boost::urls::format :: Boost Libraries Documentation

# boost::urls::format

Format arguments into a URL

## Synopsis

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

    url
    format(
        core::string_view fmt,
        std::initializer_list<format_arg> args);

## Description

Format arguments according to the format URL string into a `url`.

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

    assert(format("user/{id}", {{"id", 1}}).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.

## 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

- Format String Syntax

## Exceptions

Name | Thrown on
`system_error` | `fmt` contains an invalid format string and the result contains an invalid URL after replacements are applied.

## Return Value

A URL holding the formatted result.

## Parameters

Name | Description
--- | ---
fmt | The format URL string.
args | Arguments to be formatted.

## See Also

`format_to`.

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