mirror of
https://github.com/fmtlib/fmt.git
synced 2024-12-26 00:21:13 +00:00
342 lines
10 KiB
ReStructuredText
342 lines
10 KiB
ReStructuredText
.. _string-formatting-api:
|
|
|
|
*************
|
|
API Reference
|
|
*************
|
|
|
|
The {fmt} library API consists of the following parts:
|
|
|
|
* :ref:`fmt/core.h <core-api>`: the core API providing argument handling
|
|
facilities and a lightweight subset of formatting functions
|
|
* :ref:`fmt/format.h <format-api>`: the full format API providing compile-time
|
|
format string checks, output iterator and user-defined type support
|
|
* :ref:`fmt/time.h <time-api>`: date and time formatting
|
|
* :ref:`fmt/ostream.h <ostream-api>`: ``std::ostream`` support
|
|
* :ref:`fmt/printf.h <printf-api>`: ``printf`` formatting
|
|
|
|
All functions and types provided by the library reside in namespace ``fmt`` and
|
|
macros have prefix ``FMT_`` or ``fmt``.
|
|
|
|
.. _core-api:
|
|
|
|
Core API
|
|
========
|
|
|
|
``fmt/core.h`` defines the core API which provides argument handling facilities
|
|
and a lightweight subset of formatting functions.
|
|
|
|
The following functions use :ref:`format string syntax <syntax>`
|
|
imilar to that of Python's `str.format
|
|
<http://docs.python.org/3/library/stdtypes.html#str.format>`_.
|
|
They take *format_str* and *args* as arguments.
|
|
|
|
*format_str* is a format string that contains literal text and replacement
|
|
fields surrounded by braces ``{}``. The fields are replaced with formatted
|
|
arguments in the resulting string.
|
|
|
|
*args* is an argument list representing objects to be formatted.
|
|
|
|
.. _format:
|
|
|
|
.. doxygenfunction:: format(const S&, const Args&...)
|
|
.. doxygenfunction:: vformat(const S&, basic_format_args<typename buffer_context<Char>::type>)
|
|
|
|
.. _print:
|
|
|
|
.. doxygenfunction:: print(const S&, const Args&...)
|
|
.. doxygenfunction:: vprint(string_view, format_args)
|
|
|
|
.. doxygenfunction:: print(std::FILE *, const S&, const Args&...)
|
|
.. doxygenfunction:: vprint(std::FILE *, string_view, format_args)
|
|
.. doxygenfunction:: vprint(std::FILE *, wstring_view, wformat_args)
|
|
|
|
Named arguments
|
|
---------------
|
|
|
|
.. doxygenfunction:: fmt::arg(string_view, const T&)
|
|
|
|
Argument lists
|
|
--------------
|
|
|
|
.. doxygenfunction:: fmt::make_format_args(const Args&...)
|
|
|
|
.. doxygenclass:: fmt::format_arg_store
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::basic_format_args
|
|
:members:
|
|
|
|
.. doxygenstruct:: fmt::format_args
|
|
|
|
.. doxygenclass:: fmt::basic_format_arg
|
|
:members:
|
|
|
|
Compatibility
|
|
-------------
|
|
|
|
.. doxygenclass:: fmt::basic_string_view
|
|
:members:
|
|
|
|
.. doxygentypedef:: fmt::string_view
|
|
.. doxygentypedef:: fmt::wstring_view
|
|
|
|
.. _format-api:
|
|
|
|
Format API
|
|
==========
|
|
|
|
``fmt/format.h`` defines the full format API providing compile-time format
|
|
string checks, output iterator and user-defined type support.
|
|
|
|
Compile-time format string checks
|
|
---------------------------------
|
|
|
|
.. doxygendefine:: fmt
|
|
|
|
Formatting user-defined types
|
|
-----------------------------
|
|
|
|
To make a user-defined type formattable, specialize the ``formatter<T>`` struct
|
|
template and implement ``parse`` and ``format`` methods::
|
|
|
|
#include <fmt/format.h>
|
|
|
|
struct point { double x, y; };
|
|
|
|
namespace fmt {
|
|
template <>
|
|
struct formatter<point> {
|
|
template <typename ParseContext>
|
|
constexpr auto parse(ParseContext &ctx) { return ctx.begin(); }
|
|
|
|
template <typename FormatContext>
|
|
auto format(const point &p, FormatContext &ctx) {
|
|
return format_to(ctx.begin(), "({:.1f}, {:.1f})", p.x, p.y);
|
|
}
|
|
};
|
|
}
|
|
|
|
Then you can pass objects of type ``point`` to any formatting function::
|
|
|
|
point p = {1, 2};
|
|
std::string s = fmt::format("{}", p);
|
|
// s == "(1.0, 2.0)"
|
|
|
|
In the example above the ``formatter<point>::parse`` function ignores the
|
|
contents of the format string referred to by ``ctx.begin()`` so the object will
|
|
always be formatted in the same way. See ``formatter<tm>::parse`` in
|
|
:file:`fmt/time.h` for an advanced example of how to parse the format string and
|
|
customize the formatted output.
