* Building C++ Hello World

/justbuild/ is a true language-agnostic (there are no more-equal languages) and
multi-repository build system. As a consequence, high-level concepts (e.g., C++
binaries, C++ libraries, etc.) are not hardcoded built-ins of the tool, but
rather provided via a set of rules. These rules can be specified as a true
dependency to your project like any other external repository your project might
depend on.

For the remainder of this tutorial, we will use the rules provided in the
open-source repository of justbuild, which we assume is checked out to the path
~/tmp/justbuild~.

** Setting up the Multi-Repository Configuration

To build a project with multi-repository dependencies, we first need to provide
a configuration that declares the required repositories. Before we begin, we
need to declare where the root of our workspace is located by creating an empty
file ~ROOT~:

#+BEGIN_SRC sh
$ touch ROOT
#+END_SRC

Second, we also need to create the multi-repository configuration ~repos.json~
in the workspace root:

#+SRCNAME: repos.json
#+BEGIN_SRC js
{ "main": "tutorial"
, "repositories":
  { "just-rules":
    { "repository": {"type": "file", "path": "/tmp/justbuild/rules"}
    }
  , "tutorial":
    { "repository": {"type": "file", "path": "."}
    , "bindings": {"rules": "just-rules"}
    }
  }
}
#+END_SRC

In that configuration, two repositories are defined:

 1. The ~"just-rules"~ repository located at ~/tmp/justbuild/rules~, which
    contains the high-level concepts for building C/C++ binaries and libraries.

 2. The ~"tutorial"~ repository located at ~.~, which contains the targets that
    we want to build. It has a single dependency, which is the /rules/ that are
    needed to build the target. These rules are bound via the open name
    ~"rules"~ to the just created repository ~"just-rules"~. In this way, the
    entities provided by ~"just-rules"~ can be accessed from within the
    ~"tutorial"~ repository via ~["@", "rules", "<module>", "<rule>"]~.

The final repository configuration contains a single ~JSON~ object with the key
~"repositories"~ referring to an object of repository names as keys and
repository descriptions as values. For convenience, the main repository to pick
is set to ~"tutorial"~.

** Description of the helloworld target

For this tutorial, we want to create a target ~helloworld~ that produces a
binary from the C++ source ~main.cpp~. To define such a target, create a
~TARGETS~ file with the following content:

#+SRCNAME: TARGETS
#+BEGIN_SRC js
{ "helloworld":
  { "type": ["@", "rules", "CC", "binary"]
  , "name": ["helloworld"]
  , "srcs": ["main.cpp"]
  }
}
#+END_SRC

The ~"type"~ field refers to the rule ~"binary"~ from the module ~"CC"~ of the
~"rules"~ repository. This rule additionally requires the string field ~"name"~,
which specifies the name of the binary to produce. Furthermore, at least one
input to the binary is required, which can be specified via the target fields
~"srcs"~ or ~"deps"~. In our case, the former is used, which contains our single
source file (files are considered targets).

Now, the last file that is missing is the actual source file ~main.cpp~:

#+SRCNAME: main.cpp
#+BEGIN_SRC cpp
#include <iostream>

int main() {
  std::cout << "Hello world!\n";
  return 0;
}
#+END_SRC

** Building the helloworld target

To build the ~helloworld~ target, we need specify it on the ~just-mr~ command
line:

#+BEGIN_SRC sh
$ just-mr build helloworld
INFO: Requested target is [["@","tutorial","","helloworld"],{}]
INFO: Analysed target [["@","tutorial","",helloworld"],{}]
INFO: Export targets found: 0 cached, 0 uncached, 0 not eligible for caching
INFO: Discovered 2 actions, 1 trees, 0 blobs
INFO: Building [["@","helloworld","","helloworld"],{}].
INFO: Processed 2 actions, 0 cache hits.
INFO: Artifacts built, logical paths are:
        helloworld [b5cfca8b810adc4686f5cac00258a137c5d4a3ba:17088:x]
$
#+END_SRC

Note that the target is taken from the ~tutorial~ repository, as it specified as
the main repository in ~repos.json~. If targets from other repositories should
be build, the repository to use must be specified via the ~--main~ option.

~just-mr~ reads the repository configuration, fetches externals (if any),
generates the actual build configuration, and stores it in its cache directory
(by default under ~$HOME/.cache/just~). Afterwards, the generated configuration
is used to call the ~just~ binary, which performs the actual build.

Note that these two programs, ~just-mr~ and ~just~, can also be run
individually. To do so, first run ~just-mr~ with ~setup~ and capture the path to
the generated build configuration from stdout by assigning it to a shell
variable (e.g., ~CONF~). Afterwards, ~just~ can be called to perform the actual
build by explicitly specifying the configuration file via ~-C~:

#+BEGIN_SRC sh
$ CONF=$(just-mr setup tutorial)
$ just build -C $CONF helloworld
#+END_SRC

Note that ~just-mr~ only needs to be run the very first time and only once again
whenever the ~repos.json~ file is modified.

By default, the BSD-default compiler front-ends (which are also defined for most
Linux distributions) ~cc~ and ~c++~ are used for C and C++ (variables ~"CC"~ and
~"CXX"~). If you want to temporarily use different defaults, you can use ~-D~ to
provide a JSON object that sets different default variables. For instance, to
use Clang as C++ compiler for a single build invocation, you can use the
following command to provide an object that sets ~"CXX"~ to ~"clang++"~:

#+BEGIN_SRC sh
$ just-mr build helloworld -D'{"CXX":"clang++"}'
INFO: Requested target is [["@","tutorial","","helloworld"],{"CXX":"clang++"}]
INFO: Analysed target [["@","tutorial","","helloworld"],{"CXX":"clang++"}]
INFO: Export targets found: 0 cached, 0 uncached, 0 not eligible for caching
INFO: Discovered 2 actions, 1 trees, 0 blobs
INFO: Building [["@","tutorial","","helloworld"],{"CXX":"clang++"}].
INFO: Processed 2 actions, 0 cache hits.
INFO: Artifacts built, logical paths are:
        helloworld [b8cf7b8579d9dc7172b61660139e2c14521cedae:16944:x]
$
#+END_SRC

** Defining project defaults

To define a custom set of defaults (toolchain and compile flags) for your
project, you need to create a separate file root for providing required
~TARGETS~ file, which contains the ~"defaults"~ target that should be used by
the rules. This file root is then used as the /target root/ for the rules (i.e.,
the search path for ~TARGETS~ files). In this way, the description of the
~"defaults"~ target is provided in a separate file root, to keep the rules
repository independent of these definitions.

We will call the new file root ~tutorial-defaults~ and need to create a module
directory ~CC~ in it:

#+BEGIN_SRC sh
$ mkdir -p ./tutorial-defaults/CC
#+END_SRC

In that module, we need to create the file ~tutorial-defaults/CC/TARGETS~ that
contains the target ~"defaults"~ and specifies which toolchain and compile flags
to use:

#+SRCNAME: tutorial-defaults/CC/TARGETS
#+BEGIN_SRC js
{ "defaults":
  { "type": ["CC", "defaults"]
  , "CC": ["cc"]
  , "CXX": ["c++"]
  , "CFLAGS": ["-O2", "-Wall"]
  , "CXXFLAGS": ["-O2", "-Wall"]
  , "AR": ["ar"]
  , "PATH": ["/bin", "/usr/bin"]
  }
}
#+END_SRC

To use the project defaults, modify the existing ~repos.json~ to reflect the
following content:

#+SRCNAME: repos.json
#+BEGIN_SRC js
{ "main": "tutorial"
, "repositories":
  { "just-rules":
    { "repository": {"type": "file", "path": "/tmp/justbuild/rules"}
    , "target_root": "tutorial-defaults"
    , "rule_root": "just-rules"
    }
  , "tutorial":
    { "repository": {"type": "file", "path": "."}
    , "bindings": {"rules": "just-rules"}
    }
  , "tutorial-defaults":
    { "repository": {"type": "file", "path": "./tutorial-defaults"}
    }
  }
}
#+END_SRC

Note that the ~"defaults"~ target uses the rule ~["CC", "defaults"]~ without
specifying any external repository (e.g., ~["@", "rules", ...]~). This is
because ~"tutorial-defaults"~ is not a full-fledged repository but merely a file
root that is considered local to the ~"just-rules"~ repository. In fact, the
~"just-rules"~ repository cannot refer to any external repository as it does not
have any defined bindings.

To rebuild the project, we need to rerun ~just-mr~ (note that due to
configuration changes, rerunning only ~just~ would not suffice):

#+BEGIN_SRC sh
$ just-mr build helloworld
INFO: Requested target is [["@","tutorial","","helloworld"],{}]
INFO: Analysed target [["@","tutorial","","helloworld"],{}]
INFO: Export targets found: 0 cached, 0 uncached, 0 not eligible for caching
INFO: Discovered 2 actions, 1 trees, 0 blobs
INFO: Building [["@","tutorial","","helloworld"],{}].
INFO: Processed 2 actions, 0 cache hits.
INFO: Artifacts built, logical paths are:
        helloworld [487dc9e47b978877ed2f7d80b3395ce84b23be92:16992:x]
$
#+END_SRC

Note that the output binary may have changed due to different defaults.

