add hints about C++ standard to readmes

Author: Felix Nieuwenhuizen

runtime/Cpp/README.md

@@ -5,11 +5,11 @@ This folder contains the C++ runtime support for ANTLR.  See [the canonical antl
 ## Authors and major contributors
 
 ANTLR 4 is the result of substantial effort of the following people:
- 
+
 * [Terence Parr](http://www.cs.usfca.edu/~parrt/), [email protected]
   ANTLR project lead and supreme dictator for life
   [University of San Francisco](http://www.usfca.edu/)
-* [Sam Harwell](http://tunnelvisionlabs.com/) 
+* [Sam Harwell](http://tunnelvisionlabs.com/)
   Tool co-author, Java and C# target)
 
 The C++ target has been the work of the following people:
@@ -40,6 +40,7 @@ The minimum C++ version to compile the ANTLR C++ runtime with is C++11. The supp
 Include the antlr4-runtime.h umbrella header in your target application to get everything needed to use the library.
 
 If you are compiling with cmake, the minimum version required is cmake 2.8.
+By default, the libraries produced by the CMake build target C++11. If you want to target a different C++ standard, you can explicitly pass the standard - e.g. `-DCMAKE_CXX_STANDARD=17`.
 
 #### Compiling on Windows with Visual Studio using he Visual Studio projects
 Simply open the VS project from the runtime folder (VS 2013+) and build it.
@@ -69,5 +70,3 @@ If the CMake variable 'ANTLR4_INSTALL' is set, CMake Packages will be build and
 They expose two packages: antlr4_runtime and antlr4_generator which can be referenced to ease up the use of the
 ANTLR Generator and runtime.
 Use and Sample can be found [here](cmake/Antlr4Package.md)
-
-

runtime/Cpp/cmake/ExternalAntlr4Cpp.cmake

@@ -88,6 +88,8 @@ if(ANTLR4_ZIP_REPOSITORY)
       CMAKE_CACHE_ARGS
           -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
           -DWITH_STATIC_CRT:BOOL=${ANTLR4_WITH_STATIC_CRT}
+          # -DCMAKE_CXX_STANDARD:STRING=17 # if desired, compile the runtime with a different C++ standard
+          # -DCMAKE_CXX_STANDARD:STRING=${CMAKE_CXX_STANDARD} # alternatively, compile the runtime with the same C++ standard as the outer project
       INSTALL_COMMAND ""
       EXCLUDE_FROM_ALL 1)
 else()
@@ -105,6 +107,7 @@ else()
           -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
           -DWITH_STATIC_CRT:BOOL=${ANTLR4_WITH_STATIC_CRT}
           # -DCMAKE_CXX_STANDARD:STRING=17 # if desired, compile the runtime with a different C++ standard
+          # -DCMAKE_CXX_STANDARD:STRING=${CMAKE_CXX_STANDARD} # alternatively, compile the runtime with the same C++ standard as the outer project
       INSTALL_COMMAND ""
       EXCLUDE_FROM_ALL 1)
 endif()

runtime/Cpp/cmake/README.md

@@ -129,6 +129,8 @@ The ANTLR C++ runtime source is downloaded from GitHub by default. However, user
 
 Visual C++ compiler users may want to additionally define `ANTLR4_WITH_STATIC_CRT` before including the file. Set `ANTLR4_WITH_STATIC_CRT` to true if ANTLR4 C++ runtime library should be compiled with `/MT` flag, otherwise will be compiled with `/MD` flag. This variable has a default value of `OFF`. Changing `ANTLR4_WITH_STATIC_CRT` after building the library may require reinitialization of CMake or `clean` for the library to get rebuilt.
 
+You may need to modify your local copy of ExternalAntlr4Cpp.cpp to modify some build settings. For example, to specify the C++ standard to use when building the runtime, add `-DCMAKE_CXX_STANDARD:STRING=17` to `CMAKE_CACHE_ARGS`.
+
 ### Examples
 
 To build and link ANTLR4 static library to a target one may call: