struct, i am getting the compile time error saying use of unassigned local variable One, however when i called the field _one, it works pretty expected here what i am doing:
public struct myStruct
{
public int _one;
public int One
{
get { return _one; }
set { _one = value; }
}
public void Display()
{
Console.WriteLine(One);
}
}
static void Main(string[] args)
{
myStruct _struct;
_struct.One = 2; // Does not works
_struct._one = 2; // Works fine
}
can anyone explain whats the reason behind this, could not understand the concept.
You need to initialize the struct in order for the property to be accessible - _struct has a default value otherwise:
myStruct _struct = new myStruct();
By the way - mutable value types are evil.
This is unintuitive behavior, but it is permitted by the rules of Definite assignment checking. Described in excruciating detail in section 5.3 of the C# Language Specification. The key phrase, early in the chapter is:
In additional to the rules above, the following rules apply to struct-type variables and their instance variables:
- An instance variable is considered definitely assigned if its containing struct-type variable is considered definitely assigned.
- A struct-type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.
It is the latter rule that permits this. In other words, you can also initialize a struct by assigning all of its variables. You can see this by trying these snippets:
myStruct _struct = new myStruct();
_struct.Display(); // fine by the 1st bullet
myStruct _struct;
_struct.Display(); // bad
myStruct _struct;
_struct._one = 2;
_struct.Display(); // fine by the 2nd bullet
So you don't get CS0165 by assigning the field because that would disallow initializing the structure by assigning its variables.
The reasons which would favor using read-write properties instead of exposed fields in class definitions do not apply to structures, since they can support neither inheritance nor update notifications, and the mutability of a struct's field depends upon the mutability of the struct instance, regardless of whether the field is exposed or not. If a struct is supposed to represent a group of related but freely-independently-modifiable variables, it should simply expose those variables as fields. If a property with a backing field is supposed to be read-only, the constructor should set the backing field directly, rather than via property setter.