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435 lines
13 KiB
ReStructuredText
435 lines
13 KiB
ReStructuredText
.. _string-formatting-api:
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*************
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API Reference
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*************
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The {fmt} library API consists of the following parts:
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* :ref:`fmt/core.h <core-api>`: the core API providing argument handling
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facilities and a lightweight subset of formatting functions
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* :ref:`fmt/format.h <format-api>`: the full format API providing compile-time
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format string checks, output iterator and user-defined type support
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* :ref:`fmt/ranges.h <ranges-api>`: additional formatting support for ranges
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and tuples
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* :ref:`fmt/chrono.h <chrono-api>`: date and time formatting
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* :ref:`fmt/ostream.h <ostream-api>`: ``std::ostream`` support
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* :ref:`fmt/printf.h <printf-api>`: ``printf`` formatting
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All functions and types provided by the library reside in namespace ``fmt`` and
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macros have prefix ``FMT_``.
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.. _core-api:
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Core API
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========
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``fmt/core.h`` defines the core API which provides argument handling facilities
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and a lightweight subset of formatting functions. In the header-only mode
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include ``fmt/format.h`` instead of ``fmt/core.h``.
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The following functions use :ref:`format string syntax <syntax>`
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similar to that of Python's `str.format
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<http://docs.python.org/3/library/stdtypes.html#str.format>`_.
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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. A function taking *format_str* doesn't
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participate in an overload resolution if the latter is not a string.
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*args* is an argument list representing objects to be formatted.
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.. _format:
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.. doxygenfunction:: format(const S&, Args&&...)
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.. doxygenfunction:: vformat(const S&, basic_format_args<buffer_context<Char>>)
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.. _print:
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.. doxygenfunction:: print(const S&, Args&&...)
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.. doxygenfunction:: vprint(string_view, format_args)
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.. doxygenfunction:: print(std::FILE *, const S&, Args&&...)
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.. doxygenfunction:: vprint(std::FILE *, string_view, format_args)
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.. doxygenfunction:: vprint(std::FILE *, wstring_view, wformat_args)
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Named Arguments
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---------------
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.. doxygenfunction:: fmt::arg(const S&, const T&)
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Named arguments are not supported in compile-time checks at the moment.
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Argument Lists
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--------------
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.. doxygenfunction:: fmt::make_format_args(const Args&...)
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.. doxygenclass:: fmt::format_arg_store
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:members:
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.. doxygenclass:: fmt::basic_format_args
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:members:
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.. doxygenstruct:: fmt::format_args
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.. doxygenclass:: fmt::basic_format_arg
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:members:
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Compatibility
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-------------
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.. doxygenclass:: fmt::basic_string_view
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:members:
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.. doxygentypedef:: fmt::string_view
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.. doxygentypedef:: fmt::wstring_view
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.. _format-api:
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Format API
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==========
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``fmt/format.h`` defines the full format API providing compile-time format
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string checks, output iterator and user-defined type support.
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Compile-time Format String Checks
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---------------------------------
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.. doxygendefine:: FMT_STRING
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Formatting User-defined Types
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-----------------------------
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To make a user-defined type formattable, specialize the ``formatter<T>`` struct
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template and implement ``parse`` and ``format`` methods::
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#include <fmt/format.h>
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struct point { double x, y; };
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template <>
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struct fmt::formatter<point> {
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// Presentation format: 'f' - fixed, 'e' - exponential.
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char presentation = 'f';
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// Parses format specifications of the form ['f' | 'e'].
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constexpr auto parse(format_parse_context& ctx) {
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// [ctx.begin(), ctx.end()) is a character range that contains a part of
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// the format string starting from the format specifications to be parsed,
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// e.g. in
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//
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// fmt::format("{:f} - point of interest", point{1, 2});
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//
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// the range will contain "f} - point of interest". The formatter should
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// parse specifiers until '}' or the end of the range. In this example
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// the formatter should parse the 'f' specifier and return an iterator
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// pointing to '}'.
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// Parse the presentation format and store it in the formatter:
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auto it = ctx.begin(), end = ctx.end();
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if (it != end && (*it == 'f' || *it == 'e')) presentation = *it++;
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// Check if reached the end of the range:
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if (it != end && *it != '}')
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throw format_error("invalid format");
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// Return an iterator past the end of the parsed range:
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return it;
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}
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// Formats the point p using the parsed format specification (presentation)
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// stored in this formatter.
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template <typename FormatContext>
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auto format(const point& p, FormatContext& ctx) {
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// ctx.out() is an output iterator to write to.
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return format_to(
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ctx.out(),
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presentation == 'f' ? "({:.1f}, {:.1f})" : "({:.1e}, {:.1e})",
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p.x, p.y);
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}
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};
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Then you can pass objects of type ``point`` to any formatting function::
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point p = {1, 2};
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std::string s = fmt::format("{:f}", p);
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// s == "(1.0, 2.0)"
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You can also reuse existing formatters via inheritance or composition, for
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example::
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enum class color {red, green, blue};
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template <>
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struct fmt::formatter<color>: formatter<string_view> {
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// parse is inherited from formatter<string_view>.
