mirror of
https://github.com/fmtlib/fmt.git
synced 2024-12-26 00:21:13 +00:00
284 lines
8.1 KiB
ReStructuredText
284 lines
8.1 KiB
ReStructuredText
.. _string-formatting-api:
|
|
|
|
*************
|
|
API Reference
|
|
*************
|
|
|
|
All functions and classes provided by the fmt library reside
|
|
in namespace ``fmt`` and macros have prefix ``FMT_``. For brevity the
|
|
namespace is usually omitted in examples.
|
|
|
|
Format API
|
|
==========
|
|
|
|
The following functions defined in ``fmt/format.h`` use :ref:`format string
|
|
syntax <syntax>` similar to the one used by Python's `str.format
|
|
<http://docs.python.org/3/library/stdtypes.html#str.format>`_ function.
|
|
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 arbitrary arguments.
|
|
|
|
The `performance of the format API
|
|
<https://github.com/fmtlib/fmt/blob/master/README.rst#speed-tests>`_ is close
|
|
to that of glibc's ``printf`` and better than the performance of IOStreams.
|
|
For even better speed use the `write API`_.
|
|
|
|
.. _format:
|
|
|
|
.. doxygenfunction:: format(CStringRef, ArgList)
|
|
|
|
.. doxygenfunction:: operator""_format(const char *, std::size_t)
|
|
|
|
.. _print:
|
|
|
|
.. doxygenfunction:: print(CStringRef, ArgList)
|
|
|
|
.. doxygenfunction:: print(std::FILE *, CStringRef, ArgList)
|
|
|
|
.. doxygenclass:: fmt::BasicFormatter
|
|
:members:
|
|
|
|
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>`_.
|
|
|
|
Formatting user-defined types
|
|
-----------------------------
|
|
|
|
To make a user-defined type formattable, specialize the ``formatter<T>`` struct
|
|
template and implement ``parse`` and ``format`` methods::
|
|
|
|
struct MyStruct { double x, y; };
|
|
|
|
namespace fmt {
|
|
template <>
|
|
struct formatter<MyStruct> {
|
|
template <typename ParseContext>
|
|
auto parse(ParseContext &ctx) { return ctx.begin(); }
|
|
|
|
template <typename FormatContext>
|
|
auto format(const MyStruct &s, FormatContext &ctx) {
|
|
fmt::format_to(ctx.begin(), "[MyStruct: x={:.1f}, y={:.2f}]", s.x, s.y);
|
|
}
|
|
};
|
|
}
|
|
|
|
Then you can pass objects of type ``MyStruct`` to any formatting function::
|
|
|
|
MyStruct m = {1, 2};
|
|
std::string s = fmt::format("m={}", m);
|
|
// s == "m=[MyStruct: x=1.0, y=2.00]"
|
|
|
|
In the example above the ``formatter<MyStruct>::parse`` function ignores the
|
|
contents of the format string referred to by ``ctx.begin()`` so the object will
|
|
always be formatted as specified. 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.
|
|
|
|
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.
|
|
|
|
``std::ostream`` support
|
|
------------------------
|
|
|
|
The header ``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&, CStringRef, ArgList)
|
|
|
|
Argument formatters
|
|
-------------------
|
|
|
|
It is possible to change the way arguments are formatted by providing a
|
|
custom argument formatter class::
|
|
|
|
// A custom argument formatter that formats negative integers as unsigned
|
|
// with the ``x`` format specifier.
|
|
class CustomArgFormatter :
|
|
public fmt::BasicArgFormatter<CustomArgFormatter, char> {
|
|
public:
|
|
CustomArgFormatter(fmt::BasicFormatter<char, CustomArgFormatter> &f,
|
|
fmt::FormatSpec &s, const char *fmt)
|
|
: fmt::BasicArgFormatter<CustomArgFormatter, char>(f, s, fmt) {}
|
|
|
|
void visit_int(int value) {
|
|
if (spec().type() == 'x')
|
|
visit_uint(value); // convert to unsigned and format
|
|
else
|
|
fmt::BasicArgFormatter<CustomArgFormatter, char>::visit_int(value);
|
|
}
|
|
};
|
|
|
|
std::string custom_format(const char *format_str, fmt::ArgList args) {
|
|
fmt::MemoryWriter writer;
|
|
// Pass custom argument formatter as a template arg to BasicFormatter.
