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The direct answer
What is a reference in C++? Some specific instance of type that is not an object type.
What is a pointer in C++? Some specific instance of type that is an object type.
From the ISO C++ definition of object type:
An object type is a (possibly cv-qualified) type that is not a function type, not a reference type, and not cv void.
It may be important to know, object type is a top-level category of the type universe in C++. Reference is also a top-level category. But pointer is not.
Pointers and references are mentioned together in the context of compound type. This is basically due to the nature of the declarator syntax inherited from (and extended) C, which has no references. (Besides, there are more than one kind of declarator of references since C++ 11, while pointers are still “unityped”: &+&& vs. *.) So drafting a language specific by “extension” with similar style of C in this context is somewhat reasonable. (I will still argue that the syntax of declarators wastes the syntactic expressiveness a lot, makes both human users and implementations frustrating. Thus, all of them are not qualified to be built-in in a new language design. This is a totally different topic about PL design, though.)
Otherwise, it is insignificant that pointers can be qualified as a specific sorts of types with references together. They simply share too few common properties besides the syntax similarity, so there is no need to put them together in most cases.
Note the statements above only mentions “pointers” and “references” as types. There are some interested questions about their instances (like variables). There also come too many misconceptions.
The differences of the top-level categories can already reveal many concrete differences not tied to pointers directly:
Object types can have top-level cv qualifiers. References cannot.
Variable of object types do occupy storage as per the abstract machine semantics. Reference do not necessary occupy storage (see the section about misconceptions below for details).
…
A few more special rules on references:
Compound declarators are more restrictive on references.
References can collapse.
Special rules on && parameters (as the “forwarding references”) based on reference collapsing during template parameter deduction allow “perfect forwarding” of parameters.
References have special rules in initialization. The lifetime of variable declared as a reference type can be different to ordinary objects via extension.
BTW, a few other contexts like initialization involving std::initializer_list follows some similar rules of reference lifetime extension. It is another can of worms.
…
The misconceptions
I know references are syntactic sugar, so code is easier to read and write.
Technically, this is plain wrong. References are not syntactic sugar of any other features in C++, because they cannot be exactly replaced by other features without any semantic differences.
(Similarly, lambda-expressions are not syntactic sugar of any other features in C++ because it cannot be precisely simulated with “unspecified” properties like the declaration order of the captured variables, which may be important because the initialization order of such variables can be significant.)
C++ only has a few kinds of syntactic sugars in this strict sense. One instance is (inherited from C) the built-in (non-overloaded) operator [], which is defined exactly having same semantic properties of specific forms of combination over built-in operator unary * and binary +.
Storage
So, a pointer and a reference both use the same amount of memory.
The statement above is simply wrong. To avoid such misconceptions, look at the ISO C++ rules instead:
From [intro.object]/1:
… An object occupies a region of storage in its period of construction, throughout its lifetime, and in its period of destruction. …
From [dcl.ref]/4:
It is unspecified whether or not a reference requires storage.
Note these are semantic properties.
Pragmatics
Even that pointers are not qualified enough to be put together with references in the sense of the language design, there are still some arguments making it debatable to make choice between them in some other contexts, for example, when making choices on parameter types.
But this is not the whole story. I mean, there are more things than pointers vs references you have to consider.
If you don’t have to stick on such over-specific choices, in most cases the answer is short: you do not have the necessity to use pointers, so you don’t. Pointers are usually bad enough because they imply too many things you don’t expect and they will rely on too many implicit assumptions undermining the maintainability and (even) portability of the code. Unnecessarily relying on pointers is definitely a bad style and it should be avoided in the sense of modern C++. Reconsider your purpose and you will finally find that pointer is the feature of last sorts in most cases.
Sometimes the language rules explicitly require specific types to be used. If you want to use these features, obey the rules.
Copy constructors require specific types of cv-& reference type as the 1st parameter type. (And usually it should be const qualified.)
Move constructors require specific types of cv-&& reference type as the 1st parameter type. (And usually there should be no qualifiers.)
Overloaded operator= as special member functions requires reference types similar to 1st parameter of copy/move constructors.
Postfix ++ requires dummy int.
…
If you know pass-by-value (i.e. using non-reference types) is sufficient, use it directly, particularly when using an implementation supporting C++17 mandated copy elision. (Warning: However, to exhaustively reason about the necessity can be very complicated.)
If you want to operate some handles with ownership, use smart pointers like unique_ptr and shared_ptr (or even with homebrew ones by yourself if you require them to be opaque), rather than raw pointers.
If you are doing some iterations over a range, use iterators (or some ranges which are not provided by the standard library yet), rather than raw pointers unless you are convinced raw pointers will do better (e.g. for less header dependencies) in very specific cases.
If you know pass-by-value is sufficient and you want some explicit nullable semantics, use wrapper like std::optional, rather than raw pointers.
If you know pass-by-value is not ideal for the reasons above, and you don’t want nullable semantics, use {lvalue, rvalue, forwarding}-references.
Even when you do want semantics like traditional pointer, there are often something more appropriate, like observer_ptr in Library Fundamental TS.
The only exceptions cannot be worked around in the current language:
When you are implementing smart pointers above, you may have to deal with raw pointers.
Specific language-interoperation routines require pointers, like operator new. (However, cv-void* is still quite different and safer compared to the ordinary object pointers because it rules out unexpected pointer arithmetics unless you are relying on some non conforming extension on void* like GNU’s.)
Function pointers can be converted from lambda expressions without captures, while function references cannot. You have to use function pointers in non-generic code for such cases, even you deliberately do not want nullable values.
So, in practice, the answer is so obvious: when in doubt, avoid pointers. You have to use pointers only when there are very explicit reasons that nothing else is more appropriate. Except a few exceptional cases mentioned above, such choices are almost always not purely C++-specific (but likely to be language-implementation-specific). Such instances can be:
You have to serve to old-style (C) APIs.
You have to meet the ABI requirements of specific C++ implementations.
You have to interoperate at runtime with different language implementations (including various assemblies, language runtime and FFI of some high-level client languages) based on assumptions of specific implementations.
You have to improve efficiency of the translation (compilation & linking) in some extreme cases.
You have to avoid symbol bloat in some extreme cases.
Language neutrality caveats
If you come to see the question via some Google search result (not specific to C++), this is very likely to be the wrong place.
References in C++ is quite “odd”, as it is essentially not first-class: they will be treated as the objects or the functions being referred to so they have no chance to support some first-class operations like being the left operand of the member access operator independently to the type of the referred object. Other languages may or may not have similar restrictions on their references.
References in C++ will likely not preserve the meaning across different languages. For example, references in general do not imply nonnull properties on values like they in C++, so such assumptions may not work in some other languages (and you will find counterexamples quite easily, e.g. Java, C#, …).
There can still be some common properties among references in different programming languages in general, but let’s leave it for some other questions in SO.
(A side note: the question may be significant earlier than any “C-like” languages are involved, like ALGOL 68 vs. PL/I.)
What’S The Difference Between A Uri And A Url?
The terms “URI” and “URL” are often used interchangeably, but they are not exactly the same.
A URI is an identifier of a specific resource. Like a page, or book, or a document.
A URL is special type of identifier that also tells you how to access it, such as HTTPs, FTP, etc.-like https://www.google.com.
If the protocol (https, ftp, etc.) is either present or implied for a domain, you should call it a URL -even though it’s also a URI.
All URLs are URIs, but not all URIs are URLs.
When most people talk about a given URI, they’re also talking about a URL because the protocol is implied.
That’s really it.
TL;DR – When communicating, being more specific is usually better, and a “URL” is a specific type of URI that provides an access method/location.
That’s all you probably need to know, but if you want to see how the sausage is made (I warn you, it’s gross), feel free to read on!
A deeper explanation (let’s get technical)
This is one of the most common Nerd Fight debates in tech history, and that’s saying a lot.
One obstacle to getting to the bottom of things is that the relevant RFCs are extremely dense, confusing, and even contradictory. For example, RFC 3986 says a URI can be a name, locator, or both…
My emphasis.
A URI can be further classified as a locator, a name, or both. The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network “location”).
RFC 3986, Section 1.1.3
But just a little further down that same RFC says…
My emphasis.
The URI itself only provides identification; access to the resource is neither guaranteed nor implied by the presence of a URI.
RFC 3986, Section 1.2.2
And then, if you’re not yet completely confused, it also says…
My emphasis.
Each URI begins with a scheme name, as defined in Section 3.1, that refers to a specification for assigning identifiers within that scheme.
RFC 3986, Section 1.1.1
And it goes on to give examples:
Notice how they all their examples have schemes.
ftp://ftp.is.co.za/rfc/rfc1808.txt http://www.ietf.org/rfc/rfc2396.txt ldap://[2001:db8::7]/c=GB?objectClass?one mailto:John.Doe@example.com news:comp.infosystems.www.servers.unix tel:+1-816-555-1212 telnet://192.0.2.16:80/ urn:oasis:names:specification:docbook:dtd:xml:4.1.2
Wait…what?
