libstdc++: Fix std::format alternate form for floating-point [PR108046]

A decimal point was being added to the end of the string for {:#.0}
because the __expc character was not being set, for the _Pres_none
presentation type, so __s.find(__expc) didn't the 'e' in "1e+01" and so
we created "1e+01." by appending the radix char to the end.

This can be fixed by ensuring that __expc='e' is set for the _Pres_none
case. I realized we can also set __expc='P' and __expc='E' when needed,
to save a call to std::toupper later.

For the {:#.0g} format, __expc='e' was being set and so the 'e' was
found in "1e+10" but then __z = __prec - __sigfigs would wraparound to
SIZE_MAX. That meant we would decide not to add a radix char because the
number of extra characters to insert would be 1+SIZE_MAX i.e. zero.

This can be fixed by using __z == 0 when __prec == 0.

libstdc++-v3/ChangeLog:

	PR libstdc++/108046
	* include/std/format (__formatter_fp::format): Ensure __expc is
	always set for all presentation types. Set __z correctly for
	zero precision.
	* testsuite/std/format/functions/format.cc: Check problem cases.

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 0c6069b268181ab9ed5d42faf19a0c8d2f13cc55..1e0ef612ddd8865425c3d356ae6965943ed09b50 100644 (file)
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1430,22 +1430,24 @@ namespace __format
          chars_format __fmt{};
          bool __upper = false;
          bool __trailing_zeros = false;
-         char __expc = 0;
+         char __expc = 'e';
 
          switch (_M_spec._M_type)
          {
            case _Pres_A:
              __upper = true;
+             __expc = 'P';
              [[fallthrough]];
            case _Pres_a:
-             __expc = 'p';
+             if (_M_spec._M_type != _Pres_A)
+               __expc = 'p';
              __fmt = chars_format::hex;
              break;
            case _Pres_E:
              __upper = true;
+             __expc = 'E';
              [[fallthrough]];
            case _Pres_e:
-             __expc = 'e';
              __use_prec = true;
              __fmt = chars_format::scientific;
              break;
@@ -1455,10 +1457,10 @@ namespace __format
              break;
            case _Pres_G:
              __upper = true;
+             __expc = 'E';
              [[fallthrough]];
            case _Pres_g:
              __trailing_zeros = true;
-             __expc = 'e';
              __use_prec = true;
              __fmt = chars_format::general;
              break;
@@ -1511,7 +1513,6 @@ namespace __format
            {
              for (char* __p = __start; __p != __res.ptr; ++__p)
                *__p = std::toupper(*__p);
-             __expc = std::toupper(__expc);
            }
 
          // Add sign for non-negative values.
@@ -1545,15 +1546,15 @@ namespace __format
                  __p = __s.find(__expc);
                  if (__p == __s.npos)
                    __p = __s.size();
-                 __d = __p;
+                 __d = __p; // Position where '.' should be inserted.
                  __sigfigs = __d;
                }
 
-             if (__trailing_zeros)
+             if (__trailing_zeros && __prec != 0)
                {
                  if (!__format::__is_xdigit(__s[0]))
                    --__sigfigs;
-                 __z = __prec - __sigfigs;
+                 __z = __prec - __sigfigs; // Number of zeros to insert.
                }
 
              if (size_t __extras = int(__d == __p) + __z)

diff --git a/libstdc++-v3/testsuite/std/format/functions/format.cc b/libstdc++-v3/testsuite/std/format/functions/format.cc
index 3485535e3cba531bcba138187766e6b53211ea78..bd914df6d7c8916a6a6d89884f175df0fdcf49e3 100644 (file)
--- a/libstdc++-v3/testsuite/std/format/functions/format.cc
+++ b/libstdc++-v3/testsuite/std/format/functions/format.cc
@@ -152,6 +152,10 @@ test_alternate_forms()
 
   s = std::format("{:#.2g}", -0.0);
   VERIFY( s == "-0.0" );
+
+  // PR libstdc++/108046
+  s = std::format("{0:#.0} {0:#.1} {0:#.0g}", 10.0);
+  VERIFY( s == "1.e+01 1.e+01 1.e+01" );
 }
 
 struct euro_punc : std::numpunct<char>