bslfmt_format_string

Detailed Description

Provide a string_view wrapper for formatting library usage.

Outline

• Purpose
• Classes
• Canonical Header
• Description
  • Usage
    • Example: Own format function

Purpose

Provide a string_view wrapper for formatting library usage

Classes

• bslfmt::basic_format_string: formatting library string_view wrapper

Canonical Header

bsl_format.h

Description

This component provides an implementation of the C++20 Standard Library's basic_format_string , providing wrapper around string_view for format specification strings.

Using this rather than string_view directly serves three main purposes:

• We ensure that the interface to format and vformat are consistent with the standard library, which takes a format_string rather than a string_view .
• Under C++20 we can enforce that the source string used for construction is a compile-time constant.
• Under C++17 and earlier, by limiting construction to a const t_CHAR* we can limit the number of scenarios which would fail to compile under C++20 but succeed under earlier versions.
• It is possible to perform compile-time checking of the format string when compiled under C++20, in the same way as std::format_string (not yet implemented).

Suppression of the argument deduction that would normally result in a compile-time error requires that we use intermediate template aliases for format_string and wformat_string . As this is not possible under C++03, usage of this type necessarily differs under C++03. This is acceptable as this type is typically only used internally within the bslfmt::format family of functions.

This header is not intended to be included directly. Please include <bsl_format.h> to be able to use bsl::basic_format_string.

Usage

In this section we show the intended use of this component.

Example: Own format function

This usage example reflects how bslfmt::format typically uses this type.

Suppose we have a function that takes a format string and requires that the string be constant evaluated under C++20:

#if defined(BSLS_COMPILERFEATURES_SUPPORT_ALIAS_TEMPLATES) &&
    defined(BSLS_COMPILERFEATURES_SUPPORT_VARIADIC_TEMPLATES)
  template <class t_ARG>
  void myFormatLikeFunction(bslfmt::format_string<t_ARG> fmtstr,
                            const t_ARG&)
  {
    assert(fmtstr.get() == "{:}");
  }
#else
  template <class t_ARG>
  void myFormatLikeFunction(bslfmt::format_string        fmtstr,
                            const t_ARG&)
  {
    assert(fmtstr.get() == "{:}");
  }
#endif

We can then invoke our function:

  int value = 5;
  myFormatLikeFunction("{:}", value);