Scheme

Notation

The scheme is the top-level hierarchical element which defines the syntax and semantics of the rest of the URL. The scheme identifier is always followed by a colon when it appears in a URL.

Here are some examples of URLs with the schemes https and file:

https://www.example.com/path/to/file.txt?page=2

file:///usr/local/bin/

A scheme must start with a letter, and may contain only letters, digits, plus and minus signs, and periods:

Table 1.7. Scheme BNF

scheme          = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

absolute-URI    = scheme ":" hier-part [ "?" query ]

URI             = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

Member Functions

The functions for inspecting the scheme in a url_view are as follows:

Table 1.8. Scheme Observers

Function	Description
`has_scheme`	Return true if a scheme is present
`scheme`	Return the scheme as a string
`scheme_id`	Return the scheme as a known-scheme enumeration constant

	Note
	None of these functions throw exceptions. If the URL has no scheme, `scheme` returns an empty string. If the function `scheme_id` identifies a valid but unknown scheme, the value `scheme::unknown` is returned.

Observers

The function scheme can be used to obtain the scheme from a url_view:

Code	Output
urls::string_view s = "mailto:name@email.com"; urls::url_view u = urls::parse_uri( s ).value(); std::cout << u.scheme() << "\n";	mailto

If the URL has no scheme, this function returns an empty string. To check whether a URL contains a scheme the function url_view::has_scheme might be used.

Code	Output
urls::url_view u = urls::parse_uri( s ).value(); if (u.has_scheme()) { std::cout << u.scheme() << "\n"; }	mailto

Common schemes

The library defines an enumeration of values for some well-known scheme identifiers.

urls::string_view s = "file://host/path/to/file";
urls::url_view u = urls::parse_uri( s ).value();
if (u.scheme_id() == urls::scheme::file)
{
    // handle file
}

These may be used instead of their corresponding strings:

Table 1.9. Scheme IDs

ID	Description
`scheme::none`	Indicates no scheme present
`scheme::unknown`	A valid but unknown scheme
`scheme::ftp`	File Transfer Protocol ("FTP")
`scheme::file`	File URI Scheme
`scheme::http`	Hypertext Transfer Protocol
`scheme::https`	Secure Hypertext Transfer Protocol
`scheme::ws`	WebSocket Protocol
`scheme::wss`	Secure WebSocket Protocol

A number of schemes are used to define the semantics of URLs. For instance, the term web address is often used informally to describe URLs with the http scheme, whose semantics are defined by rfc3986. One of the conventions of the HTTP scheme is that when the port 80 is implicitly assumed when it is not provided. Such conventions are not part of the URL protocol.

Schemes are also different from protocols. Although the scheme http is used to interact with resources via the HTTP protocol, the scheme file has no corresponding protocol.

Some noteworthy IANA-registered schemes are

Table 1.10. Scheme IDs

Scheme	Resource
`cid`	SMTP/MIME messages
`data`	Inline data
`dav`	WebDAV
`dns`	Domain Name System
`file`	File systems
`ftp`	FTP resources
`git`	GIT repository
`http`	HTTP resources
`https`	HTTP secured using SSL/TLS
`mailto`	Secure WebSocket Protocol
`magnet`	Identify files by content
`nfs`	Network File System
`pop`	POP3
`s3`	Amazon S3
`sms`	SMS messages
`svn`	Subversion (SVN) repository
`tel`	Telephone number
`udp`	Streaming protocols over UDP
`urn`	Uniform Resource Names
`ws`	WebSocket Protocol
`wss`	Secure WebSocket Protocol

Many other valid but unofficial schemes are common:

Table 1.11. Scheme IDs

Scheme	Resource
`doi`	Digital Object Identifier
`javascript`	Javascript Code
`odbc`	Open Database Connectivity
`slack`	Slack Client

Scheme

Notation

Member Functions

Observers

Common schemes

Use Cases