New AddFlagSet function on FlagSet

This function allows one to create a different FlagSets and then easily
merge them together.

2 files changed, 30 insertions, 0 deletions

diff --git a/flag.go b/flag.go
index c99ccf3..603c81a 100644
--- a/flag.go
+++ b/flag.go
@@ -524,6 +524,19 @@ func (f *FlagSet) AddFlag(flag *Flag) {
 	f.shorthands[c] = flag
 }
 
+// AddFlagSet adds one FlagSet to another. If a flag is already present in f
+// the flag from newSet will be ignored
+func (f *FlagSet) AddFlagSet(newSet *FlagSet) {
+	if newSet == nil {
+		return
+	}
+	newSet.VisitAll(func(flag *Flag) {
+		if f.Lookup(flag.Name) == nil {
+			f.AddFlag(flag)
+		}
+	})
+}
+
 // Var defines a flag with the specified name and usage string. The type and
 // value of the flag are represented by the first argument, of type Value, which
 // typically holds a user-defined implementation of Value. For instance, the
diff --git a/flag_test.go b/flag_test.go
index 9ff1934..5114f28 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -110,6 +110,23 @@ func TestUsage(t *testing.T) {
 	}
 }
 
+func TestAddFlagSet(t *testing.T) {
+	oldSet := NewFlagSet("old", ContinueOnError)
+	newSet := NewFlagSet("new", ContinueOnError)
+
+	oldSet.String("flag1", "flag1", "flag1")
+	oldSet.String("flag2", "flag2", "flag2")
+
+	newSet.String("flag2", "flag2", "flag2")
+	newSet.String("flag3", "flag3", "flag3")
+
+	oldSet.AddFlagSet(newSet)
+
+	if len(oldSet.formal) != 3 {
+		t.Errorf("Unexpected result adding a FlagSet to a FlagSet %v", oldSet)
+	}
+}
+
 func TestAnnotation(t *testing.T) {
 	f := NewFlagSet("shorthand", ContinueOnError)