boost::urls::format_to

Format arguments into a URL

Synopsis

Declared in header <boost/url/format.hpp>

void
format_to(
    url_base& u,
    core::string_view fmt,
    std::initializer_list<see-below> args);

Description

Format arguments according to the format URL string into a url_base .

This overload allows type-erased arguments to be passed as an initializer_list, which is mostly convenient for named parameters.

All arguments must be convertible to a implementation defined type able to store a type-erased reference to any valid format argument.

The rules for a format URL string are the same as for a std::format_string, where replacement fields are delimited by curly braces.

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.

Hence, the delimiters between URL components, such as :, //, ?, and #, should be included in the URL format string. Likewise, a format string with a single "{}" is interpreted as a path and any replacement characters invalid in this component will be encoded to form a valid URL.

Example

static_url<30> u;
format_to(u, "user/{id}", {{"id", 1}})
assert(u.buffer() == "user/1");

Preconditions

All replacement fields must be valid and the resulting URL should be valid after arguments are formatted into the URL.

Because any invalid characters for a URL component are encoded by this function, only replacements in the scheme and port components might be invalid, as these components do not allow percent-encoding of arbitrary characters.

Exception Safety

Strong guarantee.

BNF

replacement_field ::=  "{" [arg_id] [":" (format_spec | chrono_format_spec)] "}"
arg_id            ::=  integer | identifier
integer           ::=  digit+
digit             ::=  "0"..."9"
identifier        ::=  id_start id_continue*
id_start          ::=  "a"..."z" | "A"..."Z" | "_"
id_continue       ::=  id_start | digit

Specification

Exceptions

Name Thrown on

system_error

fmt contains an invalid format string and u contains an invalid URL after replacements are applied.

Parameters

Name Description

u

An object that derives from url_base .

fmt

The format URL string.

args

Arguments to be formatted.

See Also

format .