ENH: rationalize some string methods.

- silently deprecate 'startsWith', 'endsWith' methods
  (added in 2016: 2b14360662), in favour of
  'starts_with', 'ends_with' methods, corresponding to C++20 and
  allowing us to cull then in a few years.

- handle single character versions of starts_with, ends_with.

- add single character version of removeEnd and silently deprecate
  removeTrailing which did the same thing.

- drop the const versions of removeRepeated, removeTrailing.
  Unused and with potential confusion.

STYLE: use shrink_to_fit(), erase()

applications/utilities/mesh/conversion/tetgenToFoam/tetgenToFoam.C

@@ -175,7 +175,7 @@ int main(int argc, char *argv[])
     {
         nodeStream.getLine(line);
     }
-    while (line.size() && line[0] == '#');
+    while (line.starts_with('#'));
 
     IStringStream nodeLine(line);
 
@@ -252,7 +252,7 @@ int main(int argc, char *argv[])
     {
         eleStream.getLine(line);
     }
-    while (line.size() && line[0] == '#');
+    while (line.starts_with('#'));
 
     IStringStream eleLine(line);
 
@@ -368,7 +368,7 @@ int main(int argc, char *argv[])
         {
             faceStream.getLine(line);
         }
-        while (line.size() && line[0] == '#');
+        while (line.starts_with('#'));
 
         IStringStream faceLine(line);
 

applications/utilities/mesh/manipulation/objToVTK/objToVTK.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
+    Copyright (C) 2019 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,7 +52,7 @@ string getLine(std::ifstream& is)
     {
         std::getline(is, line);
     }
-    while (line.size() && line[0] == '#');
+    while (line.starts_with('#'));
 
     return line;
 }

applications/utilities/miscellaneous/foamHelp/helpTypes/helpType/helpTypeNew.C

@@ -40,7 +40,7 @@ Foam::autoPtr<Foam::helpType> Foam::helpType::New
     {
         // special treatment for -help
         // exit without stack trace
-        if (helpTypeName.startsWith("-help"))
+        if (helpTypeName.starts_with("-help"))
         {
             FatalErrorInFunction
                 << "Valid helpType selections:" << nl

applications/utilities/postProcessing/graphics/PVReaders/foamReader/library/vtkPVFoam.C

@@ -678,7 +678,7 @@ void Foam::vtkPVFoam::Update
         // Suppress caching of Lagrangian since it normally always changes.
         cachedVtp_.filterKeys
         (
-            [](const word& k){ return k.startsWith("lagrangian/"); },
+            [](const word& k){ return k.starts_with("lagrangian/"); },
             true // prune
         );
     }

applications/utilities/postProcessing/graphics/PVReaders/foamReader/library/vtkPVFoamMesh.C

@@ -179,7 +179,7 @@ void Foam::vtkPVFoam::convertMeshPatches()
             }
         }
 
-        if (longName.startsWith("group/"))
+        if (longName.starts_with("group/"))
         {
             // Patch group. Collect patch faces.