Replacing string += "." by '.'

[Wolfgang Bangerth <bangerth at ticam dot utexas dot edu>]

Hi all,
triggered by a recent patch of Paolo, I thought about the implications of 
replacing adding single characters like
  __str += '=';
by
  __str += "=";

It seemed to me that the latter is more expensive. The reasoning is the 
following call chain:
  string::operator += (_CharT *)
  string::append (_CharT *)
This calls
  append(__s, traits_type::length(__s))
which basically needs to call strlen on __s.

On the other hand, adding the single character (i.e. '.' instead of ".") 
has the following call chain:
  string::operator += (_CharT c)
  string::append (1,c)             // length already known

The rest should have equal costs.

The consequence is to add single characters to strings wherever possible. 
The same reasoning also applies when writing
  cout << ".";
or
  cout << '.';
The appended patch does this throughout the libstdc++ directory (sans 
testsuite). There are not so terribly many places where this applies, but 
there are several more where operations like
  strcmp(__s, "C")
and
  strcpy(_M_name_messages, "C");
are performed, usually with the "C" locale. The first could be replaced by
  (__s[0] == 'C' && __s[1]=='\0')
and for the second something similar might be possible, removing the loop 
that would be necessary for these operations otherwise. I have not done 
that in this patch.

I don't know how to test the patch. In case someone would like to consider 
it, please test it according to the your principles before applying it.

Thanks
  Wolfgang

-------------------------------------------------------------------------
Wolfgang Bangerth              email:           bangerth@ticam.utexas.edu
                               www: http://www.ticam.utexas.edu/~bangerth

Index: include/std/std_complex.h
===================================================================
RCS file: /cvsroot/gcc/gcc/libstdc++-v3/include/std/std_complex.h,v
retrieving revision 1.2
diff -u -3 -r1.2 std_complex.h
--- include/std/std_complex.h   28 Jan 2002 22:13:10 -0000      1.2
+++ include/std/std_complex.h   25 Nov 2002 16:30:10 -0000
@@ -390,7 +390,7 @@
       __s.flags(__os.flags());
       __s.imbue(__os.getloc());
       __s.precision(__os.precision());
-      __s << '(' << __x.real() << "," << __x.imag() << ')';
+      __s << '(' << __x.real() << ',' << __x.imag() << ')';
       return __os << __s.str();
     }

Index: src/locale.cc
===================================================================
RCS file: /cvsroot/gcc/gcc/libstdc++-v3/src/locale.cc,v
retrieving revision 1.71
diff -u -3 -r1.71 locale.cc
--- src/locale.cc       24 Nov 2002 18:48:35 -0000      1.71
+++ src/locale.cc       25 Nov 2002 16:30:10 -0000
@@ -235,14 +235,14 @@
                    for (size_t __j = 0; __j < __i; ++__j)
                      {
                        __str += _S_categories[__j];
-                       __str += "=";
+                       __str += '=';
                        __str += __res;
-                       __str += ";";
+                       __str += ';';
                      }
                    __str += _S_categories[__i];
-                   __str += "=";
+                   __str += '=';
                    __str += __env;
-                   __str += ";";
+                   __str += ';';
                    __i++;
                    for (; __i < _S_categories_size
                           + _S_extra_categories_size; ++__i)
@@ -251,9 +251,9 @@
                        if (!__env || strcmp(__env, "") == 0)
                          {
                            __str += _S_categories[__i];
-                           __str += "=";
+                           __str += '=';
                            __str += __res;
-                           __str += ";";
+                           __str += ';';
                          }
                        else if (strcmp(__env, "C") == 0
                                 || strcmp(__env, "POSIX") == 0)
@@ -264,9 +264,9 @@
                        else
                          {
                            __str += _S_categories[__i];
-                           __str += "=";
+                           __str += '=';
                            __str += __env;
-                           __str += ";";
+                           __str += ';';
                          }
                      }
                    __str.erase(__str.end() - 1);
@@ -347,15 +347,15 @@
     else
       {
        __ret += _S_categories[0];
-       __ret += "=";
+       __ret += '=';
        __ret += _M_impl->_M_names[0];
        for (size_t __i = 1;
             __i < _S_categories_size + _S_extra_categories_size;
             ++__i)
          {
-           __ret += ";";
+           __ret += ';';
            __ret += _S_categories[__i];
-           __ret += "=";
+           __ret += '=';
            __ret += _M_impl->_M_names[__i];
          }
       }