** Modeling target dependencies

For demonstration purposes, we will separate the print statements into a static
library ~greet~, which will become a dependency to our binary. Therefore, we
create a new subdirectory ~greet~ with the files ~greet/greet.hpp~:

#+SRCNAME: greet/greet.hpp
#+BEGIN_SRC cpp
#include <string>

void greet(std::string const& s);
#+END_SRC

and ~greet/greet.cpp~:

#+SRCNAME: greet/greet.cpp
#+BEGIN_SRC cpp
#include "greet.hpp"
#include <iostream>

void greet(std::string const& s) {
  std::cout << "Hello " << s << "!\n";
}
#+END_SRC

These files can now be used to create a static library ~libgreet.a~. To do so,
we need to create the following target description in ~greet/TARGETS~:

#+SRCNAME: greet/TARGETS
#+BEGIN_SRC js
{ "greet":
  { "type": ["@", "rules", "CC", "library"]
  , "name": ["greet"]
  , "hdrs": ["greet.hpp"]
  , "srcs": ["greet.cpp"]
  , "stage": ["greet"]
  }
}
#+END_SRC

Similar to ~"binary"~, we have to provide a name and source file. Additionally,
a library has public headers defined via ~"hdrs"~ and an optional staging
directory ~"stage"~ (default value ~"."~). The staging directory specifies where
the consumer of this library can expect to find the library's artifacts. Note
that this does not need to reflect the location on the file system (i.e., a
full-qualified path like ~["com", "example", "utils", "greet"]~ could be used to
distinguish it from greeting libraries of other projects). The staging directory
does not only affect the main artifact ~libgreet.a~ but also it's runfiles;
hence, the public header will be staged to ~"greet/greet.hpp"~. With that
knowledge, we can now perform the necessary modifications to ~main.cpp~:

#+SRCNAME: main.cpp
#+BEGIN_SRC cpp
#include "greet/greet.hpp"

int main() {
  greet("Universe");
  return 0;
}
#+END_SRC

The target ~"helloworld"~ will have a direct dependency to the target ~"greet"~
of the module ~"greet"~ in the top-level ~TARGETS~ file:

#+SRCNAME: TARGETS
#+BEGIN_SRC js
{ "helloworld":
  { "type": ["@", "rules", "CC", "binary"]
  , "name": ["helloworld"]
  , "srcs": ["main.cpp"]
  , "deps": [["greet", "greet"]]
  }
}
#+END_SRC

Note that there is no need to explicitly specify ~"greet"~'s public headers here
as the appropriate artifacts of dependencies are automatically added to the
inputs of compile and link actions. The new binary can be built with the same
command as before (no need to rerun ~just-mr~):

#+BEGIN_SRC sh
$ just-mr build helloworld
INFO: Requested target is [["@","tutorial","","helloworld"],{}]
INFO: Analysed target [["@","tutorial","","helloworld"],{}]
INFO: Export targets found: 0 cached, 0 uncached, 0 not eligible for caching
INFO: Discovered 4 actions, 2 trees, 0 blobs
INFO: Building [["@","tutorial","","helloworld"],{}].
INFO: Processed 4 actions, 0 cache hits.
INFO: Artifacts built, logical paths are:
        helloworld [2b81e3177afc382452a2df9f294d3df90a9ccaf0:17664:x]
$
#+END_SRC

To only build the static library target ~"greet"~ from module ~"greet"~, run the
following command:

#+BEGIN_SRC sh
$ just-mr build greet greet
INFO: Requested target is [["@","tutorial","greet","greet"],{}]
INFO: Analysed target [["@","tutorial","greet","greet"],{}]
INFO: Export targets found: 0 cached, 0 uncached, 0 not eligible for caching
INFO: Discovered 2 actions, 1 trees, 0 blobs
INFO: Building [["@","tutorial","greet","greet"],{}].
INFO: Processed 2 actions, 2 cache hits.
INFO: Artifacts built, logical paths are:
        greet/libgreet.a [83ed406e21f285337b0c9bd5011f56f656bba683:2992:f]
      (1 runfiles omitted.)
$
#+END_SRC