A URI Library for C++¶

Example Usage¶

std::network::uri uri("http://www.example.com/glynos/?key=value#frag");
assert(uri.is_valid());
assert(uri.is_absolute());
assert(!uri.is_opaque());
assert(uri.scheme().string() == "http");
assert(uri.host().string() == "www.example.com");
assert(uri.path().string() == "/glynos/");
assert(uri.query().string() == "?key=value");
assert(uri.fragment().string() == "frag");

The code excerpt above shows a simple of how the proposed uri will work. The URI string is parsed during object construction and broken down into its component parts. HTTP URIs are absolute and hierarchical (i.e. not opaque).

std::network::uri uri(U"xmpp:example-node@example.com?message;subject=Hello%20World");
assert(uri.is_valid());
assert(uri.is_absolute());
assert(uri.is_opaque());
assert(uri.scheme().string() == "xmpp");
assert(uri.path().string() == "example-node@example.com");
assert(uri.query().string() == "?message;subject=Hello%20World");

The uri in this proposal supports encoded strings and supports encoding conversion. The example above shows a uri object constructed using a std::u32string and allow the parts to be accessed as std::string objects in UTF-8 encoding.

Percent Encoding¶

URI percent encoding is described in RFC 3986, section 2.1 and RFC 3986, section 2.4.

Percent encoding is the mechanism used to encode reserved characters in a URI. According to RFC 3986, section 2.2, the set of reserved characters are:

Percent encoding is not limited to reserved characters. Any character data may be percent encoded:

Converting the Scheme / Host to Lower Case¶

The scheme and host are case-insensitive. The proposed normalization solution will convert these to lowercase.

HTTP://Example.com/ --> http://example.com/

The user info, path, query and fragment are case-sensitive and so must not be converted.

Capitalizing Characters in Escape Sequences¶

Characters in a percent-encoded triplet are case-insensitive. The proposed normalization solution will convert these to lowercase.

http://example.com/%5b%5d --> http://example.com/%5B%5D

Decoding Unreserved Characters¶

Unreserved characters that have been encoded will be decoded.

http://example.com/%7Eglynos/ --> http://example.com/~glynos/

Adding Trailing /¶

If a path refers to a directory, it should be indicated with a trailing slash.

http://example.com/glynos --> http://example.com/glynos/

But not if the path refers to a file.

http://example.com/glynos/page.html --> http://example.com/glynos/page.html

Removing dot-segments from the Path¶

The segments ”..” and ”.” can be removed according to the algorithm specified in RFC 3986, Section 5.2.4.

http://example.com/glynos/./proposals/../ --> http://example.com/glynos/

Removing the default port¶

Some schemes may have a default port (for HTTP it is 80). The default port can be removed.

http://example.com:80/ --> http://example.com/
http://example.com:/ --> http://example.com/

The Comparison Ladder¶

The Comparison Ladder is described in RFC 3986, Section 6.2. It explains that comparing URIs using normalization can be implemented in different ways according to the complexity of the method and the number of false negatives which may arise.

The final two steps in the Comparison Ladder require more information than can be provided within the limits of the proposal in order to be implemented comprehensively, and will not form part of the proposal at this stage.

Declarations¶

The <network/uri> header contains a declaration for a single uri class in the std::network namespace.

At this stage, the sub-namespace network namespace should be regarded as a placeholder for a namespace specified for network components during the standardization process (should such as sub-namespace be specified).

Equality and Comparison Operators¶

namespace std {
namespace network {
bool operator == (const uri &lhs, const uri &rhs);
bool operator != (const uri &lhs, const uri &rhs);
bool operator <  (const uri &lhs, const uri &rhs);
bool operator <= (const uri &lhs, const uri &rhs);
bool operator >  (const uri &lhs, const uri &rhs);
bool operator >= (const uri &lhs, const uri &rhs);
} // namespace network
} // namespace std

Percent Encoding and Decoding¶

namespace std {
namespace network {
template <class String>
String pct_encode(const String &source);
} // namespace network
} // namespace std

std::string s = "string with spaces";
assert(std::network::pct_encode(s) == "string%20with%20spaces");

namespace std {
namespace network {
template <class String>
String pct_decode(const String &source);
} // namespace network
} // namespace std

Stream Operators¶

namespace std {
namespace network {
std::ostream &operator << (std::ostream &os, const uri &u);
std::wostream &operator << (std::wostream &os, const uri &u);
} // namespace network
} // namespace std

This proposal specifies output stream operators for character and wide character streams.

namespace std {
namespace network {
std::istream &operator >> (std::istream &is, uri &u);
std::wistream &operator >> (std::wistream &is, uri &u);
} // namespace network
} // namespace std

This proposal specifies input stream operators for character and wide character streams.

