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 use format string 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.

std::string fmt::format(CStringRef format_str, ArgList args)

Formats arguments and returns the result as a string.

Example:

std::string message = format("The answer is {}", 42);

internal::UdlFormat<char> fmt::literals::operator""_format(const char *s, std::size_t)

C++11 literal equivalent of fmt::format().

Example:

using namespace fmt::literals;
std::string message = "The answer is {}"_format(42);

void fmt::print(CStringRef format_str, ArgList args)

Prints formatted data to stdout.

Example:

print("Elapsed time: {0:.2f} seconds", 1.23);

void fmt::print(std::FILE *f, CStringRef format_str, ArgList args)

Prints formatted data to the file f.

Example:

print(stderr, "Don't {}!", "panic");

template <typename CharType, typename ArgFormatter>
class fmt::BasicFormatter

This template formats data and writes the output to a writer.

Public Types

typedef CharType Char

The character type for the output.

Public Functions

BasicFormatter(const ArgList &args, BasicWriter<Char> &w)

Constructs a BasicFormatter object. References to the arguments and the writer are stored in the formatter object so make sure they have appropriate lifetimes.

BasicWriter<Char> &writer()

Returns a reference to the writer associated with this formatter.

void format(BasicCStringRef<Char> format_str)

Formats stored arguments and writes the output to the writer.

Date and time formatting

The library supports 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.

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"
void fmt::print(std::ostream &os, CStringRef format_str, ArgList args)

Prints formatted data to the stream os.

Example:

print(cerr, "Don't {}!", "panic");

int fmt::fprintf(std::ostream &os, CStringRef format_str, ArgList args)

Prints formatted data to the stream os.

Example:

fprintf(cerr, "Don't %s!", "panic");

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"
template <typename Impl, typename Result>
class fmt::ArgVisitor

An argument visitor based on the curiously recurring template pattern.

To use ArgVisitor define a subclass that implements some or all of the visit methods with the same signatures as the methods in ArgVisitor, for example, visit_int(). Pass the subclass as the Impl template parameter. Then calling visit() for some argument will dispatch to a visit method specific to the argument type. For example, if the argument type is double then the visit_double() method of a subclass will be called. If the subclass doesn’t contain a method with this signature, then a corresponding method of ArgVisitor will be called.

Example:

class MyArgVisitor : public fmt::ArgVisitor<MyArgVisitor, void> {
 public:
  void visit_int(int value) { fmt::print("{}", value); }
  void visit_double(double value) { fmt::print("{}", value ); }
};

Public Functions

Result visit_int(int value)

Visits an int argument.

Result visit_long_long(LongLong value)

Visits a long long argument.

Result visit_uint(unsigned value)

Visits an unsigned argument.

Result visit_ulong_long(ULongLong value)

Visits an unsigned long long argument.

Result visit_bool(bool value)

Visits a bool argument.

Result visit_char(int value)

Visits a char or wchar_t argument.

template <typename T>
Result visit_any_int(T)

Visits an argument of any integral type.

Result visit_double(double value)

Visits a double argument.

Result visit_long_double(long double value)

Visits a long double argument.

template <typename T>
Result visit_any_double(T)

Visits a double or long double argument.

Result visit_cstring(const char *)

Visits a null-terminated C string (const char *) argument.

Result visit_string(Arg::StringValue<char>)

Visits a string argument.

Result visit_wstring(Arg::StringValue<wchar_t>)

Visits a wide string argument.

Result visit_pointer(const void *)

Visits a pointer argument.

Result visit_custom(Arg::CustomValue)

Visits an argument of a custom (user-defined) type.

Result visit(const Arg &arg)

Visits an argument dispatching to the appropriate visit method based on the argument type. For example, if the argument type is double then the visit_double() method of the Impl class will be called.

template <typename Impl, typename Char>
class fmt::BasicArgFormatter

An argument formatter based on the curiously recurring template pattern.

