implement an utils::print() function similar to fmt::print()

this doesn't have the constexpr requirement for the format string.
also it will help porting to std::format in C++20, which doesn't
have a similar functionality either.

doc/src/Developer_code_design.rst

@@ -300,18 +300,19 @@ Formatting with the {fmt} library
 
 The LAMMPS source code includes a copy of the `{fmt} library
 <https://fmt.dev>`_, which is preferred over formatting with the
-"printf()" family of functions.  The primary reason is that it allows
+"printf()" family of functions.  The primary reason is that it allows a
-a typesafe default format for any type of supported data.  This is
+typesafe default format for any type of supported data.  This is
 particularly useful for formatting integers of a given size (32-bit or
-64-bit) which may require different format strings depending on
+64-bit) which may require different format strings depending on compile
-compile time settings or compilers/operating systems.  Furthermore,
+time settings or compilers/operating systems.  Furthermore, {fmt} gives
-{fmt} gives better performance, has more functionality, a familiar
+better performance, has more functionality, a familiar formatting syntax
-formatting syntax that has similarities to ``format()`` in Python, and
+that has similarities to ``format()`` in Python, and provides a facility
-provides a facility that can be used to integrate format strings and a
+that can be used to integrate format strings and a variable number of
-variable number of arguments into custom functions in a much simpler
+arguments into custom functions in a much simpler way than the varargs
-way than the varargs mechanism of the C library.  Finally, {fmt} has
+mechanism of the C library.  Finally, {fmt} has been included into the
-been included into the C++20 language standard, so changes to adopt it
+C++20 language standard as ``std::format()``, so changes to adopt it are
-are future-proof.
+future-proof, for as long as they are not using any extensions that are
+not (yet) included into C++.
 
 Formatted strings are frequently created by calling the
 ``fmt::format()`` function, which will return a string as a
@@ -319,11 +320,13 @@ Formatted strings are frequently created by calling the
 ``printf()``, the {fmt} library uses ``{}`` to embed format descriptors.
 In the simplest case, no additional characters are needed, as {fmt} will
 choose the default format based on the data type of the argument.
-Otherwise, the ``fmt::print()`` function may be used instead of
+Otherwise, the :cpp:func:`utils::print() <LAMMPS_NS::utils::print>`
-``printf()`` or ``fprintf()``.  In addition, several LAMMPS output
+function may be used instead of ``printf()`` or ``fprintf()``.  In
-functions, that originally accepted a single string as argument have
+addition, several LAMMPS output functions, that originally accepted a
-been overloaded to accept a format string with optional arguments as
+single string as argument have been overloaded to accept a format string
-well (e.g., ``Error::all()``, ``Error::one()``, ``utils::logmesg()``).
+with optional arguments as well (e.g., ``Error::all()``,
+``Error::one()``, :cpp:func:`utils::logmesg()
+<LAMMPS_NS::utils::logmesg>`).
 
 Summary of the {fmt} format syntax
 ==================================

doc/src/Developer_utils.rst

@@ -238,6 +238,12 @@ Convenience functions
 .. doxygenfunction:: logmesg(LAMMPS *lmp, const std::string &mesg)
    :project: progguide
 
+.. doxygenfunction:: print(FILE *fp, const std::string &format, Args&&... args)
+   :project: progguide
+
+.. doxygenfunction:: print(FILE *fp, const std::string &mesg)
+   :project: progguide
+
 .. doxygenfunction:: errorurl
    :project: progguide
 

doc/src/Developer_write_fix.rst

@@ -96,8 +96,8 @@ Here the we specify which methods of the fix should be called during
      MPI_Allreduce(localAvgVel, globalAvgVel, 4, MPI_DOUBLE, MPI_SUM, world);
      scale3(1.0 / globalAvgVel[3], globalAvgVel);
      if ((comm->me == 0) && screen) {
-       fmt::print(screen,"{}, {}, {}\n",
+       utils::print(screen, "{}, {}, {}\n",
-                  globalAvgVel[0], globalAvgVel[1], globalAvgVel[2]);
+                    globalAvgVel[0], globalAvgVel[1], globalAvgVel[2]);
      }
    }
 

doc/utils/sphinx-config/false_positives.txt

@@ -1236,6 +1236,7 @@ fp
 fphi
 fPIC
 fplo
+fprintf
 Fqq
 Fraige
 framerate

src/utils.cpp

@@ -262,6 +262,22 @@ void utils::fmtargs_logmesg(LAMMPS *lmp, fmt::string_view format, fmt::format_ar
   }
 }
 
+/* specialization for the case of just a single string argument */
+
+void utils::print(FILE *fp, const std::string &mesg)
+{
+  fputs(mesg.c_str(), fp);
+}
+
+void utils::fmtargs_print(FILE *fp, fmt::string_view format, fmt::format_args args)
+{
+  try {
+    print(fp, fmt::vformat(format, args));
+  } catch (fmt::format_error &) {
+    ; // do nothing
+  }
+}
+
 std::string utils::errorurl(int errorcode)
 {
   return fmt::format("\nFor more information see https://docs.lammps.org/err{:04d}", errorcode);

src/utils.h

@@ -92,8 +92,8 @@ namespace utils {
    *
    * This function simplifies the repetitive task of outputting some
    * message to both the screen and/or the log file. The template
-   * wrapper with fmtlib format and argument processing allows
+   * wrapper with {fmt} formatting and argument processing allows
-   * this function to work similar to ``fmt::print()``.
+   * this function to work similar to :cpp:func:`utils::print() <LAMMPS_NS::utils::print>`.
    *
    *  \param lmp    pointer to LAMMPS class instance
    *  \param format format string of message to be printed
@@ -111,6 +111,40 @@ namespace utils {
 
   void logmesg(LAMMPS *lmp, const std::string &mesg);
 
+  /*! Return text redirecting the user to a specific paragraph in the manual
+   *
+   * The LAMMPS manual contains detailed explanations for errors and
+   * warnings where a simple error message may not be sufficient.  These can
+   * be reached through URLs with a numeric code.  This function creates the
+   * corresponding text to be included into the error message that redirects
+   * the user to that URL.
+   *
+   *  \param errorcode   number pointing to a paragraph in the manual */
+
+  /*! Internal function handling the argument list for print(). */
+
+  void fmtargs_print(FILE *fp, fmt::string_view format, fmt::format_args args);
+
+  /*! Write formatted message to file
+   *
+   * This function implements a version of fprintf() that uses {fmt} formatting
+   *
+   *  \param fp     stdio FILE pointer
+   *  \param format format string of message to be printed
+   *  \param args   arguments to format string */
+
+  template <typename... Args> void print(FILE *fp, const std::string &format, Args &&...args)
+  {
+    fmtargs_print(fp, format, fmt::make_format_args(args...));
+  }
+
+  /*! \overload
+   *
+   *  \param fp     stdio FILE pointer
+   *  \param mesg   string with message to be printed */
+
+  void print(FILE *fp, const std::string &mesg);
+
   /*! Return text redirecting the user to a specific paragraph in the manual
    *
    * The LAMMPS manual contains detailed explanations for errors and