Features

While the library is general purpose, special care has been taken to ensure that the implementation and data representation are friendly to network programs which need to handle URLs efficiently and securely, including the case where the inputs come from untrusted sources.Interfaces are provided for using error codes instead of exceptions as needed, and most algorithms have the means to opt-out of dynamic memory allocation.Another feature of the library is that all modifications leave the URL in a valid state.Code which uses this library is easy to read, flexible, and performant.

Network programs such as those using Boost.Asio or Boost.Beast often encounter the need to process, generate, or modify URLs.This library provides a very much needed modular component for handling these use-cases.

Boost.URL offers these features:

• C++11 as only requirement

• Fast compilation, few templates

• Strict compliance withrfc3986

• Containers that maintain valid URLs

• Parsing algorithms that work without exceptions

• Control over storage and allocation for URLs

• Support for-fno-exceptions, detected automatically

• Features that work well on embedded devices

Requirements

The library requires a compiler supporting at least C++11.

Aliases for standard types, such aserror_code orstring_view, use their Boost equivalents.

Boost.URL works great on embedded devices.It can be used in a way that avoids all dynamic memory allocations.Furthermore, it offers alternative interfaces that work without exceptions if desired.

Tested Compilers

Boost.URL has been tested with the following compilers:

• clang: 3.8, 4, 5, 6, 7, 8, 9, 10, 11, 12

• gcc: 4.8, 4.9, 5, 6, 7, 8, 9, 10, 11

• msvc: 14.1, 14.2, 14.3

and these architectures: x86, x64, ARM64, S390x.

We do not test and support gcc 8.0.1.

Quality Assurance

The development infrastructure for the library includes these per-commit analyses:

• Coverage reports

• Compilation and tests on Drone.io and GitHub Actions

• Regular code audits for security

Nomenclature

Various names have been used historically to refer to different flavors of resource identifiers, includingURI,URL,URN, and evenIRI.Over time, the distinction between URIs and URLs has disappeared when discussed in technical documents and informal works.In this library we use the termURL to refer to all strings which are valid according to the top-level grammar rules found inrfc3986.

Visual Studio Solution Generation

Integration

We begin by including the library header file which brings all the symbols into scope.

Alternatively, individual headers may be included to obtain the declarations for specific types.

Boost.URL is a compiled library.You need to install binaries in a location that can be found by your linker and link your program with the Boost.URL built library.If you followed the [@http://www.boost.org/doc/libs/release/more/getting_started/index.html Boost Getting Started]instructions, that’s already been done for you.

For example, if you are using CMake, you can use the following commands to find and link the library:

Parsing

Say you have the following URL that you want to parse:

In this example,string_view is an alias toboost::core::string_view, astring_view implementation that is implicitly convertible from and tostd::string_view.

You can parse the string by calling this function:

The functionparse_uri returns an object of typeresult<url_view>which is a container resembling a variant that holds either an error or an object.A number of functions are available to parse different types of URL.

We can immediately callresult::value to obtain aurl_view.

Or simply

for unchecked access.

When there are no errors,result::valuereturns an instance ofurl_view, which holds the parsed result.

result::value throws an exception on a parsing error.Alternatively, the functionsresult::has_value andresult::has_error could also be used to check if the string has been parsed without errors.

Accessing

Accessing the parts of the URL is easy:

URL paths can be further divided into path segments with the functionurl_view::segments.

Although URL query strings are often used to represent key/value pairs, this interpretation is not defined byrfc3986.Users can treat the query as a single entity.url_view provides the functionurl_view::params to extract this view of key/value pairs.

The output is:

These functions return views referring to substrings and sub-ranges of the underlying URL.By simply referencing the relevant portion of the URL string internally, its components can represent percent-decoded strings and be converted to other types without any previous memory allocation.

A specialstring_token type can also be used to specify how a portion of the URL should be encoded and returned.

These functions might also return empty strings

for both empty and absent components

Many components do not have corresponding functions such ashas_authority to check for their existence.This happens because some URL components are mandatory.

When applicable, the encoded components can also be directly accessed through astring_view without any need to allocate memory:

The output is:

Percent-Encoding

An instance ofdecode_view provides a number of functions to persist a decoded string:

The output is:

decode_view and its decoding functions are designed to perform no memory allocations unless the algorithm where its being used needs the result to be in another container.The design also permits recycling objects to reuse their memory, and at least minimize the number of allocations by deferring them until the result is in fact needed by the application.

In the example above, the memory owned bystr can be reused to store other results.This is also useful when manipulating URLs:

Ifu2.host() returned a value type, then two memory allocations would be necessary for this operation.Another common use case is converting URL path segments into filesystem paths:

The output is:

In this example, only the internal allocations offilesystem::path need to happen.In many common use cases, no allocations are necessary at all, such as finding the appropriate route for a URL in a web server:

This allows us to easily match files in the document root directory of a web server:

Compound elements

The path and query parts of the URL are treated specially by the library.While they can be accessed as individual encoded strings, they can also be accessed through special view types.

This code callsencoded_segments to obtain the path segments as a container that returns encoded strings:

The output is:

As with otherurl_view functions which return encoded strings, the encoded segments container does not allocate memory.Instead, it returns views to the corresponding portions of the underlying encoded buffer referenced by the URL.

As with other library functions,decode_view permits accessing elements of composed elements while avoiding memory allocations entirely:

The output is:

Or with the encoded query parameters:

The output is:

Modifying

The library provides the containersurl andstatic_url which supporting modification of the URL contents.Aurl orstatic_url must be constructed from an existingurl_view.

Unlike theurl_view, which does not gain ownership of the underlying character buffer, theurl container uses the default allocator to control a resizable character buffer which it owns.

On the other hand, astatic_url has fixed-capacity storage and does not require dynamic memory allocations.

Objects of typeurl arestd::regular.Similarly to built-in types, such asint, aurl is copyable, movable, assignable, default constructible, and equality comparable.They support all the inspection functions ofurl_view, and also provide functions to modify all components of the URL.

Changing the scheme is easy:

Or we can use a predefined constant:

The scheme must be valid, however, or an exception is thrown.All modifying functions perform validation on their input.

• Attempting to set the URL scheme or port to an invalid string results in an exception.

• Attempting to set other URL components to invalid strings will get the original input properly percent-encoded for that component.

It is not possible for aurl to hold syntactically illegal text.

Modification functions return a reference to the object, so chaining is possible:

The output is:

All non-const operations offer the strong exception safety guarantee.

The path segment and query parameter containers returned by aurl offer modifiable range functionality, using member functions of the container:

The output is:

Formatting

Algorithms to format URLs construct a mutable URL by parsing and applying arguments to a URL template.The following example uses theformatfunction to construct an absolute URL:

The rules for a format URL string are the same as for astd::format_string, where replacement fields are delimited by curly braces.The URL type is inferred from the format string.

The URL components to which replacement fields belong are identified before replacement is applied and any invalid characters for that formatted argument are percent-escaped:

Delimiters in the URL template, such as":","//","?", and"#", unambiguously associate each replacement field to a URL component.All other characters are normalized to ensure the URL is valid:

The functionformat_to can be used to format URLs into any modifiable URL container.

As withstd::format, positional and named arguments are supported.

Thearg function can be used to associate names with arguments:

A second overload based onstd::initializer_listis provided for bothformat andformat_to.

These overloads can help with lists of named arguments: