zigからC/C++を呼び出す方法

# 初めに
今回用いるzigのversionは以下の通りです.
```
$ zig versoin
0.12.0-dev.2825+dd1fc1cb8
``` 

# 呼び出し方
今回はc言語とc++の両方をzigから呼び出してみたいと思います.
## c言語の準備
``` hoge.h
void hoge(void);
```

``` hoge.c
#include <stdio.h>
#include "hoge.h"

void hoge(void) {
	printf("Hello, World! in C\n");
}
```
## C++の準備
``` foo.h
#ifdef __cplusplus
extern "C" {
#endif

void foo(void);

#ifdef __cplusplus
}
#endif
```

``` foo.cpp
#include <iostream>
#include "foo.h"

void foo(void) {
  std::cout << "Hello from C++!" << std::endl;
}
```
## zigのbuild.zigの追記
``` build.zig
const std = @import("std");

// Although this function looks imperative, note that its job is to
// declaratively construct a build graph that will be executed by an external
// runner.
pub fn build(b: *std.Build) void {
    // Standard target options allows the person running `zig build` to choose
    // what target to build for. Here we do not override the defaults, which
    // means any target is allowed, and the default is native. Other options
    // for restricting supported target set are available.
    const target = b.standardTargetOptions(.{});

    // Standard optimization options allow the person running `zig build` to select
    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
    // set a preferred release mode, allowing the user to decide how to optimize.
    const optimize = b.standardOptimizeOption(.{});

    const lib = b.addStaticLibrary(.{
        .name = "hoge",
        // In this case the main source file is merely a path, however, in more
        // complicated build scripts, this could be a generated file.
        .root_source_file = .{ .path = "src/root.zig" },
        .target = target,
        .optimize = optimize,
    });

    // This declares intent for the library to be installed into the standard
    // location when the user invokes the "install" step (the default step when
    // running `zig build`).
    b.installArtifact(lib);

    const exe = b.addExecutable(.{
        .name = "hoge",
        .root_source_file = .{ .path = "src/main.zig" },
        .target = target,
        .optimize = optimize,
    });
    exe.addIncludePath(.{ .path = "src" }); // include pathの追加
    exe.addCSourceFiles(.{
        .files = &.{
            "src/hoge.c",
            "src/foo.cpp",
        },
    }); // cもcppもここに突っ込んで大丈夫
    // addCppSourceFilesみたいな関数はない
    
    exe.linkLibC(); // cの標準ライブラリ使うときはこれをつける
    exe.linkLibCpp(); // cppの標準のライブラリを使うときはこれをつける
    // This declares intent for the executable to be installed into the
    // standard location when the user invokes the "install" step (the default
    // step when running `zig build`).
    b.installArtifact(exe);

    // This *creates* a Run step in the build graph, to be executed when another
    // step is evaluated that depends on it. The next line below will establish
    // such a dependency.
    const run_cmd = b.addRunArtifact(exe);

    // By making the run step depend on the install step, it will be run from the
    // installation directory rather than directly from within the cache directory.
    // This is not necessary, however, if the application depends on other installed
    // files, this ensures they will be present and in the expected location.
    run_cmd.step.dependOn(b.getInstallStep());

    // This allows the user to pass arguments to the application in the build
    // command itself, like this: `zig build run -- arg1 arg2 etc`
    if (b.args) |args| {
        run_cmd.addArgs(args);
    }

    // This creates a build step. It will be visible in the `zig build --help` menu,
    // and can be selected like this: `zig build run`
    // This will evaluate the `run` step rather than the default, which is "install".
    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_cmd.step);

    // Creates a step for unit testing. This only builds the test executable
    // but does not run it.
    const lib_unit_tests = b.addTest(.{
        .root_source_file = .{ .path = "src/root.zig" },
        .target = target,
        .optimize = optimize,
    });

    const run_lib_unit_tests = b.addRunArtifact(lib_unit_tests);

    const exe_unit_tests = b.addTest(.{
        .root_source_file = .{ .path = "src/main.zig" },
        .target = target,
        .optimize = optimize,
    });

    const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests);

    // Similar to creating the run step earlier, this exposes a `test` step to
    // the `zig build --help` menu, providing a way for the user to request
    // running the unit tests.
    const test_step = b.step("test", "Run unit tests");
    test_step.dependOn(&run_lib_unit_tests.step);
    test_step.dependOn(&run_exe_unit_tests.step);
}
```
## 呼び出し方
``` main.zig
const std = @import("std");
const c = @cImport({
    @cInclude("hoge.h");
    @cInclude("foo.h");
});

pub fn main() !void {
    c.hoge();
    c.foo();
}
```

# 最後に
オプションどうやって書くのかは追記します