# Extra Clang Tools 22.1.2 documentation

## clang-tidy - modernize-use-std-format

# modernize-use-std-format

Converts calls to `absl::StrFormat`, or other functions via configuration options, to C++20’s `std::format`, or another function via a configuration option, modifying the format string appropriately and removing now-unnecessary calls to `std::string::c_str()` and `std::string::data()`.

For example, it turns lines like

    return absl::StrFormat("The %s is %3d", description.c_str(), value);

into:

    return std::format("The {} is {:3}", description, value);

The check uses the same format-string-conversion algorithm as modernize-use-std-print and its shortcomings and behaviour in combination with macros are described in the documentation for that check.

## Options

StrictMode

  > When true, the check will add casts when converting from variadic functions and printing signed or unsigned integer types (including fixed-width integer types from `<cstdint>`, `ptrdiff_t`, `size_t` and `ssize_t`) as the opposite signedness to ensure that the output would matches that of a simple wrapper for `std::sprintf` that accepted a C-style variable argument list. For example, with StrictMode enabled,

      extern std::string strprintf(const char *format, ...);
      int i = -42;
      unsigned int u = 0xffffffff;
      return strprintf("%u %d\n", i, u);

  would be converted to

      return std::format("{} {}\n", static_cast<unsigned int>(i), static_cast<int>(u));

  to ensure that the output will continue to be the unsigned representation of -42 and the signed representation of 0xffffffff (often 4294967254 and -1 respectively). When false (which is the default), these casts will not be added which may cause a change in the output. Note that this option makes no difference for the default value of StrFormatLikeFunctions since `absl::StrFormat` takes a function parameter pack and is not a variadic function.

StrFormatLikeFunctions

  A semicolon-separated list of regular expressions matching the (fully qualified) names of functions to replace, with the requirement that the first parameter contains the printf-style format string and the arguments to be formatted follow immediately afterwards. Qualified member function names are supported, but the replacement function name must be unqualified. The default value is absl::StrFormat.

ReplacementFormatFunction

  The function that will be used to replace the function set by the StrFormatLikeFunctions option rather than the default std::format. It is expected that the function provides an interface that is compatible with `std::format`. A suitable candidate would be fmt::format.

FormatHeader

  The header that must be included for the declaration of ReplacementFormatFunction so that a `#include` directive can be added if required. If ReplacementFormatFunction is std::format then this option will default to `<format>`, otherwise this option will default to nothing and no `#include` directive will be added.