These three contradictions are the source of this entire long-lived debate.
The same RFC just told us that a URI can be a name, a locator, or both-but a URI only provides identification, and a way to access isn’t guaranteed or implied-oh and also each URI begins with a scheme name (which in many cases tells you exactly how to access the resource).
It’s no wonder everyone is confused!
The reason the internet’s been fighting about this for over a decade is that the RFC is poorly written.
Salvaging practical rules from all this
Being the top search result for this topic means I have the conversation a lot.
Ok, so given the fact that the RFC adds to confusion rather than eliminating it, what-if anything-can we use from them?
In the vein of language being here for communication rather than pedantry, here are my own practical interpretations of the RFCs that will hopefully synchronize people and result in fewer swordfights.
All butterflies fly, but not everything that flies is a butterfly.
A Uniform Resource Identifier (URI) provides a simple and extensible means for identifying a resource (straight from RFC 3986). It’s just an identifier; don’t overthink it.
For most debates about this that matter, URI is the superset, so the question is just whether a given URI is formally a URL or not. All URLs are URIs, but not all URIs are URLs. In general, if you see http(s)://, it’s a URL.
Fragments like file.htm actually are not URNs, because URNs are required to use a special notation with urn: in the beginning.
A little-known section of RFC 3986 actually speaks directly to the religious part of the argument, and seems to say we should say URI instead of URL.
RFC 3986 is from 2005, so presumably they’re saying URI is the preferred term after that point.
Future specifications and related documentation should use the general term “URI” rather than the more restrictive terms “URL” and “URN”
RFC 3986, Section 1.1.3
So that’s support for the “URI” denomination, but in my opinion it’s even more support for those who say, “stop looking for the answers in 15-year-old RFCs”.
It’s like another widely-read text in this way.
There’s just so much contradictory content that there’s partial backing for multiple conclusions.
Summary
What a mess. Here’s the TL;DR…
The RFCs are ancient, poorly written, and not worth debating until they’re updated.
A URI is an identifier.
A URL is an identifier that tells you how to get to it.
Use the term that is best understood by the recipient.
I’d welcome a new version of the RFC that simplifies and clarifies the distinction, with modern examples.
These RFCs were written a very long time ago, and they’re written with the academic weakness of not being optimized for reading.
The best thing I can possibly tell you about this debate is not to over-index on it. I’ve not once in 20 years seen a situation where the confusion between URI or URL actually mattered.
The irony is that RFCs are supposed to remove confusion, not add to it.
So while there is some direct support that “URI” is preferred by the RFCs, and “URL” seems most accurate for full addresses with http(s) schemes (because it’s most specific), I’ve chosen to prioritize the Principle of Communication Clarity higher than that of pedantic nuance.
It’s taken me a long time to get to this point.
As a result, I personally use “URL” in most cases because it’s least likely to cause confusion, but if I hear someone use “URI” I’ll often switch immediately to using that instead.
Notes
May 3, 2019 – I’ve done a major update to the article, including correcting some errors I had had in previous versions. Namely, I had fragments such as file.html shown as a URN, which is not right. This version of the article is the best version, especially since it fully explores the conflicting language within the RFC and how little we should actually be paying attention to such an old document. I’d definitely read and follow an update, though.
RFC 3986 Link
The Wikipedia article on URI Link
What Is The Difference Between Depart And Leave?
As verbs the difference between depart and leave
is that depart is to leave; to set out on a journey while leave is to cause or allow (something) to remain as available; to refrain from taking (something) away; to stop short of consuming or otherwise depleting (something) entirely or leave can be to give leave to; allow; permit; let; grant or leave can be (rare) to produce leaves or foliageoxford english dictionary , 2nd ed.
As nouns the difference between depart and leave
is that depart is (obsolete) division; separation, as of compound substances while leave is (cricket) the action of the batsman not attempting to play at the ball or leave can be permission to be absent; time away from one’s work.
Other Comparisons: What’s the difference?Verb
()
To leave.
*Shakespeare
*:He which hath no stomach to this fight, / Let him depart .
*2009 , George Monbiot, The Guardian , 7 September:
*:The government maintains that if its regulations are too stiff, British bankers will leave the country. It’s true that they have been threatening to depart in droves, but the obvious answer is: “Sod off then.”
To set out on a journey.