Transformers¶

This proposal specifies three transformer functions: normalize, relativize and resolve.

namespace std {
namespace network {
uri normalize(const uri &u);
} // namespace network
} // namespace std

namespace std {
namespace network {
uri relativize(const uri &u1, const uri &u2);
} // namespace network
} // namespace std

std::network::uri base_uri("http://www.example.com/");
std::network::uri uri("http://www.example.com/glynos/?key=value#fragment");
std::network::uri rel_uri(base_uri.relativize(uri));
assert(rel_uri.string() == "?key=value#fragment");

namespace std {
namespace network {
uri resolve(const uri &u1, const uri &u2);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class String>
uri resolve(const uri &u1, const String &u2);
} // namespace network
} // namespace std

uri Requirements¶

Template parameters named InputIterator are required meet the requirements for a C++ standard library RandomIterator compliant iterator. The iterator’s value type is required to be char, wchar_t, char16_t, or char32_t.

Template parameters named Source are required to be one of:

A container with a value type of char, wchar_t, char16_t, or char32_t.

An iterator for a null terminated byte-string. The value type is required to be char, wchar_t, char16_t, or char32_t.

A C-array. The value type is required to be char, wchar_t, char16_t, or char32_t.

This is identical wording to that found in the filesystem proposal (N3365).

typedef s¶

typedef ... value_type;
typedef basic_string<value_type> string_type;
typedef string_type::const_iterator iterator;
typedef string_type::const_iterator const_iterator;

The value_type is left unspecified in this proposal and is intended to implementation defined. This may be either char for POSIX systems and wchar_t for Windows. This is influenced by the standard filesystem proposal.

Constructors and Destructors¶

uri();

template <typename InputIterator>
uri(const InputIterator &first, const InputIterator &last);

template <class Source>
uri(const Source &source);

Assignment¶

uri &operator = (const uri &other);
uri &operator = (uri &&other);
template <typename InputIterator>
void assign(const InputIterator &first, const InputIterator &last);
template <class Source>
void assign(const Source &source);

Swap¶

void swap(uri &other) noexcept;

Iterators¶

const_iterator begin() const;

const_iterator end() const;

Accessors¶

part_range scheme() const;

part_range user_info() const;

part_range host() const;

part_range port() const;

part_range path() const;

part_range authority() const;

part_range query() const;

part_range fragment() const;

Query¶

bool empty() const noexcept;

bool is_valid() const;

bool is_absolute() const;

bool is_opaque() const;

String Accessors¶

string_type native() const noexcept;

const value_type *c_str() const noexcept;

string string() const;

wstring wstring() const;

u16string u16string() const;

u32string u32string() const;

Constructor¶

namespace std {
namespace network {
uri::builder::builder(uri &u);
} // namespace network
} // namespace std

Builder functions¶

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::scheme(const Source &scheme);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::user_info(const Source &user_info);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::host(const Source &host);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::port(const Source &port);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
builder &authority(const Source &authority);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
builder &authority(const Source &user_info, const Source &host, const Source &port);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::path(const Source &path);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::append_path(const Source &path);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::query(const Source &query);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Key, class Param>
uri::builder &uri::builder::query(const Key &key, const Param &param);
} // namespace network
} // namespace std

namespace std {
namespace network {
template <class Source>
uri::builder &uri::builder::fragment(const Source &fragment);
} // namespace network
} // namespace std

Issue 1 - Text Encoding and Interoperability¶

The most important open issue for this proposal is how to deal with text encoding in a portable way. The main difficulty is that different platforms, libraries and applications use different encodings, Unicode or otherwise. Interoperability is therefore extremely hard.

With the advent of char16_t and char32_t, and of std::u16string and std::u32string, this problem has been acknowledged and encoding can done more explicitly, but issues with interoperability have not been addressed.

This proposal currently partially resolves interoperability issues by using a template for functions and member functions when string arguments are used. e.g. The uri constructor:

namespace std {
namespace network {
template <class Source>
uri::uri(const Source &source);
} // namespace network
} // namespace std

Internally, the uri constructor can handle strings in different encodings by using template specialization to perform the correct transformation depending on the source type.

This can help simplify the library interface but is not completely satisfactory. For example, template specialization is limited to only those types known to the standard - std::basic_string and its variants, plus character arrays. A proposal exists to add a string_ref type to the standard (N3334), which could provide better flexibility and performance. This would replace part_range in the current proposal.

This approach is limited when returning strings from functions. e.g.:

namespace std {
namespace network {
template <class Result>
Result scheme(const uri &uri_);
} // namespace network
} // namespace std

...

std::network::uri uri("http://example.com/");
auto scheme = std::network::scheme<std::u32string>(uri);

The above excerpt cannot work with character array, character pointers or string_ref since memory allocation is required.

Secondly, RFC 3987 is not well supported and often unicode text is converted using percent encoding:

std::network::uri uri(
    "http://ja.wikipedia.org/wiki/%E3%83%A1%E3%82%A4%E3%83%B3%E3%83%9A%E3%83%BC%E3%82%B8");

The pct_encode and pct_decode functions should take this into account.