|
|
|
|
You can also reuse existing formatters, for example::
|
|
|
|
enum class color {red, green, blue};
|
|
|
|
template <>
|
|
struct fmt::formatter<color>: formatter<string_view> {
|
|
// parse is inherited from formatter<string_view>.
|
|
template <typename FormatContext>
|
|
auto format(color c, FormatContext &ctx) {
|
|
string_view name = "unknown";
|
|
switch (c) {
|
|
case color::red: name = "red"; break;
|
|
case color::green: name = "green"; break;
|
|
case color::blue: name = "blue"; break;
|
|
}
|
|
return formatter<string_view>::format(name, ctx);
|
|
}
|
|
};
|
|
|
|
This section shows how to define a custom format function for a user-defined
|
|
type. The next section describes how to get ``fmt`` to use a conventional stream
|
|
output ``operator<<`` when one is defined for a user-defined type.
|
|
|
|
Output iterator support
|
|
-----------------------
|
|
|
|
.. doxygenfunction:: fmt::format_to(OutputIt, const String&, const Args&...)
|
|
.. doxygenfunction:: fmt::format_to_n(OutputIt, std::size_t, string_view, const Args&...)
|
|
.. doxygenstruct:: fmt::format_to_n_result
|
|
:members:
|
|
|
|
Literal-based API
|
|
-----------------
|
|
|
|
The following user-defined literals are defined in ``fmt/format.h``.
|
|
|
|
.. doxygenfunction:: operator""_format(const char *, std::size_t)
|
|
|
|
.. doxygenfunction:: operator""_a(const char *, std::size_t)
|
|
|
|
Utilities
|
|
---------
|
|
|
|
.. doxygenfunction:: fmt::formatted_size(string_view, const Args&...)
|
|
|
|
.. doxygenfunction:: fmt::to_string(const T&)
|
|
|
|
.. doxygenfunction:: fmt::to_wstring(const T&)
|
|
|
|
.. doxygenclass:: fmt::basic_memory_buffer
|
|
:protected-members:
|
|
:members:
|
|
|
|
System errors
|
|
-------------
|
|
|
|
fmt does not use ``errno`` to communicate errors to the user, but it may call
|
|
system functions which set ``errno``. Users should not make any assumptions about
|
|
the value of ``errno`` being preserved by library functions.
|
|
|
|
.. doxygenclass:: fmt::system_error
|
|
:members:
|
|
|
|
.. doxygenfunction:: fmt::format_system_error
|
|
|
|
.. doxygenclass:: fmt::windows_error
|
|
:members:
|
|
|
|
.. _formatstrings:
|
|
|
|
Custom allocators
|
|
-----------------
|
|
|
|
The {fmt} library supports custom dynamic memory allocators.
|
|
A custom allocator class can be specified as a template argument to
|
|
:class:`fmt::basic_memory_buffer`::
|
|
|
|
using custom_memory_buffer =
|
|
fmt::basic_memory_buffer<char, fmt::inline_buffer_size, custom_allocator>;
|
|
|
|
It is also possible to write a formatting function that uses a custom
|
|
allocator::
|
|
|
|
using custom_string =
|
|
std::basic_string<char, std::char_traits<char>, custom_allocator>;
|
|
|
|
custom_string vformat(custom_allocator alloc, fmt::string_view format_str,
|
|
fmt::format_args args) {
|
|
custom_memory_buffer buf(alloc);
|
|
fmt::vformat_to(buf, format_str, args);
|
|
return custom_string(buf.data(), buf.size(), alloc);
|
|
}
|
|
|
|
template <typename ...Args>
|
|
inline custom_string format(custom_allocator alloc,
|
|
fmt::string_view format_str,
|
|
const Args & ... args) {
|
|
return vformat(alloc, format_str, fmt::make_format_args(args...));
|
|
}
|
|
|
|
The allocator will be used for the output container only. If you are using named
|
|
arguments, the container that stores pointers to them will be allocated using
|
|
the default allocator. Also floating-point formatting falls back on ``sprintf``
|
|
which may do allocations.