*:
*:And soo she receyued hym vpon suffysaunt seurte / so alle her hurtes were wel restored of al that she coude complayne / and thenne he departed vnto the Courte of kyne Arthur / and there openly the reed knyghte of the reed laundes putte hym in the mercy of syre Launcelot and syr Gawayne
To die.
*Bible, Luke ii. 29:
*:Lord, now lettest thou thy servant depart in peace.
To deviate (from).
:His latest statements seemed to depart from party policy somewhat.
:to depart from a title or defence in legal pleading
*Madison
*:if the plan of the convention be found to depart from republican principles
To go away from; to leave.
*Bible, 1 Sam. iv. 2:
*:The glory is departed from Israel.
*2009 , The Guardian , Sport Blog, 9 September:
*:The build-up to Saturday’s visit of Macedonia and this encounter with the Dutch could be construed as odd in the sense that there seemed a basic acceptance, inevitability even, that Burley would depart office in their immediate aftermath.
(obsolete) To divide up; to distribute, share.
*:
*:and so all the worlde seythe that betwyxte three knyghtes is departed clerely knyghthode, that is Sir Launcelot du Lake, Sir Trystrams de Lyones and Sir Lamerok de Galys-thes bere now the renowne.
(obsolete) To separate, part.
*:
:(Shakespeare)
Synonyms
* ( to leave) duck out, go, go away, leave, part, pull up stakes, start, start out, set forth, split, set off, set out, take off, take leave, quit * ( to die) die * ( to deviate) deviate, digress, diverge, sidetrack, straggle, vary * ( to go away from) leave
Antonyms
* ( to leave): arrive, come, stay * ( to die): live * ( to deviate): conform
Noun
(obsolete) division; separation, as of compound substances
* Francis Bacon
(obsolete) A going away; departure.
* Shakespeare
Etymology 1
From ( etyl) leven, from ( etyl) (whence Danish levne). More at .
Verb
To have a consequence or remnant.
#To cause or allow (something) to remain as available; to refrain from taking (something) away; to stop short of consuming or otherwise depleting (something) entirely.
#:
#*, chapter=7
, title= The Mirror and the Lamp , passage=[…] St.?Bede’s at this period of its history was perhaps the poorest and most miserable parish in the East End of London. Close-packed, crushed by the buttressed height of the railway viaduct, rendered airless by huge walls of factories, it at once banished lively interest from a stranger’s mind and left only a dull oppression of the spirit.}}
#*{{quote-magazine, year=2013, month=May-June, author= David Van Tassel], [http://www.americanscientist.org/authors/detail/lee-dehaan Lee DeHaan
, title= Wild Plants to the Rescue , volume=101, issue=3, magazine=( American Scientist) , passage=Plant breeding is always a numbers game.
#To cause, to result in.
#:
#*{{quote-book, year=1899, author=(Stephen Crane)
, title=, chapter=1 , passage=There was some laughter, and Roddle was left free to expand his ideas on the periodic visits of cowboys to the town. “Mason Rickets, he had ten big punkins a-sittin’ in front of his store, an’ them fellers from the Upside-down-F ranch shot ’em up
#*, chapter=23
, title= The Mirror and the Lamp , passage=The slightest effort made the patient cough. He would stand leaning on a stick and holding a hand to his side, and when the paroxysm had passed it left him shaking.}}
#*{{quote-magazine, date=2013-07-20, volume=408, issue=8845, magazine=(The Economist)
, title= Out of the gloom , passage=[Rural solar plant] schemes are of little help to industry or other heavy users of electricity. Nor is solar power yet as cheap as the grid. For all that, the rapid arrival of electric light to Indian villages is long overdue. When the national grid suffers its next huge outage, as it did in July 2012 when hundreds of millions were left in the dark, look for specks of light in the villages.}}
#(lb) To put; to place; to deposit; to deliver, with a sense of withdrawing oneself.
#:
#*Bible, (w) v. 24
#*:Leave there thy gift before the altar and go thy way.
#*(William Shakespeare) (1564-1616)
#*:The foot / That leaves the print of blood where’er it walks.
(lb) To depart; to separate from.
#To let be or do without interference.
#:
#(lb) To depart from; to end one’s connection or affiliation with.
#:
#*
, title=( The Celebrity), chapter=1 , passage=I was about to say that I had known the Celebrity from the time he wore kilts. But I see I will have to amend that, because he was not a celebrity then, nor, indeed, did he achieve fame until some time after I left New York for the West.}}
#(lb) To end one’s membership in (a group); to terminate one’s affiliation with (an organization); to stop participating in (a project).