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template <typename FormatContext>
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auto format(color c, FormatContext& ctx) {
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string_view name = "unknown";
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switch (c) {
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case color::red: name = "red"; break;
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case color::green: name = "green"; break;
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case color::blue: name = "blue"; break;
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}
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return formatter<string_view>::format(name, ctx);
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}
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};
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You can also write a formatter for a hierarchy of classes::
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#include <type_traits>
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#include <fmt/format.h>
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struct A {
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virtual ~A() {}
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virtual std::string name() const { return "A"; }
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};
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struct B : A {
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virtual std::string name() const { return "B"; }
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};
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template <typename T>
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struct fmt::formatter<T, std::enable_if_t<std::is_base_of<A, T>::value, char>> :
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fmt::formatter<std::string> {
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template <typename FormatCtx>
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auto format(const A& a, FormatCtx& ctx) {
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return fmt::formatter<std::string>::format(a.name(), ctx);
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}
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};
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int main() {
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B b;
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A& a = b;
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fmt::print("{}", a); // prints "B"
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}
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.. doxygenclass:: fmt::basic_format_parse_context
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:members:
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Output Iterator Support
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-----------------------
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.. doxygenfunction:: fmt::format_to(OutputIt, const S&, Args&&...)
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.. doxygenfunction:: fmt::format_to_n(OutputIt, std::size_t, string_view, Args&&...)
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.. doxygenstruct:: fmt::format_to_n_result
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:members:
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Literal-based API
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-----------------
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The following user-defined literals are defined in ``fmt/format.h``.
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.. doxygenfunction:: operator""_format(const char *, std::size_t)
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.. doxygenfunction:: operator""_a(const char *, std::size_t)
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Utilities
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---------
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.. doxygenstruct:: fmt::is_char
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.. doxygentypedef:: fmt::char_t
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.. doxygenfunction:: fmt::formatted_size(string_view, const Args&...)
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.. doxygenfunction:: fmt::to_string(const T&)
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.. doxygenfunction:: fmt::to_wstring(const T&)
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.. doxygenfunction:: fmt::to_string_view(const Char *)
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.. doxygenfunction:: fmt::join(const Range&, string_view)
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.. doxygenfunction:: fmt::join(It, It, string_view)
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.. doxygenclass:: fmt::basic_memory_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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fmt does not use ``errno`` to communicate errors to the user, but it may call
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system functions which set ``errno``. Users should not make any assumptions about
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the value of ``errno`` being preserved by library functions.
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.. doxygenclass:: fmt::system_error
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:members:
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.. doxygenfunction:: fmt::format_system_error
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.. doxygenclass:: fmt::windows_error
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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::basic_memory_buffer`::
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using custom_memory_buffer =
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fmt::basic_memory_buffer<char, fmt::inline_buffer_size, custom_allocator>;
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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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using custom_string =
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std::basic_string<char, std::char_traits<char>, custom_allocator>;
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custom_string vformat(custom_allocator alloc, fmt::string_view format_str,
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fmt::format_args args) {
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custom_memory_buffer buf(alloc);
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fmt::vformat_to(buf, format_str, args);
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return custom_string(buf.data(), buf.size(), alloc);
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}
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template <typename ...Args>
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inline custom_string format(custom_allocator alloc,
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fmt::string_view format_str,
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const Args& ... args) {
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return vformat(alloc, format_str, fmt::make_format_args(args...));
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}
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The allocator will be used for the output container only. If you are using named
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arguments, the container that stores pointers to them will be allocated using
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the default allocator. Also floating-point formatting falls back on ``sprintf``
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which may do allocations.
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Custom Formatting of Built-in Types
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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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using arg_formatter = fmt::arg_formatter<fmt::buffer_range<char>>;
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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 custom_arg_formatter : public arg_formatter {
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public:
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custom_arg_formatter(fmt::format_context& ctx,
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fmt::format_parse_context* parse_ctx = nullptr,
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fmt::format_specs* spec = nullptr)
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: arg_formatter(ctx, parse_ctx, spec) {}
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using arg_formatter::operator();
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auto operator()(int value) {
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if (specs() && specs()->type == 'x')
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return (*this)(static_cast<unsigned>(value)); // convert to unsigned and format
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return arg_formatter::operator()(value);
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}
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};
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std::string custom_vformat(fmt::string_view format_str, fmt::format_args args) {
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fmt::memory_buffer buffer;
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// Pass custom argument formatter as a template arg to vformat_to.
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fmt::vformat_to<custom_arg_formatter>(buffer, format_str, args);
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return fmt::to_string(buffer);
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}
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template <typename ...Args>
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inline std::string custom_format(
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fmt::string_view format_str, const Args&... args) {
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return custom_vformat(format_str, fmt::make_format_args(args...));
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}
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std::string s = custom_format("{:x}", -42); // s == "ffffffd6"
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.. doxygenclass:: fmt::arg_formatter
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:members:
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.. _ranges-api:
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Ranges and Tuple Formatting
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===========================
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The library also supports convenient formatting of ranges and tuples::
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#include <fmt/ranges.h>
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std::tuple<char, int, float> t{'a', 1, 2.0f};
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// Prints "('a', 1, 2.0)"
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fmt::print("{}", t);
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NOTE: currently, the overload of ``fmt::join`` for iterables exists in the main
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``format.h`` header, but expect this to change in the future.
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Using ``fmt::join``, you can separate tuple elements with a custom separator::
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#include <fmt/ranges.h>
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std::tuple<int, char> t = {1, 'a'};
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// Prints "1, a"
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fmt::print("{}", fmt::join(t, ", "));
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.. _chrono-api:
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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/chrono.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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.. _ostream-api:
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``std::ostream`` Support
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========================
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``fmt/ostream.h`` provides ``std::ostream`` support including formatting of
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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::basic_ostream<Char>&, const S&, Args&&...)
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.. _printf-api:
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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(const S&, const Args&...)
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.. doxygenfunction:: fprintf(std::FILE *, const S&, const Args&...)
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.. doxygenfunction:: fprintf(std::basic_ostream<Char>&, const S&, const Args&...)
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.. doxygenfunction:: sprintf(const S&, const Args&...)
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