|
|
fmt::BasicFormatter<char, CustomArgFormatter> formatter(args, writer);
|
|
formatter.format(format_str);
|
|
return writer.str();
|
|
}
|
|
FMT_VARIADIC(std::string, custom_format, const char *)
|
|
|
|
std::string s = custom_format("{:x}", -42); // s == "ffffffd6"
|
|
|
|
.. doxygenclass:: fmt::ArgVisitor
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicArgFormatter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::ArgFormatter
|
|
:members:
|
|
|
|
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(CStringRef, ArgList)
|
|
|
|
.. doxygenfunction:: fprintf(std::FILE *, CStringRef, ArgList)
|
|
|
|
.. doxygenfunction:: fprintf(std::ostream&, CStringRef, ArgList)
|
|
|
|
.. doxygenfunction:: sprintf(CStringRef, ArgList)
|
|
|
|
.. doxygenclass:: fmt::PrintfFormatter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicPrintfArgFormatter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::PrintfArgFormatter
|
|
:members:
|
|
|
|
Write API
|
|
=========
|
|
|
|
The write API provides classes for writing formatted data into character
|
|
streams. It is usually faster than the `format API`_ but, as IOStreams,
|
|
may result in larger compiled code size. The main writer class is
|
|
`~fmt::BasicMemoryWriter` which stores its output in a memory buffer and
|
|
provides direct access to it. It is possible to create custom writers that
|
|
store output elsewhere by subclassing `~fmt::BasicWriter`.
|
|
|
|
.. doxygenclass:: fmt::BasicWriter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicMemoryWriter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicArrayWriter
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicStringWriter
|
|
:members:
|
|
|
|
.. doxygenfunction:: bin(int)
|
|
|
|
.. doxygenfunction:: oct(int)
|
|
|
|
.. doxygenfunction:: hex(int)
|
|
|
|
.. doxygenfunction:: hexu(int)
|
|
|
|
.. doxygenfunction:: pad(int, unsigned, Char)
|
|
|
|
Utilities
|
|
=========
|
|
|
|
.. doxygenfunction:: fmt::arg(StringRef, const T&)
|
|
|
|
.. doxygenfunction:: operator""_a(const char *, std::size_t)
|
|
|
|
.. doxygendefine:: FMT_CAPTURE
|
|
|
|
.. doxygendefine:: FMT_VARIADIC
|
|
|
|
.. doxygenclass:: fmt::ArgList
|
|
:members:
|
|
|
|
.. doxygenfunction:: fmt::to_string(const T&)
|
|
|
|
.. doxygenclass:: fmt::BasicStringRef
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::BasicCStringRef
|
|
:members:
|
|
|
|
.. doxygenclass:: fmt::Buffer
|
|
:protected-members:
|
|
:members:
|
|
|
|
System errors
|
|
=============
|
|
|
|
.. doxygenclass:: fmt::SystemError
|
|
:members:
|
|
|
|
.. doxygenfunction:: fmt::format_system_error
|
|
|
|
.. doxygenclass:: fmt::WindowsError
|
|
: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::BasicMemoryWriter`::
|
|
|
|
typedef fmt::BasicMemoryWriter<char, CustomAllocator> CustomMemoryWriter;
|
|
|
|
It is also possible to write a formatting function that uses a custom
|
|
allocator::
|
|
|
|
typedef std::basic_string<char, std::char_traits<char>, CustomAllocator>
|
|
CustomString;
|
|
|
|
CustomString format(CustomAllocator alloc, fmt::CStringRef format_str,
|
|
fmt::ArgList args) {
|
|
CustomMemoryWriter writer(alloc);
|
|
writer.write(format_str, args);
|
|
return CustomString(writer.data(), writer.size(), alloc);
|
|
}
|
|
FMT_VARIADIC(CustomString, format, CustomAllocator, fmt::CStringRef)
|