|
|
|
|
Custom formatting of built-in types
|
|
-----------------------------------
|
|
|
|
It is possible to change the way arguments are formatted by providing a
|
|
custom argument formatter class::
|
|
|
|
using arg_formatter =
|
|
fmt::arg_formatter<fmt::back_insert_range<fmt::internal::buffer>>;
|
|
|
|
// A custom argument formatter that formats negative integers as unsigned
|
|
// with the ``x`` format specifier.
|
|
class custom_arg_formatter : public arg_formatter {
|
|
public:
|
|
custom_arg_formatter(fmt::format_context &ctx,
|
|
fmt::format_specs *spec = nullptr)
|
|
: arg_formatter(ctx, spec) {}
|
|
|
|
using arg_formatter::operator();
|
|
|
|
auto operator()(int value) {
|
|
if (spec().type() == 'x')
|
|
return (*this)(static_cast<unsigned>(value)); // convert to unsigned and format
|
|
return arg_formatter::operator()(value);
|
|
}
|
|
};
|
|
|
|
std::string custom_vformat(fmt::string_view format_str, fmt::format_args args) {
|
|
fmt::memory_buffer buffer;
|
|
// Pass custom argument formatter as a template arg to vformat_to.
|
|
fmt::vformat_to<custom_arg_formatter>(buffer, format_str, args);
|
|
return fmt::to_string(buffer);
|
|
}
|
|
|
|
template <typename ...Args>
|
|
inline std::string custom_format(
|
|
fmt::string_view format_str, const Args &... args) {
|
|
return custom_vformat(format_str, fmt::make_format_args(args...));
|
|
}
|
|
|
|
std::string s = custom_format("{:x}", -42); // s == "ffffffd6"
|
|
|
|
.. doxygenclass:: fmt::arg_formatter
|
|
:members:
|
|
|
|
.. _time-api:
|
|
|
|
Date and time formatting
|
|
========================
|
|
|
|
The library supports `strftime
|
|
<http://en.cppreference.com/w/cpp/chrono/c/strftime>`_-like date and time
|
|
formatting::
|
|
|
|
#include <fmt/time.h>
|
|
|
|
std::time_t t = std::time(nullptr);
|
|
// Prints "The date is 2016-04-29." (with the current date)
|
|
fmt::print("The date is {:%Y-%m-%d}.", *std::localtime(&t));
|
|
|
|
The format string syntax is described in the documentation of
|
|
`strftime <http://en.cppreference.com/w/cpp/chrono/c/strftime>`_.
|
|
|
|
.. _ostream-api:
|
|
|
|
``std::ostream`` support
|
|
========================
|
|
|
|
``fmt/ostream.h`` provides ``std::ostream`` support including formatting of
|
|
user-defined types that have overloaded ``operator<<``::
|
|
|
|
#include <fmt/ostream.h>
|
|
|
|
class date {
|
|
int year_, month_, day_;
|
|
public:
|
|
date(int year, int month, int day): year_(year), month_(month), day_(day) {}
|
|
|
|
friend std::ostream &operator<<(std::ostream &os, const date &d) {
|
|
return os << d.year_ << '-' << d.month_ << '-' << d.day_;
|
|
}
|
|
};
|
|
|
|
std::string s = fmt::format("The date is {}", date(2012, 12, 9));
|
|
// s == "The date is 2012-12-9"
|
|
|
|
.. doxygenfunction:: print(std::ostream&, string_view, const Args&...)
|
|
|
|
.. _printf-api:
|
|
|
|
``printf`` formatting
|
|
=====================
|
|
|
|
The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
|
|
The following functions use `printf format string syntax
|
|
<http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html>`_ with
|
|
the POSIX extension for positional arguments. Unlike their standard
|
|
counterparts, the ``fmt`` functions are type-safe and throw an exception if an
|
|
argument type doesn't match its format specification.
|
|
|
|
.. doxygenfunction:: printf(string_view, const Args&...)
|
|
|
|
.. doxygenfunction:: fprintf(std::FILE *, string_view, const Args&...)
|
|
|
|
.. doxygenfunction:: fprintf(std::ostream&, string_view, const Args&...)
|
|
|
|
.. doxygenfunction:: sprintf(string_view, const Args&...)
|