To use BasicArgFormatter define a subclass that implements some or all of the visit methods with the same signatures as the methods in ArgVisitor, for example, visit_int(). Pass the subclass as the Impl template parameter. When a formatting function processes an argument, it will dispatch to a visit method specific to the argument type. For example, if the argument type is double then the visit_double() method of a subclass will be called. If the subclass doesn’t contain a method with this signature, then a corresponding method of BasicArgFormatter or its superclass will be called.

Public Functions

BasicArgFormatter(BasicFormatter<Char, Impl> &formatter, FormatSpec &spec, const Char *fmt)

Constructs an argument formatter object. formatter is a reference to the main formatter object, spec contains format specifier information for standard argument types, and fmt points to the part of the format string being parsed for custom argument types.

void visit_custom(internal::Arg::CustomValue c)

Formats argument of a custom (user-defined) type.

template <typename Char>
class fmt::ArgFormatter

The default argument formatter.

Public Functions

ArgFormatter(BasicFormatter<Char> &formatter, FormatSpec &spec, const Char *fmt)

Constructs an argument formatter object.

Printf formatting functions

The following functions use printf format string syntax with a POSIX extension for positional arguments.

int fmt::printf(CStringRef format, ArgList args)

Prints formatted data to stdout.

Example:

fmt::printf("Elapsed time: %.2f seconds", 1.23);

int fmt::fprintf(std::FILE *f, CStringRef format, ArgList args)

Prints formatted data to the file f.

Example:

fmt::fprintf(stderr, "Don't %s!", "panic");

std::string fmt::sprintf(CStringRef format, ArgList args)

Formats arguments and returns the result as a string.

Example:

std::string message = fmt::sprintf("The answer is %d", 42);

Write API

template <typename Char>
class fmt::BasicWriter

This template provides operations for formatting and writing data into a character stream. The output is stored in a buffer provided by a subclass such as fmt::BasicMemoryWriter.

You can use one of the following typedefs for common character types:

Type Definition
Writer BasicWriter<char>
WWriter BasicWriter<wchar_t>

Public Functions

virtual ~BasicWriter()

Destroys a BasicWriter object.

std::size_t size() const

Returns the total number of characters written.

const Char *data() const

Returns a pointer to the output buffer content.

No terminating null character is appended.

const Char *c_str() const

Returns a pointer to the output buffer content with terminating null character appended.

std::basic_string<Char> str() const

Returns the content of the output buffer as an std::string.

void write(BasicCStringRef<Char> format, ArgList args)

Writes formatted data.

args is an argument list representing arbitrary arguments.

Example:

MemoryWriter out;
out.write("Current point:\n");
out.write("({:+f}, {:+f})", -3.14, 3.14);

This will write the following output to the out object:

Current point:
(-3.140000, +3.140000)

The output can be accessed using data(), c_str() or str() methods.

See also Format String Syntax.

BasicWriter &operator<<(ULongLong value)

Formats value and writes it to the stream.

BasicWriter &operator<<(long double value)

Formats value using the general format for floating-point numbers ('g') and writes it to the stream.

BasicWriter &operator<<(char value)

Writes a character to the stream.

BasicWriter &operator<<(fmt::BasicStringRef<Char> value)

Writes value to the stream.

template <typename Char, typename Allocator = std::allocator<Char>>
class fmt::BasicMemoryWriter

This class template provides operations for formatting and writing data into a character stream. The output is stored in a memory buffer that grows dynamically.

You can use one of the following typedefs for common character types and the standard allocator:

Type Definition
MemoryWriter BasicMemoryWriter<char, std::allocator<char>>
WMemoryWriter BasicMemoryWriter<wchar_t, std::allocator<wchar_t>>

Example:

MemoryWriter out;
out << "The answer is " << 42 << "\n";
out.write("({:+f}, {:+f})", -3.14, 3.14);

This will write the following output to the out object:

The answer is 42
(-3.140000, +3.140000)

The output can be converted to an std::string with out.str() or accessed as a C string with out.c_str().

