C++ string formatting library.

Sean Middleditch [email protected]

formatxx is a modern C++ string formatting library. Its intended goals are to offer fast compilation times, minimal binary bloat, and reasonable speed. Final measurements of these goals are not yet available; the library is still in preliminary development.

The library supports writing primitive types as well as user-defined types into string formatting buffers. The libray has as little dependence on the C++ standard library as possible, which is intended to make it a very light header to include throughout a larger C++ project.

The included string writers allow for formatting into a std::string-compatible type, a fixed- size buffer with guaranteed no allocations, or a buffer that initially has a fixed-size buffer but can grow to accomodate larger strings. The combination of these buffers allow for easy use in three major cases: quick creation of std::string values, use in assert handlers that cannot allocate, and use in log systems where allocation should be avoided but is allowed when necessary. Users can easily write their own buffer systems as well.

The underlying method of operation of formatxx is to collect a list of arguments via variadic templates, lookup a format_value function for each of those arguments, and then pass the format string, an array of format functions, and an array of void pointers to arguments into the the actual formatting function. Builtin C++ types go through a slightly different mechanism for the sake of avoiding excessing ADL noise. The header mechanisms that generate these lists of functions and pointers are intended to be light-weight on the compiler to the extent reasonable. The actual formatting work is all implemented in a source file and not the header, to keep the header small and cheap to include.

formatxx can write into user-defined buffers, or a user may use one of the provided buffer types. Formatting is support for any type that has an appropriate format_value free function with the signature void format_value(formatxx::IWriter&, TheType, formatxx::format_options const&). For instance:

#include <formatxx/format.h>
#include <string>

struct Foo { int value };
	
void format_value(formatxx::writer& out, Foo const& foo, formatxx::format_options const& opts) {
	format_to(out, "Foo({})", foo.value);
}
	
int main() {
	std::cout << formatxx::format_as<std::string>("testing {0}", Foo{123});
}

The above will print testing Foo(123) to standard output.

The options argument are additional options passed to the formatter. These are parsed from format string options. In the format_to case user-provided arguments may be provided, otherwise they are interpreted by Python or printf rules.

The functions formatxx::parse_format_spec and formatxx::parse_printf_spec can be used to interpret format specs from string inputs.

The formatxx::format_as<ResultT>(string_view, ...) template can be used for formatting a series of arguments into any result type that implements a string-like append method. Including formatxx/std_string.h also provides a format_string function that defaults to returning std::string results.

The formatxx::format_to(formatxx::writer&, string_view, ...) template can be used to write into a write buffer. This is the recommended way of formatting.

The provided write buffers are:

• formatxx::append_writer<StringT> - writes to a string-like object using append.
• fmt::container_writer<ContainerT> - writes to a container using insert at the end.
• fmt::span_writer<CharT> - writes to a pre-allocated buffer.

All three of the provided write buffers guarantee NUL-terminated strings, but support use with string types that are not NUL-terminated (another important use case for formatxx).

The library that motivated this author to write formatxx is the excellent fmtlib (previously cppformat) by Victor Zverovich and contributors. fmtlib does much of what this library does, has excellent runtime speed, and responsive and talented authors. Unfortunately at the time the cppformat library failed to meet a few needs that formatxx seeks to address: it had relied on some heavy standard headers, its error handling mechanisms were and are limited to C++ exceptions or abort, the compilation time overhead was large, and its support for formatting user-defined types relied on complicated and expensive mechanisms like std::ostream wrappers. For those reasons, research into formatxx began.

The initial design did not use a format_value function for each type. Instead, an enumeration for the basic categories of primitives (bool, signed long, double, etc.) was computed via a template, values would be cast into their desired "native" representation, and that would be passed into the format function. Unfortunately, this added a lot of heavy template machinery to the header: the enum selection, a std::tuple for storing the converted inputs, etc. It also proved to be difficult to get good support for format_value functions for user-defined types with clean and concise error messages.

We also try to very heavily minimize header dependencies, especially on certain problematic C++ headers. For example, the core library tries to avoid including <string> due to that header's particularly heavy cost in some implementations. This likewise precludes formatxx from using other standard library types like std::string_view or so on. In some cases we try to avoid standard headers but fallback on including them for wide compatibility; for example, we attempt to use the common __builtin_strlen support on major compilers but will fallback to std::char_traits when necessary.

The current header relies on a function template wrapper around the real formatting functions. This is one template more than is desired that will lead to object file bloat, and for unoptimized debug builds essentially means that all format_value functions get an extra unnecessary "trampoline" that can be very heavyweight with many compilers' debug checks enabled. Ideally, these templates would be externed or converted away from wrappers. One solution would be to make the templates' functions for primitive types directly be the formatters for those types and then make format_value be the template wrapper (since it is expected to be more rarely used; it mostly exists to keep the same API between primitives and user-defined types for generic user code).

Each format_value is responsible currently for its own formatting and even its own format specifier parsing. This is not necessarily ideal and may change in the long run to standardize better for at least things like alignment and padding.

A particular limitation with printf-like features right now is that the argument array abstracts away integral arguments such that argument precision (e.g. %.*s) cannot be supported. A fix for this may be to bind a set of simple types instead of binding format functions directly to instead form a simple struct with unique identifiers for primitive types and only bind the format functions for user-provided types. This would allow the format code to parse out integral types efficiently, and possibly also better-optimize primitive formatting.

To the point possible, we use modern C++ and only work with recent compilers. In some cases, we're held back to slightly older compilers. We currently require C++17 support and so require recent Visual C++ 19.x (2015 SP 3 or later), Clang 3.9, and GCC 6.3

A final note is that floating point formatting currently bounces through snprintf which necessarily means that our floating point performance is slower and more complicated than calling the CRT directly. Writing a correct floating point formatter is incredibly complex. C++ now includes helpers like to_chars but it is not yet widely available on enough standard library implementations for us to rely on exclusively.

• Performance pass
  • Benchmarks
• Correctness to wide/unicode char support
  • u8/u16/u32?
  • maybe just remove?

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