Add C/C++ diff: scope, modules, packages. (#37167)

* Add C/C++ diff: scope, modules, packages.

* Tweak C/C++ info & wrap lines.

* More tweaks to the C++ differences.

* Update doc/src/manual/noteworthy-differences.md

Co-authored-by: Stefan Karpinski <stefan@karpinski.org>

* correction: cwd is not a package repo by default.

* Apply suggestions from code review

Co-authored-by: Stefan Karpinski <stefan@karpinski.org>
Co-authored-by: Viral B. Shah <ViralBShah@users.noreply.github.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: 4 people

doc/src/manual/noteworthy-differences.md

@@ -351,6 +351,97 @@ For users coming to Julia from R, these are some noteworthy differences:
     it's more general than that since methods are dispatched on every argument type, not only `this`,
     using the most-specific-declaration rule).
 
+### Julia ⇔ C/C++: Namespaces
+  * C/C++ `namespace`s correspond roughly to Julia `module`s.
+  * There are no private globals or fields in Julia.  Everything is publicly accessible
+    through fully qualified paths (or relative paths, if desired).
+  * `using MyNamespace::myfun` (C++) corresponds roughly to `import MyModule: myfun` (Julia).
+  * `using namespace MyNamespace` (C++) corresponds roughly to `using MyModule` (Julia)
+    * In Julia, only `export`ed symbols are made available to the calling module.
+    * In C++, only elements found in the included (public) header files are made available.
+  * Caveat: `import`/`using` keywords (Julia) also *load* modules (see below).
+  * Caveat: `import`/`using` (Julia) works only at the global scope level (`module`s)
+    * In C++, `using namespace X` works within arbitrary scopes (ex: function scope).
+
+### Julia ⇔ C/C++: Module loading
+  * When you think of a C/C++ "**library**", you are likely looking for a Julia "**package**".
+    * Caveat: C/C++ libraries often house multiple "software modules" whereas Julia
+      "packages" typically house one.
+    * Reminder: Julia `module`s are global scopes (not necessarily "software modules").
+  * **Instead of build/`make` scripts**, Julia uses "Project Environments" (sometimes called
+    either "Project" or "Environment").
+    * Build scripts are only needed for more complex applications
+      (like those needing to compile or download C/C++ executables).
+    * To develop application or project in Julia, you can initialize its root directory
+      as a "Project Environment", and house application-specific code/packages there.
+      This provides good control over project dependencies, and future reproducibility.
+    * Available packages are added to a "Project Environment" with the `Pkg.add()` function or Pkg REPL mode.
+      (This does not **load** said package, however).
+    * The list of available packages (direct dependencies) for a "Project Environment" are
+      saved in its `Project.toml` file.
+    * The *full* dependency information for a "Project Environment" is auto-generated & saved
+      in its `Manifest.toml` file by `Pkg.resolve()`.
+  * Packages ("software modules") available to the "Project Environment" are loaded with
+    `import` or `using`.
+    * In C/C++, you `#include <moduleheader>` to get object/function delarations, and link in
+      libraries when you build the executable.
+    * In Julia, calling using/import again just brings the existing module into scope, but does not load it again
+      (similar to adding the non-standard `#pragma once` to C/C++).
+  * **Directory-based package repositories** (Julia) can be made available by adding repository
+    paths to the `Base.LOAD_PATH` array.
+    * Packages from directory-based repositories do not require the `Pkg.add()` tool prior to
+      being loaded with `import` or `using`. They are simply available to the project.
+    * Directory-based package repositories are the **quickest solution** to developping local
+      libraries of "software modules".
+
+### Julia &hArr; C/C++: Assembling modules
+  * In C/C++, `.c`/`.cpp` files are compiled & added to a library with build/`make` scripts.
+    * In Julia, `import [PkgName]`/`using [PkgName]` statements load `[PkgName].jl` located
+      in a package's `[PkgName]/src/` subdirectory.
+    * In turn, `[PkgName].jl` typically loads associated source files with calls to
+      `include "[someotherfile].jl"`.
+  * `include "./path/to/somefile.jl"` (Julia) is very similar to
+    `#include "./path/to/somefile.jl"` (C/C++).
+    * However `include "..."` (Julia) is not used to include header files (not required).
+    * **Do not use** `include "..."` (Julia) to load code from other "software modules"
+      (use `import`/`using` instead).
+    * `include "path/to/some/module.jl"` (Julia) would instantiate multiple versions of the
+      same code in different modules (creating *distinct* types (etc.) with the *same* names).
+    * `include "somefile.jl"` is typically used to assemble multiple files *within the same
+      Julia package* ("software module"). It is therefore relatively straightforward to ensure
+      file are `include`d only once (No `#ifdef` confusion).
+
+### Julia &hArr; C/C++: Module interface
+  * C++ exposes interfaces using "public" `.h`/`.hpp` files whereas Julia `module`s `export`
+    symbols that are intended for their users.
+    * Often, Julia `module`s simply add functionality by generating new "methods" to existing
+      functions (ex: `Base.push!`).
+    * Developers of Julia packages therefore cannot rely on header files for interface
+      documentation.
+    * Interfaces for Julia packages are typically described using docstrings, README.md,
+      static web pages, ...
+  * Some developers choose not to `export` all symbols required to use their package/module.
+    * Users might be expected to access these components by qualifying functions/structs/...
+      with the package/module name (ex: `MyModule.run_this_task(...)`).
+
+### Julia &hArr; C/C++: Quick reference
+
+| Software Concept   | Julia | C/C++ |
+| :---               | :---  | :---  |
+| unnamed scope      | `begin` ... `end`        | `{` ... `}`                                  |
+| function scope     | `function x()` ... `end` | `int x() {` ... `}`                          |
+| global scope       | `module MyMod` ... `end` | `namespace MyNS {` ... `}`                   |
+| software module    | A Julia "package"        | `.h`/`.hpp` files<br>+compiled `somelib.a`   |
+| assembling<br>software modules | `SomePkg.jl`: ...<br>`import("subfile1.jl")`<br>`import("subfile2.jl")`<br>... | `$(AR) *.o` &rArr; `somelib.a` |
+| import<br>software module | `import SomePkg`  | `#include <somelib>`<br>+link in `somelib.a` |
+| module library     | `LOAD_PATH[]`, \*Git repository,<br>\*\*custom package registry  | more `.h`/`.hpp` files<br>+bigger compiled `somebiglib.a` |
+
+\* The Julia package manager supports registering multiple packages from a single Git repository.<br>
+\* This allows users to house a library of related packages in a single repository.<br>
+\*\* Julia registries are primarily designed to provide versionning \& distribution of packages.<br>
+\*\* Custom package registries can be used to create a type of module library.
+
+
 ## Noteworthy differences from Common Lisp
 
 - Julia uses 1-based indexing for arrays by default, and it can also handle arbitrary [index offsets](@ref man-custom-indices).