Overview¶
fmt (formerly cppformat) is an open-source formatting library. It can be used as a fast and safe alternative to printf and IOStreams.
Format API¶
The replacement-based Format API provides a safe alternative to printf
,
sprintf
and friends with comparable or better performance.
The format string syntax is similar to the one used by
str.format
in Python:
fmt::format("The answer is {}.", 42);
The fmt::format
function returns a string “The answer is 42.”. You can use
fmt::memory_buffer
to avoid constructing std::string
:
fmt::memory_buffer out;
format_to(out, "For a moment, {} happened.", "nothing");
out.data(); // returns a pointer to the formatted data
The fmt::print
function performs formatting and writes the result to a file:
fmt::print(stderr, "System error code = {}\n", errno);
The file argument can be omitted in which case the function prints to
stdout
:
fmt::print("Don't {}\n", "panic");
The Format API also supports positional arguments useful for localization:
fmt::print("I'd rather be {1} than {0}.", "right", "happy");
Named arguments can be created with fmt::arg
. This makes it easier to track
what goes where when multiple values are being inserted:
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
fmt::arg("name", "World"), fmt::arg("number", 42));
If your compiler supports C++11 user-defined literals, the suffix _a
offers
an alternative, slightly terser syntax for named arguments:
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
"name"_a="World", "number"_a=42);
The _format
suffix may be used to format string literals similar to Python:
std::string message = "{0}{1}{0}"_format("abra", "cad");
Other than the placement of the format string on the left of the operator,
_format
is functionally identical to fmt::format
. In order to use the
literal operators, they must be made visible with the directive
using namespace fmt::literals;
. Note that this brings in only _a
and
_format
but nothing else from the fmt
namespace.
Safety¶
The library is fully type safe, automatic memory management prevents buffer overflow, errors in format strings are reported using exceptions or at compile tim. For example, the code
fmt::format("The answer is {:d}", "forty-two");
throws a format_error
exception with description “unknown format code ‘d’ for
string”, because the argument "forty-two"
is a string while the format code
d
only applies to integers, while
format(fmt("The answer is {:d}"), "forty-two");
reports a compile-time error for the same reason on compilers that support
relaxed constexpr
.
The following code
fmt::format("Cyrillic letter {}", L'\x42e');
produces a compile-time error because wide character L'\x42e'
cannot be
formatted into a narrow string. You can use a wide format string instead:
fmt::format(L"Cyrillic letter {}", L'\x42e');
For comparison, writing a wide character to std::ostream
results in
its numeric value being written to the stream (i.e. 1070 instead of letter ‘ю’
which is represented by L'\x42e'
if we use Unicode) which is rarely what is
needed.
Compact binary code¶
The library is designed to produce compact per-call compiled code. For example (godbolt),
#include <fmt/core.h>
int main() {
fmt::print("The answer is {}.", 42);
}
compiles to just
main: # @main
sub rsp, 24
mov qword ptr [rsp], 42
mov rcx, rsp
mov edi, offset .L.str
mov esi, 17
mov edx, 2
call fmt::v5::vprint(fmt::v5::basic_string_view<char>, fmt::v5::format_args)
xor eax, eax
add rsp, 24
ret
.L.str:
.asciz "The answer is {}."
Portability¶
The library is highly portable and relies only on a small set of C++11 features:
variadic templates
type traits
rvalue references
decltype
trailing return types
deleted functions
These are available since GCC 4.4, Clang 2.9 and MSVC 18.0 (2013). For older compilers use fmt version 4.x which continues to be maintained and only requires C++98.
The output of all formatting functions is consistent across platforms. In
particular, formatting a floating-point infinity always gives inf
while the
output of printf
is platform-dependent in this case. For example,
fmt::print("{}", std::numeric_limits<double>::infinity());
always prints inf
.
Ease of Use¶
fmt has a small self-contained code base with the core library consisting of just three header files and no external dependencies. A permissive BSD license allows using the library both in open-source and commercial projects.
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