## Getting started with Antlr4Cpp

Here is how you can use this external project to create the antlr4cpp demo to start your project off.

1. Create your project source folder somewhere. e.g. ~/srcfolder/
   1. Make a subfolder cmake
   2. Copy the files in this folder to srcfolder/cmake
   3. Cut below and use it to create srcfolder/CMakeLists.txt
   4. Copy main.cpp, TLexer.g4 and TParser.g4 to srcfolder/ from [here](https://github.com/antlr/antlr4/tree/master/runtime/Cpp/demo)
2. Make a build folder e.g. ~/buildfolder/
3. From the buildfolder, run `cmake ~/srcfolder; make`

```cmake
# minimum required CMAKE version
CMAKE_MINIMUM_REQUIRED(VERSION 3.7 FATAL_ERROR)

list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)

# compiler must be 17
set(CMAKE_CXX_STANDARD 17)

# required if linking to static library
add_definitions(-DANTLR4CPP_STATIC)

# using /MD flag for antlr4_runtime (for Visual C++ compilers only)
set(ANTLR4_WITH_STATIC_CRT OFF)

# Specify the version of the antlr4 library needed for this project.
# By default the latest version of antlr4 will be used.  You can specify a
# specific, stable version by setting a repository tag value or a link
# to a zip file containing the libary source.
# set(ANTLR4_TAG 4.13.2)
# set(ANTLR4_ZIP_REPOSITORY https://github.com/antlr/antlr4/archive/refs/tags/4.13.2.zip)

# add external build for antlrcpp
include(ExternalAntlr4Cpp)
# add antlr4cpp artifacts to project environment
include_directories(${ANTLR4_INCLUDE_DIRS})

# set variable pointing to the antlr tool that supports C++
# this is not required if the jar file can be found under PATH environment
set(ANTLR_EXECUTABLE /home/user/antlr-4.13.2-complete.jar)
# add macros to generate ANTLR Cpp code from grammar
find_package(ANTLR REQUIRED)

# Call macro to add lexer and grammar to your build dependencies.
antlr_target(SampleGrammarLexer TLexer.g4 LEXER
             PACKAGE antlrcpptest)
antlr_target(SampleGrammarParser TParser.g4 PARSER
             PACKAGE antlrcpptest
             DEPENDS_ANTLR SampleGrammarLexer
             COMPILE_FLAGS -lib ${ANTLR_SampleGrammarLexer_OUTPUT_DIR})

# include generated files in project environment
include_directories(${ANTLR_SampleGrammarLexer_OUTPUT_DIR})
include_directories(${ANTLR_SampleGrammarParser_OUTPUT_DIR})

# add generated grammar to demo binary target
add_executable(demo main.cpp
               ${ANTLR_SampleGrammarLexer_CXX_OUTPUTS}
               ${ANTLR_SampleGrammarParser_CXX_OUTPUTS})
target_link_libraries(demo antlr4_static)
```

## Documentation for FindANTLR

The module defines the following variables:

```
ANTLR_FOUND - true is ANTLR jar executable is found
ANTLR_EXECUTABLE - the path to the ANTLR jar executable
ANTLR_VERSION - the version of ANTLR
```

If ANTLR is found, the module will  provide the macros:

```
ANTLR_TARGET(<name> <input>
             [PACKAGE namespace]
             [OUTPUT_DIRECTORY dir]
             [DEPENDS_ANTLR <target>]
             [COMPILE_FLAGS [args...]]
             [DEPENDS [depends...]]
             [LEXER]
             [PARSER]
             [LISTENER]
             [VISITOR])
```

which creates a custom command to generate C++ files from `<input>`. Running the macro defines the following variables:

```
ANTLR_${name}_INPUT - the ANTLR input used for the macro
ANTLR_${name}_OUTPUTS - the outputs generated by ANTLR
ANTLR_${name}_CXX_OUTPUTS - the C++ outputs generated by ANTLR
ANTLR_${name}_OUTPUT_DIR - the output directory for ANTLR
```

The options are:

* `PACKAGE` - defines a namespace for the generated C++ files
* `OUTPUT_DIRECTORY` - the output directory for the generated files. By default it uses `${CMAKE_CURRENT_BINARY_DIR}`
* `DEPENDS_ANTLR` - the dependent target generated from antlr_target for the current call
* `COMPILE_FLAGS` - additional compile flags for ANTLR tool
* `DEPENDS` - specify the files on which the command depends. It works the same way `DEPENDS` in [`add_custom_command()`](https://cmake.org/cmake/help/v3.11/command/add_custom_command.html)
* `LEXER` - specify that the input file is a lexer grammar
* `PARSER` - specify that the input file is a parser grammar
* `LISTENER` - tell ANTLR tool to generate a parse tree listener
* `VISITOR` - tell ANTLR tool to generate a parse tree visitor

### Examples

To generate C++ files from an ANTLR input file T.g4, which defines both lexer and parser grammar one may call:

```cmake
find_package(ANTLR REQUIRED)
antlr_target(Sample T.g4)
```

Note that this command will do nothing unless the outputs of `Sample`, i.e. `ANTLR_Sample_CXX_OUTPUTS` gets used by some target.

## Documentation for ExternalAntlr4Cpp

Including ExternalAntlr4Cpp will add `antlr4_static` and `antlr4_shared` as an optional target. It will also define the following variables:

```
ANTLR4_INCLUDE_DIRS - the include directory that should be included when compiling C++ source file
ANTLR4_STATIC_LIBRARIES - path to antlr4 static library
ANTLR4_SHARED_LIBRARIES - path to antlr4 shared library
ANTLR4_RUNTIME_LIBRARIES - path to antlr4 shared runtime library (such as DLL, DYLIB and SO file)
ANTLR4_TAG - branch/tag used for building antlr4 library
```

`ANTLR4_TAG` is set to master branch by default to keep the antlr4 library up to date. However, this will require a rebuild after every `clean` is called. Set `ANTLR4_TAG` to a desired commit hash value to avoid rebuilding after every `clean` and keep the build stable, at the cost of not automatically updating to latest commit.

By defualt the ANTLR C++ runtime source is cloned from GitHub. However, users may specify `ANTLR4_ZIP_REPOSITORY` in order to download source as a zip file from [ANTLR downloads](http://www.antlr.org/download.html) (under *C++ Target*) or other locations. For example, this variable could list a zip file included in your the project directory.  This is useful for maintaining a canonical source tree for each new build.

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:

```cmake
include(ExternalAntlr4Cpp)
include_directories(${ANTLR4_INCLUDE_DIRS})
add_executable(output main.cpp)
target_link_libraries(output antlr4_static)
```

It may also be a good idea to copy the runtime libraries (DLL, DYLIB or SO file) to the executable for it to run properly after build. i.e. To build and link antlr4 shared library to a target one may call:

```cmake
include(ExternalAntlr4Cpp)
include_directories(${ANTLR4_INCLUDE_DIRS})
add_executable(output main.cpp)
target_link_libraries(output antlr4_shared)
add_custom_command(TARGET output
                   POST_BUILD
                   COMMAND ${CMAKE_COMMAND}
                           -E copy ${ANTLR4_RUNTIME_LIBRARIES} .
                   WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
```
