mirror of
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564 lines
18 KiB
ReStructuredText
564 lines
18 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 main formatting functions
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for ``char``/UTF-8 with compile-time checks and minimal dependencies
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* :ref:`fmt/format.h <format-api>`: the full format API providing additional
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formatting functions and locale support
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* :ref:`fmt/ranges.h <ranges-api>`: formatting of ranges and tuples
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* :ref:`fmt/chrono.h <chrono-api>`: date and time formatting
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* :ref:`fmt/compile.h <compile-api>`: format string compilation
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* :ref:`fmt/color.h <color-api>`: terminal color and text style
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* :ref:`fmt/os.h <os-api>`: system APIs
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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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* :ref:`fmt/xchar.h <xchar-api>`: optional ``wchar_t`` support
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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 main formatting functions for
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``char``/UTF-8 with compile-time checks. It has minimal include dependencies for
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better compile times. This header is only beneficial when using {fmt} as a
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library and not in the header-only mode.
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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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<https://docs.python.org/3/library/stdtypes.html#str.format>`_.
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They take *fmt* and *args* as arguments.
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*fmt* is a format string that contains literal text and replacement fields
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surrounded by braces ``{}``. The fields are replaced with formatted arguments
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in the resulting string. `~fmt::format_string` is a format string which can be
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implicitly constructed from a string literal or a ``constexpr`` string and is
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checked at compile time in C++20. To pass a runtime format string wrap it in
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`fmt::runtime`.
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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(format_string<T...> fmt, T&&... args) -> std::string
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.. doxygenfunction:: vformat(string_view fmt, format_args args) -> std::string
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.. doxygenfunction:: format_to(OutputIt out, format_string<T...> fmt, T&&... args) -> OutputIt
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.. doxygenfunction:: format_to_n(OutputIt out, size_t n, format_string<T...> fmt, T&&... args) -> format_to_n_result<OutputIt>
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.. doxygenfunction:: formatted_size(format_string<T...> fmt, T&&... args) -> size_t
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.. doxygenstruct:: fmt::format_to_n_result
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:members:
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.. _print:
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.. doxygenfunction:: fmt::print(format_string<T...> fmt, T&&... args)
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.. doxygenfunction:: fmt::vprint(string_view fmt, format_args args)
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.. doxygenfunction:: print(std::FILE *f, format_string<T...> fmt, T&&... args)
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.. doxygenfunction:: vprint(std::FILE *f, string_view fmt, format_args args)
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Compile-time Format String Checks
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---------------------------------
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Compile-time checks are enabled when using ``FMT_STRING``. They support built-in
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and string types as well as user-defined types with ``constexpr`` ``parse``
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functions in their ``formatter`` specializations.
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Requires C++14 and is a no-op in C++11.
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.. doxygendefine:: FMT_STRING
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To force the use of compile-time checks, define the preprocessor variable
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``FMT_ENFORCE_COMPILE_STRING``. When set, functions accepting ``FMT_STRING``
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will fail to compile with regular strings. Runtime-checked
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formatting is still possible using ``fmt::vformat``, ``fmt::vprint``, etc.
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.. doxygenclass:: fmt::basic_format_string
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:members:
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.. doxygentypedef:: fmt::format_string
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.. doxygenfunction:: fmt::runtime(const S&)
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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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You can create your own formatting function with compile-time checks and small
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binary footprint, for example (https://godbolt.org/z/oba4Mc):
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.. code:: c++
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#include <fmt/format.h>
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void vlog(const char* file, int line, fmt::string_view format,
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fmt::format_args args) {
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fmt::print("{}: {}: ", file, line);
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fmt::vprint(format, args);
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}
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template <typename S, typename... Args>
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void log(const char* file, int line, const S& format, Args&&... args) {
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vlog(file, line, format,
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fmt::make_args_checked<Args...>(format, args...));
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}
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#define MY_LOG(format, ...) \
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log(__FILE__, __LINE__, FMT_STRING(format), __VA_ARGS__)
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MY_LOG("invalid squishiness: {}", 42);
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Note that ``vlog`` is not parameterized on argument types which improves compile
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times and reduces binary code size compared to a fully parameterized version.
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.. doxygenfunction:: fmt::make_args_checked(const S&, const remove_reference_t<Args>&...)
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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::dynamic_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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.. doxygentypedef:: fmt::format_args
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.. doxygenclass:: fmt::basic_format_arg
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:members:
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.. doxygenclass:: fmt::basic_format_context
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:members:
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.. doxygentypedef:: fmt::format_context
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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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Locale
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------
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All formatting is locale-independent by default. Use the ``'L'`` format
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specifier to insert the appropriate number separator characters from the
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locale::
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#include <fmt/core.h>
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#include <locale>
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std::locale::global(std::locale("en_US.UTF-8"));
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auto s = fmt::format("{:L}", 1000000); // s == "1,000,000"
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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 additional formatting
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functions and locale support.
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.. _udt:
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Formatting User-defined Types
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-----------------------------
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The {fmt} library provides formatters for many standard C++ types.
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See :ref:`fmt/ranges.h <ranges-api>` for ranges and tuples including standard
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containers such as ``std::vector`` and :ref:`fmt/chrono.h <chrono-api>` for date
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and time formatting.
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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 {
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double x, y;
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};
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template <> 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) -> decltype(ctx.begin()) {
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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 != '}') 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) -> decltype(ctx.out()) {
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// ctx.out() is an output iterator to write to.
