Ensure convert directory appears in Dirs

This is attempting to track down a bug where convert.h isn't generated
properly in some cases.
Sadly, this test passes, so it isn't caused by a missing directory.

internal/gen/config/cpp/cpp.go

@@ -16,6 +16,31 @@ type Config struct {
 	Catkin   *catkin.Config     `xml:"catkin"`   // Catkin contains Catkin generator configurations.
 }
 
+// New constructs a Config programmatically with the given variant and options.
+func New(variant Variant, options ...Option) *Config {
+	var cfg Config
+
+	cfg.Variant = variant
+
+	pcfg := &cfg
+
+	for _, option := range options {
+		option(pcfg)
+	}
+
+	return pcfg
+}
+
+// Option is a functional option for building a Config.
+type Option func(*Config)
+
+// WithChannel binds channel name to type ty in the configuration.
+func WithChannel(name, ty string) Option {
+	return func(config *Config) {
+		config.Channels = append(config.Channels, Channel{Name: name, Type: ty})
+	}
+}
+
 // ChannelMap gets the Channels field of this Config as a map from channel names to type overrides.
 func (c *Config) ChannelMap() map[string]string {
 	cmap := make(map[string]string, len(c.Channels))

internal/gen/cpp/cpp_test.go

@@ -0,0 +1,62 @@
+package cpp_test
+
+import (
+	"path/filepath"
+	"reflect"
+	"sort"
+	"testing"
+
+	cfg "github.com/UoY-RoboStar/rtcg/internal/gen/config/cpp"
+	"github.com/UoY-RoboStar/rtcg/internal/gen/cpp"
+	"github.com/UoY-RoboStar/rtcg/internal/gen/gencommon"
+	"github.com/UoY-RoboStar/rtcg/internal/stm"
+)
+
+// TestOnSuite_Dirs tests the Dirs method of OnSuite.
+func TestOnSuite_Dirs(t *testing.T) {
+	t.Parallel()
+
+	rtcgConvertDir := filepath.Join("out", "src", "convert")
+	rtcgSrcDir := filepath.Join("out", "src", "rtcg")
+
+	for name, input := range map[string]struct {
+		config *cfg.Config
+		want   []string
+	}{
+		"empty-animate": {
+			config: cfg.New(cfg.VariantAnimate),
+			want:   []string{rtcgSrcDir},
+		},
+		"empty-animate-convert": {
+			config: cfg.New(cfg.VariantAnimate, cfg.WithChannel("foo", "bar")),
+			want:   []string{rtcgConvertDir, rtcgSrcDir},
+		},
+		// TODO: add tests for non-empty cases
+	} {
+		config := input.config
+		want := input.want
+
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			dirs := gencommon.DirSet{Input: "in", Output: "out"}
+
+			gen, err := cpp.New(config, dirs)
+			if err != nil {
+				t.Fatalf("unexpected error creating generator: %s", err)
+			}
+
+			var suite stm.Suite
+			onSuite := gen.OnSuite(&suite)
+
+			got := onSuite.Dirs()
+
+			sort.Strings(got)
+			sort.Strings(want)
+
+			if !reflect.DeepEqual(got, want) {
+				t.Errorf("unexpected directories: got %v, want %v", got, want)
+			}
+		})
+	}
+}