Public Functions

BasicMemoryWriter(BasicMemoryWriter &&other)

Constructs a fmt::BasicMemoryWriter object moving the content of the other object to it.

BasicMemoryWriter &operator=(BasicMemoryWriter &&other)

Moves the content of the other BasicMemoryWriter object to this one.

template <typename Char>
class fmt::BasicArrayWriter

This class template provides operations for formatting and writing data into a fixed-size array. For writing into a dynamically growing buffer use fmt::BasicMemoryWriter.

Any write method will throw std::runtime_error if the output doesn’t fit into the array.

You can use one of the following typedefs for common character types:

Type Definition
ArrayWriter BasicArrayWriter<char>
WArrayWriter BasicArrayWriter<wchar_t>

Public Functions

BasicArrayWriter(Char *array, std::size_t size)

Constructs a fmt::BasicArrayWriter object for array of the given size.

template <std::size_t SIZE>
BasicArrayWriter(Char (&array)[SIZE])

Constructs a fmt::BasicArrayWriter object for array of the size known at compile time.

IntFormatSpec<int, TypeSpec<'b'>> fmt::bin(int value)

Returns an integer format specifier to format the value in base 2.

IntFormatSpec<int, TypeSpec<'o'>> fmt::oct(int value)

Returns an integer format specifier to format the value in base 8.

IntFormatSpec<int, TypeSpec<'x'>> fmt::hex(int value)

Returns an integer format specifier to format the value in base 16 using lower-case letters for the digits above 9.

IntFormatSpec<int, TypeSpec<'X'>> fmt::hexu(int value)

Returns an integer formatter format specifier to format in base 16 using upper-case letters for the digits above 9.

template <char TYPE_CODE, typename Char>
IntFormatSpec<int, AlignTypeSpec<TYPE_CODE>, Char> fmt::pad(int value, unsigned width, Char fill)

Returns an integer format specifier to pad the formatted argument with the fill character to the specified width using the default (right) numeric alignment.

Example:

MemoryWriter out;
out << pad(hex(0xcafe), 8, '0');
// out.str() == "0000cafe"

Utilities

template <typename T>
internal::NamedArg<char> fmt::arg(StringRef name, const T &arg)

Returns a named argument for formatting functions.

Example:

print("Elapsed time: {s:.2f} seconds", arg("s", 1.23));

internal::UdlArg<char> fmt::literals::operator""_a(const char *s, std::size_t)

C++11 literal equivalent of fmt::arg().

Example:

using namespace fmt::literals;
print("Elapsed time: {s:.2f} seconds", "s"_a=1.23);

FMT_CAPTURE(...)

Convenient macro to capture the arguments’ names and values into several fmt::arg(name, value).

Example:

int x = 1, y = 2;
print("point: ({x}, {y})", FMT_CAPTURE(x, y));
// same as:
// print("point: ({x}, {y})", arg("x", x), arg("y", y));

FMT_VARIADIC(ReturnType, func, ...)

Defines a variadic function with the specified return type, function name and argument types passed as variable arguments to this macro.

Example:

void print_error(const char *file, int line, const char *format,
                 fmt::ArgList args) {
  fmt::print("{}: {}: ", file, line);
  fmt::print(format, args);
}
FMT_VARIADIC(void, print_error, const char *, int, const char *)

FMT_VARIADIC is used for compatibility with legacy C++ compilers that don’t implement variadic templates. You don’t have to use this macro if you don’t need legacy compiler support and can use variadic templates directly:

template <typename... Args>
void print_error(const char *file, int line, const char *format,
                 const Args & ... args) {
  fmt::print("{}: {}: ", file, line);
  fmt::print(format, args...);
}

class fmt::ArgList

An argument list.

Public Functions

internal::Arg operator[](unsigned index) const

Returns the argument at specified index.

template <typename Char>
class fmt::BasicStringRef

A string reference. It can be constructed from a C string or std::string.