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return presentation == 'f'
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? format_to(ctx.out(), "({:.1f}, {:.1f})", p.x, p.y)
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: format_to(ctx.out(), "({:.1e}, {:.1e})", 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 <> 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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Since ``parse`` is inherited from ``formatter<string_view>`` it will recognize
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all string format specifications, for example
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.. code-block:: c++
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fmt::format("{:>10}", color::blue)
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will return ``" blue"``.
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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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If a type provides both a ``formatter`` specialization and an implicit
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conversion to a formattable type, the specialization takes precedence over the
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conversion.
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.. doxygenclass:: fmt::basic_format_parse_context
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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 *s, size_t n) -> detail::udl_formatter<char>
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.. doxygenfunction:: operator""_a(const char *s, size_t) -> detail::udl_arg<char>
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Utilities
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---------
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.. doxygenfunction:: fmt::ptr(T p) -> const void*
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.. doxygenfunction:: fmt::ptr(const std::unique_ptr<T> &p) -> const void*
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.. doxygenfunction:: fmt::ptr(const std::shared_ptr<T> &p) -> const void*
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.. doxygenfunction:: fmt::underlying(Enum e) -> typename std::underlying_type<Enum>::type
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.. doxygenfunction:: fmt::to_string(const T &value) -> std::string
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.. doxygenfunction:: fmt::to_string_view(const Char *s) -> basic_string_view<Char>
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.. doxygenfunction:: fmt::join(Range &&range, string_view sep) -> join_view<detail::iterator_t<Range>, detail::sentinel_t<Range>>
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.. doxygenfunction:: fmt::join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel>
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.. doxygenfunction:: fmt::group_digits(T value) -> group_digits_view<T>
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.. doxygenclass:: fmt::detail::buffer
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:members:
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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
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about the value of ``errno`` being preserved by library functions.
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.. doxygenfunction:: fmt::system_error
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.. doxygenfunction:: fmt::format_system_error
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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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auto buf = custom_memory_buffer(alloc);
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fmt::vformat_to(std::back_inserter(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. Formatting functions
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normally don't do any allocations for built-in and string types except for
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non-default floating-point formatting that occasionally falls back on
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``sprintf``.
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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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``fmt/chrono.h`` provides formatters for
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* `std::chrono::duration <https://en.cppreference.com/w/cpp/chrono/duration>`_
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* `std::chrono::time_point
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<https://en.cppreference.com/w/cpp/chrono/time_point>`_
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* `std::tm <https://en.cppreference.com/w/cpp/chrono/c/tm>`_
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The format syntax is described in :ref:`chrono-specs`.
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**Example**::
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#include <fmt/chrono.h>
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int main() {
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std::time_t t = std::time(nullptr);
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// Prints "The date is 2020-11-07." (with the current date):
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fmt::print("The date is {:%Y-%m-%d}.", fmt::localtime(t));
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using namespace std::literals::chrono_literals;
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// Prints "Default format: 42s 100ms":
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fmt::print("Default format: {} {}\n", 42s, 100ms);
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// Prints "strftime-like format: 03:15:30":
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fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);
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}
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.. doxygenfunction:: localtime(std::time_t time)
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.. doxygenfunction:: gmtime(std::time_t time)
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.. _compile-api:
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Format string compilation
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=========================
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``fmt/compile.h`` provides format string compilation support when using
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``FMT_COMPILE``. Format strings are parsed, checked and converted into efficient
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formatting code at compile-time. This supports arguments of built-in and string
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types as well as user-defined types with ``constexpr`` ``parse`` functions in
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their ``formatter`` specializations. Format string compilation can generate more
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binary code compared to the default API and is only recommended in places where
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formatting is a performance bottleneck.
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.. doxygendefine:: FMT_COMPILE
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.. _color-api:
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Terminal color and text style
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=============================
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``fmt/color.h`` provides support for terminal color and text style output.
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.. doxygenfunction:: print(const text_style &ts, const S &format_str, const Args&... args)
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.. doxygenfunction:: fg(detail::color_type)
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.. doxygenfunction:: bg(detail::color_type)
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.. _os-api:
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System APIs
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===========
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.. doxygenclass:: fmt::ostream
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:members:
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.. doxygenfunction:: fmt::windows_error
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:members:
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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 an overloaded insertion operator (``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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{fmt} only supports insertion operators that are defined in the same namespaces
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as the types they format and can be found with the argument-dependent lookup.
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.. doxygenfunction:: print(std::basic_ostream<Char> &os, const S &format_str, Args&&... 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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<https://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 &format_str, const T&... args)
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.. doxygenfunction:: fprintf(std::FILE *f, const S &fmt, const T&... args) -> int
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.. doxygenfunction:: sprintf(const S&, const T&...)
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.. _xchar-api:
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``wchar_t`` Support
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===================
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The optional header ``fmt/xchar.h`` provides support for ``wchar_t`` and exotic
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character types.
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.. doxygenstruct:: fmt::is_char
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.. doxygentypedef:: fmt::wstring_view
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.. doxygentypedef:: fmt::wformat_context
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.. doxygenfunction:: fmt::to_wstring(const T &value)
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Compatibility with C++20 ``std::format``
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========================================
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{fmt} implements nearly all of the `C++20 formatting library
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<https://en.cppreference.com/w/cpp/utility/format>`_ with the following
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differences:
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* Names are defined in the ``fmt`` namespace instead of ``std`` to avoid
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collisions with standard library implementations.
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* Width calculation doesn't use grapheme clusterization. The latter has been
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implemented in a separate branch but hasn't been integrated yet.
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* Most C++20 chrono types are not supported yet.
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