#:
#(lb) To depart; to go away from a certain place or state.
#:
(lb) To transfer something.
#(lb) To transfer possession of after death.
#:
#(lb) To give (something) to someone; to deliver (something) to a repository; to deposit.
#:
#(lb) To transfer responsibility or attention of (something) (to someone); to stop being concerned with.
#:
To remain (behind); to stay.
*:
*:And whanne sire launcelot sawe them fare soo / he gat a spere in his hand / and there encountred with hym al attones syr bors sir Ector and sire Lyonel / and alle they thre smote hym atte ones with their speres
*
*:Carried somehow, somewhither, for some reason, on these surging floods, were these travelers,. Even such a boat as the Mount Vernon offered a total deck space so cramped as to leave secrecy or privacy well out of the question, even had the motley and democratic assemblage of passengers been disposed to accord either.
To stop, desist from; to “leave off” (+ noun / gerund).
*:When he had leeft speakynge, he sayde vnto Simon: Cary vs into the depe, and lett slippe thy nette to make a draught.
*(Alexander Pope) (1688-1744)
*:Now leave complaining and begin your tea.
Derived terms
* beleave * forleave * leave behind * leave for dead * leave no stone unturned * leave nothing in the tank * leave someone hanging * leave someone high and dry * leave someone holding the bag * leave off * leave out * leave in the lurch * leave well enough alone * not leave one’s thought * overleave * up and leave
Noun
()
(cricket) The action of the batsman not attempting to play at the ball.
(billiards) The arrangement of balls in play that remains after a shot is made (which determines whether the next shooter – who may be either the same player, or an opponent – has good options, or only poor ones).
* 1890 February 27, “Slosson’s Close Shave”], in [[w:New York Times, The New York Times] :
Etymology 2
From ( etyl) leve, from ( etyl) . Related to ( etyl) verlof, ( etyl) Erlaubnis. See also ( l).
Noun
(–)
Permission to be absent; time away from one’s work.
I’ve been given three weeks’ leave by my boss.(senseid)(dated, or, legal) Permission.
The applicant now seeks leave to appeal and, if leave be granted, to appeal against these sentences.(dated) Farewell, departure.
I took my leave of the gentleman without a backward glance.Derived terms
* administrative leave * annual leave * by your leave * compassionate leave * leave of absence * maternity leave * on leave * parental leave * paternity leave * shore leave * sick leave * take French leave * take leave * ticket-of-leave
Verb
To give leave to; allow; permit; let; grant.
Verb
(rare) To produce leaves or foliage.Oxford English Dictionary , 2nd ed.
* 1868 , , The Rubáiyát of Omar Khayyám , 2nd edition:
Each Morn a thousand Roses brings, you say:
What Is The Difference Between Shipping And Delivery?
What Is Shipping?
There are two main definitions of shipping when it comes to logistics. The first definition speaks to the size of the package. Smaller objects, such as shoes, clothes, and accessories, can fall under the category of shipping since they can be boxed and sent to the customer using a postal service. The second definition refers to when items are required to be sent to customers. As such, consumers can understand the shipping date to be the day on which the item was dispatched and began making its way to them.
What Is Delivery?
Delivery often refers to larger objects, like major appliances and furniture, which require installation or a delivery person to fetch it inside your home—items that need to be delivered are usually too big to be shipped. Delivery is also the term used to refer to the date on which the package will arrive at the customer’s doorstep. However, this date is usually a general estimate since it’s uncontrollable by the vendor and can be delayed by unforeseen circumstances.
Shipping vs. Delivery: What’s the Difference?
Overall, there are two main differences between shipping and delivery. The first being the size: smaller items are shipped while larger items are delivered. The second difference is the date on which each takes place. Shipping dates usually refer to when an item leaves the warehouse while the delivery date specifies when it should reach the customer.
If you’re looking for a fulfillment company that’ll make your delivery date almost the same as the shipping date, then choose APS Fulfillment Inc. We serve a variety of industries, so whether you’re looking to ship retail, healthcare or financial items, or commercial furniture, we’ve got the solution for you. One of the main benefits of allowing APS to handle your fulfillment needs is the flexibility it allows your business. You’ll be able to not only save money and time by eliminating the need to set up your own fulfillment system, but you’ll also be able to scale your fulfillment services depending on the growth of your business. Fast and dependable fulfillment services is integral to maintaining a good rapport with customers, which is why you should trust APS—we take the extra care to ensure your customers receive their packages on time, and in good condition. Contact us today to learn more about how we can help your small business continue to grow.
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