You can use one of the following typedefs for common character types:

Type Definition
StringRef BasicStringRef<char>
WStringRef BasicStringRef<wchar_t>

This class is most useful as a parameter type to allow passing different types of strings to a function, for example:

template <typename... Args>
std::string format(StringRef format_str, const Args & ... args);

format("{}", 42);
format(std::string("{}"), 42);

Public Functions

BasicStringRef(const Char *s, std::size_t size)

Constructs a string reference object from a C string and a size.

BasicStringRef(const Char *s)

Constructs a string reference object from a C string computing the size with std::char_traits<Char>::length.

BasicStringRef(const std::basic_string<Char> &s)

Constructs a string reference from an std::string object.

std::basic_string<Char> to_string() const

Converts a string reference to an std::string object.

const Char *data() const

Returns a pointer to the string data.

std::size_t size() const

Returns the string size.

template <typename Char>
class fmt::BasicCStringRef

A reference to a null terminated string. It can be constructed from a C string or std::string.

You can use one of the following typedefs for common character types:

Type Definition
CStringRef BasicCStringRef<char>
WCStringRef BasicCStringRef<wchar_t>

This class is most useful as a parameter type to allow passing different types of strings to a function, for example:

template <typename... Args>
std::string format(CStringRef format_str, const Args & ... args);

format("{}", 42);
format(std::string("{}"), 42);

Public Functions

BasicCStringRef(const Char *s)

Constructs a string reference object from a C string.

BasicCStringRef(const std::basic_string<Char> &s)

Constructs a string reference from an std::string object.

const Char *c_str() const

Returns the pointer to a C string.

template <typename T>
class fmt::Buffer

A buffer supporting a subset of std::vector‘s operations.

Public Functions

std::size_t size() const

Returns the size of this buffer.

std::size_t capacity() const

Returns the capacity of this buffer.

void resize(std::size_t new_size)

Resizes the buffer.

If T is a POD type new elements may not be initialized.

void reserve(std::size_t capacity)

Reserves space to store at least capacity elements.

template <typename U>
void append(const U *begin, const U *end)

Appends data to the end of the buffer.

Protected Functions

virtual void grow(std::size_t size) = 0

Increases the buffer capacity to hold at least size elements updating ptr_ and capacity_.

System errors

class fmt::SystemError

An error returned by an operating system or a language runtime, for example a file opening error.

Public Functions

SystemError(int error_code, CStringRef message)

Constructs a fmt::SystemError object with the description of the form

<message>: <system-message>

where <message> is the formatted message and <system-message> is the system message corresponding to the error code. error_code is a system error code as given by errno. If error_code is not a valid error code such as -1, the system message may look like “Unknown error -1” and is platform-dependent.

Example:

// This throws a SystemError with the description
//   cannot open file 'madeup': No such file or directory
// or similar (system message may vary).
const char *filename = "madeup";
std::FILE *file = std::fopen(filename, "r");
if (!file)
  throw fmt::SystemError(errno, "cannot open file '{}'", filename);

class fmt::WindowsError

A Windows error.

Public Functions

WindowsError(int error_code, CStringRef message)

Constructs a fmt::WindowsError object with the description of the form

<message>: <system-message>

where <message> is the formatted message and <system-message> is the system message corresponding to the error code. error_code is a Windows error code as given by GetLastError. If error_code is not a valid error code such as -1, the system message will look like “error -1”.

Example:

// This throws a WindowsError with the description
//   cannot open file 'madeup': The system cannot find the file specified.
// or similar (system message may vary).
const char *filename = "madeup";
LPOFSTRUCT of = LPOFSTRUCT();
HFILE file = OpenFile(filename, &of, OF_READ);
if (file == HFILE_ERROR) {
  throw fmt::WindowsError(GetLastError(),
                          "cannot open file '{}'", filename);
}

Custom allocators

The fmt library supports custom dynamic memory allocators. A custom allocator class can be specified as a template argument to 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)