# Extra Clang Tools 19.1.7 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 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("%d %u\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 (fully qualified) function names to replace, with the requirement that the first parameter contains the printf-style format string and the arguments to be formatted follow immediately afterwards. The default value for this option 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.