Thirdly, the iterators returned by begin and end are not portable with the proposal in its current form. This proposal allows different internal character types, but the iterator types are completely unaware of the encoding. So the following examples are not portable:

std::network::uri uri(
    "http://ja.wikipedia.org/wiki/%E3%83%A1%E3%82%A4%E3%83%B3%E3%83%9A%E3%83%BC%E3%82%B8");
std::u32string u32;
std::copy(std::begin(uri), std::end(uri), std::back_inserter(u32));

std::network::uri uri(
    "http://ja.wikipedia.org/wiki/%E3%83%A1%E3%82%A4%E3%83%B3%E3%83%9A%E3%83%BC%E3%82%B8");
auto path_range = uri.path();
std::u32string path;
std::copy(std::begin(path_range), std::end(path_range), std::back_inserter(path));

This proposal as it stands needs better library or language support in order offer better portability. One promising solution is given in another standard proposal, N3336.

Issue 2 - The Parser Implementation¶

At this stage it is important to note that there are many compatibility issues with URIs. If this proposal were to specify full standard compliance, it may not work in practice. For example, the uri parser implementation must correctly deal with escape control characters. Furthermore, no RFC fully specifies how to parse Windows file-system paths (e.g. file:///C:\path\to\a\file.txt). Finally, it is the belief of the author of this proposal that its success depends strongly on the correct portable behavior of the parser implementation. Without wishing to run the risk of over-specifying the parser, leaving it as “implementation defined” will not be sufficient. There will be more issues that may need to specified in further detail in future revisions of this proposal.

This proposal includes internationalized URIs (RFC 2732). Since this RFC is not widely supported, and as many applications deal with unicode characters through percent encoding, the parser implementation could be simplified if appropriate by removing RFC 2732 from this proposal.

Issue 3 - Error and Exception Handling¶

The proposal in its current form does not propose a detailed error handling mechanism, beyond providing a uri::is_valid() accessor to determine if the URI is valid according to the parser.

There several potential sources for errors:

std::network::uri uri;
std::network::uri::builder builder(uri);
builder.scheme("http");
assert(!uri.is_valid());
builder.authority("example.com")
       .path("/glynos/");
assert(uri.is_valid());

Secondly, passing illegal arguments to the builder has the same effect: the resultant uri object is simply invalid again. This proposal leaves a lot of scope for improvement in better error reporting when building uri objects.

It may be appropriate to develop a uri_parse_error exception type that can be thrown when a parse error occurs.

Issue 4 - Equivalence¶

As explained in the previous section on normalization, there is more than one way to test if two URIs are equivalent depending on different accuracy, performance and complexity trade-offs.

Providing a single operator == is sufficient to allow developers to choose different ways of testing URI equivalence, although choosing a good default comparison will be sufficient for the vast majority of cases. Future revisions of this proposal could provide functionality to allow a library to use different parts of the Comparison Ladder.

Issue 5 - Extending the URI¶

The interface can be extended to allow more flexibility for accessing parts of the URI. For example, an accessor could be provided which converts all query elements into a map:

namespace std {
namespace network {
template <class QueryMap>
QueryMap query(const uri &u);
} // namespace network
} // namespace std

...

typedef std::map<std::string, std::string> QueryMap;
QueryMap query = std::network::query<QueryMap>(uri);

Additionally, his proposal can be extended to include factory functions for common operations, such as constructing a URI from a filesystem path:

namespace std {
namespace network {
uri uri::from_path(const filesystem::path &path);
} // namespace network
} // namespace std

...

std::filesystem::path path("/usr/bin/c++");
auto uri_path = std::network::uri::from_path(path);
assert(uri_path.string() == "file:///usr/bin/c++");

Furthermore, there may be new proposals for types that represent IP addresses. If such types can be accepted into the standard, it would be possible to accommodate them in future revisions of this proposal:

namespace std {
namespace network {
uri::builder &uri::builder::host(const std::network::address_ipv4 &host);
uri::builder &uri::builder::host(const std::network::address_ipv6 &host);
} // namespace network
} // namespace std

Issue 6 - Allocator Support¶

This proposal does not specify any kind of allocator support.

Issue 7 - Percent Encoding and Iterators¶

It may be possible to gain extra flexibility and performance with iterators that are aware of percent encoding and decoding. The following untested examples give an illustration:

// assume we're on Windows, using wchar_t
std::network::uri uri(L"http://example.com/path%20with%20spaces/");
// the iterators in the type returned by pct_decoded_path still refer to the native type
auto pct_decoded_path_range = std::network::pct_decoded_path(uri);
assert(std::equal(std::begin(pct_decoded_path_range), std::end(pct_decoded_path_range),
                  L"/path with spaces/"));

This may even be improved by providing an overload for std::network::pct_decode:

std::network::uri uri(L"http://example.com/path%20with%20spaces/");
auto pct_decoded_path_range = std::network::pct_decode(uri.path());
assert(std::equal(std::begin(pct_decoded_path_range), std::end(pct_decoded_path_range),
                  L"/path with spaces/"));

For better expressiveness this can be improved further by replacing std::equal with a range-based equivalent available in Boost:

std::network::uri uri(L"http://example.com/path%20with%20spaces/");
assert(boost::equal(std::network::pct_decode(uri.path()), L"/path with spaces/"));