247 lines
6.8 KiB
ReStructuredText
247 lines
6.8 KiB
ReStructuredText
.. _string-formatting-api:
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*************
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API Reference
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*************
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All functions and classes provided by the fmt library reside
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in namespace ``fmt`` and macros have prefix ``FMT_``. For brevity the
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namespace is usually omitted in examples.
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Format API
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==========
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The following functions use :ref:`format string syntax <syntax>` similar
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to the one used by Python's `str.format
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<http://docs.python.org/3/library/stdtypes.html#str.format>`_ function.
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They take *format_str* and *args* as arguments.
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*format_str* is a format string that contains literal text and replacement
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fields surrounded by braces ``{}``. The fields are replaced with formatted
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arguments in the resulting string.
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*args* is an argument list representing arbitrary arguments.
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The `performance of the format API
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<https://github.com/fmtlib/fmt/blob/master/README.rst#speed-tests>`_ is close
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to that of glibc's ``printf`` and better than the performance of IOStreams.
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For even better speed use the `write API`_.
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.. _format:
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.. doxygenfunction:: format(CStringRef, ArgList)
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.. doxygenfunction:: operator""_format(const char *, std::size_t)
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.. _print:
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.. doxygenfunction:: print(CStringRef, ArgList)
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.. doxygenfunction:: print(std::FILE *, CStringRef, ArgList)
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.. doxygenclass:: fmt::BasicFormatter
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:members:
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Date and time formatting
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------------------------
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The library supports `strftime
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<http://en.cppreference.com/w/cpp/chrono/c/strftime>`_-like date and time
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formatting::
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#include "fmt/time.h"
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std::time_t t = std::time(nullptr);
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// Prints "The date is 2016-04-29." (with the current date)
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fmt::print("The date is {:%Y-%m-%d}.", *std::localtime(&t));
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The format string syntax is described in the documentation of
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`strftime <http://en.cppreference.com/w/cpp/chrono/c/strftime>`_.
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``std::ostream`` support
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------------------------
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The header ``fmt/ostream.h`` provides ``std::ostream`` support including
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formatting of user-defined types that have overloaded ``operator<<``::
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#include "fmt/ostream.h"
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class Date {
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int year_, month_, day_;
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public:
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Date(int year, int month, int day): year_(year), month_(month), day_(day) {}
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friend std::ostream &operator<<(std::ostream &os, const Date &d) {
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return os << d.year_ << '-' << d.month_ << '-' << d.day_;
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}
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};
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std::string s = fmt::format("The date is {}", Date(2012, 12, 9));
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// s == "The date is 2012-12-9"
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.. doxygenfunction:: print(std::ostream&, CStringRef, ArgList)
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Argument formatters
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-------------------
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It is possible to change the way arguments are formatted by providing a
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custom argument formatter class::
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// A custom argument formatter that formats negative integers as unsigned
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// with the ``x`` format specifier.
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class CustomArgFormatter :
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public fmt::BasicArgFormatter<CustomArgFormatter, char> {
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public:
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CustomArgFormatter(fmt::BasicFormatter<char, CustomArgFormatter> &f,
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fmt::FormatSpec &s, const char *fmt)
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: fmt::BasicArgFormatter<CustomArgFormatter, char>(f, s, fmt) {}
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void visit_int(int value) {
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if (spec().type() == 'x')
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visit_uint(value); // convert to unsigned and format
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else
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fmt::BasicArgFormatter<CustomArgFormatter, char>::visit_int(value);
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}
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};
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std::string custom_format(const char *format_str, fmt::ArgList args) {
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fmt::MemoryWriter writer;
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// Pass custom argument formatter as a template arg to BasicFormatter.
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fmt::BasicFormatter<char, CustomArgFormatter> formatter(args, writer);
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formatter.format(format_str);
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return writer.str();
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}
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FMT_VARIADIC(std::string, custom_format, const char *)
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std::string s = custom_format("{:x}", -42); // s == "ffffffd6"
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.. doxygenclass:: fmt::ArgVisitor
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:members:
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.. doxygenclass:: fmt::BasicArgFormatter
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:members:
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.. doxygenclass:: fmt::ArgFormatter
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:members:
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Printf formatting
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-----------------
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The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
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The following functions use `printf format string syntax
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<http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html>`_ with
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the POSIX extension for positional arguments. Unlike their standard
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counterparts, the ``fmt`` functions are type-safe and throw an exception if an
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argument type doesn't match its format specification.
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.. doxygenfunction:: printf(CStringRef, ArgList)
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.. doxygenfunction:: fprintf(std::FILE *, CStringRef, ArgList)
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.. doxygenfunction:: fprintf(std::ostream&, CStringRef, ArgList)
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.. doxygenfunction:: sprintf(CStringRef, ArgList)
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.. doxygenclass:: fmt::PrintfFormatter
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:members:
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.. doxygenclass:: fmt::BasicPrintfArgFormatter
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:members:
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.. doxygenclass:: fmt::PrintfArgFormatter
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:members:
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Write API
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=========
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The write API provides classes for writing formatted data into character
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streams. It is usually faster than the `format API`_ but, as IOStreams,
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may result in larger compiled code size. The main writer class is
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`~fmt::BasicMemoryWriter` which stores its output in a memory buffer and
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provides direct access to it. It is possible to create custom writers that
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store output elsewhere by subclassing `~fmt::BasicWriter`.
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.. doxygenclass:: fmt::BasicWriter
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:members:
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.. doxygenclass:: fmt::BasicMemoryWriter
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:members:
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.. doxygenclass:: fmt::BasicArrayWriter
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:members:
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.. doxygenclass:: fmt::BasicStringWriter
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:members:
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.. doxygenfunction:: bin(int)
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.. doxygenfunction:: oct(int)
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.. doxygenfunction:: hex(int)
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.. doxygenfunction:: hexu(int)
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.. doxygenfunction:: pad(int, unsigned, Char)
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Utilities
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=========
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.. doxygenfunction:: fmt::arg(StringRef, const T&)
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.. doxygenfunction:: operator""_a(const char *, std::size_t)
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.. doxygendefine:: FMT_CAPTURE
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.. doxygendefine:: FMT_VARIADIC
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.. doxygenclass:: fmt::ArgList
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:members:
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.. doxygenfunction:: fmt::to_string(const T&)
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.. doxygenclass:: fmt::BasicStringRef
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:members:
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.. doxygenclass:: fmt::BasicCStringRef
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:members:
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.. doxygenclass:: fmt::Buffer
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:protected-members:
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:members:
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System errors
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=============
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.. doxygenclass:: fmt::SystemError
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:members:
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.. doxygenfunction:: fmt::format_system_error
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.. doxygenclass:: fmt::WindowsError
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:members:
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.. _formatstrings:
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Custom allocators
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=================
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The fmt library supports custom dynamic memory allocators.
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A custom allocator class can be specified as a template argument to
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:class:`fmt::BasicMemoryWriter`::
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typedef fmt::BasicMemoryWriter<char, CustomAllocator> CustomMemoryWriter;
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It is also possible to write a formatting function that uses a custom
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allocator::
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typedef std::basic_string<char, std::char_traits<char>, CustomAllocator>
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CustomString;
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CustomString format(CustomAllocator alloc, fmt::CStringRef format_str,
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fmt::ArgList args) {
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CustomMemoryWriter writer(alloc);
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writer.write(format_str, args);
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return CustomString(writer.data(), writer.size(), alloc);
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}
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FMT_VARIADIC(CustomString, format, CustomAllocator, fmt::CStringRef)
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