blob: 9647c90b6ef6780dcdef0143e15e4c7b491b73d0 [file] [log] [blame]
.. _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
<>`_ 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
<>`_ 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
Date and time formatting
The library supports `strftime
<>`_-like date and time
#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 <>`_.
Formatting user-defined types
A custom ``format_arg`` function may be implemented and used to format any
user-defined type. That is how date and time formatting described in the
previous section is implemented in :file:`fmt/time.h`. The following example
shows how to implement custom formatting for a user-defined structure.
struct MyStruct { double a, b; };
void format_arg(fmt::BasicFormatter<char> &f,
const char *&format_str, const MyStruct &s) {
f.writer().write("[MyStruct: a={:.1f}, b={:.2f}]", s.a, s.b);
MyStruct m = { 1, 2 };
std::string s = fmt::format("m={}", n);
// s == "m=[MyStruct: a=1.0, b=2.00]"
Note in the example above the ``format_arg`` function ignores the contents of
``format_str`` so the type will always be formatted as specified. See
``format_arg`` in :file:`fmt/time.h` for an advanced example of how to use
the ``format_str`` argument to customize the formatted output.
This technique can also be used for formatting class hierarchies::
namespace local {
struct Parent {
Parent(int p) : p(p) {}
virtual void write(fmt::Writer &w) const {
w.write("Parent : p={}", p);
int p;
struct Child : Parent {
Child(int c, int p) : Parent(p), c(c) {}
virtual void write(fmt::Writer &w) const {
w.write("Child c={} : ", c);
int c;
void format_arg(fmt::BasicFormatter<char> &f,
const char *&format_str, const Parent &p) {
Local::Child c(1,2);
Local::Parent &p = c;
fmt::print("via ref to base: {}\n", p);
fmt::print("direct to child: {}\n", c);
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_;
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> {
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
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);
return writer.str();
FMT_VARIADIC(std::string, custom_format, const char *)
std::string s = custom_format("{:x}", -42); // s == "ffffffd6"
.. doxygenclass:: fmt::ArgVisitor
.. doxygenclass:: fmt::BasicArgFormatter
.. doxygenclass:: fmt::ArgFormatter
Printf formatting
The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
The following functions use `printf format string syntax
<>`_ 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
.. doxygenclass:: fmt::BasicPrintfArgFormatter
.. doxygenclass:: fmt::PrintfArgFormatter
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
.. doxygenclass:: fmt::BasicMemoryWriter
.. doxygenclass:: fmt::BasicArrayWriter
.. doxygenclass:: fmt::BasicStringWriter
.. doxygenclass:: fmt::BasicContainerWriter
.. doxygenfunction:: bin(int)
.. doxygenfunction:: oct(int)
.. doxygenfunction:: hex(int)
.. doxygenfunction:: hexu(int)
.. doxygenfunction:: pad(int, unsigned, Char)
.. doxygenfunction:: fmt::arg(StringRef, const T&)
.. doxygenfunction:: operator""_a(const char *, std::size_t)
.. doxygendefine:: FMT_CAPTURE
.. doxygendefine:: FMT_VARIADIC
.. doxygenclass:: fmt::ArgList
.. doxygenfunction:: fmt::to_string(const T&)
.. doxygenfunction:: fmt::to_wstring(const T&)
.. doxygenclass:: fmt::BasicStringRef
.. doxygenclass:: fmt::BasicCStringRef
.. doxygenclass:: fmt::Buffer
System errors
.. doxygenclass:: fmt::SystemError
.. doxygenfunction:: fmt::format_system_error
.. doxygenclass:: fmt::WindowsError
.. _formatstrings:
Custom allocators
The fmt library supports custom dynamic memory allocators.
A custom allocator class can be specified as a template argument to
typedef fmt::BasicMemoryWriter<char, CustomAllocator> CustomMemoryWriter;
It is also possible to write a formatting function that uses a custom
typedef std::basic_string<char, std::char_traits<char>, CustomAllocator>
CustomString format(CustomAllocator alloc, fmt::CStringRef format_str,
fmt::ArgList args) {
CustomMemoryWriter writer(alloc);
writer.write(format_str, args);
return CustomString(, writer.size(), alloc);
FMT_VARIADIC(CustomString, format, CustomAllocator, fmt::CStringRef)