Text Formatting

2016-08-19

Introduction
Design
    Format String Syntax
    Locale Support
Wording
References

Introduction

This paper proposes a new text formatting functionality that can be used as a safe and extensible alternative to the printf family of functions. It is intended to complement the existing C++ I/O streams library and reuse some of its infrastructure such as overloaded insertion operators for user-defined types.

Example:

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

Design

Format String Syntax

Variations of the printf format string syntax are arguably the most popular among the programming languages and C++ itself inherits printf from C [1]. The advantage of the printf syntax is that many programmers are familiar with it. However, in its current form it has a number of issues:

• Many format specifiers like hh, h, l, j, etc. are used only to convey type information. They are redundant in type-safe formatting and would unnecessarily complicate specification and parsing.
• There is no standard way to extend the syntax for user-defined types.
• There are subtle differences between different implementations. For example, POSIX positional arguments [2] are not supported on MSVC.
• Using '%' in a custom format specifier, e.g. for put_time-like time formatting, poses difficulties.

Although it is possible to address these issues, this will break compatibility and can potentially be more confusing to users than introducing a different syntax.

• Consistent and easy to parse mini-language focused on formatting rather than conveying type information
• Extensibility and support for custom format strings for user-defined types
• Positional arguments
• Support for both locale-specific and locale-independent formatting (see Locale Support)
• Minor formatting improvements such as center alignment and binary format

The syntax is expressive enough to enable translation, possibly automated, of most printf format strings. TODO: table of correspondence between printf and the new syntax

Locale Support

TODO

Wording

TODO

References

Implementation

The ideas proposed in this paper have been implemented in the open-source fmt library. TODO: link

[1] The fprintf function. ISO/IEC 9899:2011. 7.21.6.1.
[2] fprintf, printf, snprintf, sprintf - print formatted output. The Open Group Base Specifications Issue 6 IEEE Std 1003.1, 2004 Edition.
[3] 6.1.3. Format String Syntax. Python 3.5.2 documentation.
[4] String.Format Method. .NET Framework Class Library.
[5] Module std::fmt. The Rust Standard Library.