New flag integration

integrate.go

diff --git a/integrate.go b/integrate.go
new file mode 100644
index 0000000..1e97290
--- /dev/null
+++ b/integrate.go
@@ -0,0 +1,30 @@
+package properties
+
+import "flag"
+
+// MustFlag sets flags that are skipped by dst.Parse when p contains
+// the respective key for flag.Flag.Name.
+//
+// It's use is recommended with command line arguments as in:
+// 	flag.Parse()
+// 	p.MustFlag(flag.CommandLine)
+func (p *Properties) MustFlag(dst *flag.FlagSet) {
+	m := make(map[string]*flag.Flag)
+	dst.VisitAll(func(f *flag.Flag) {
+		m[f.Name] = f
+	})
+	dst.Visit(func(f *flag.Flag) {
+		delete(m, f.Name) // overridden
+	})
+
+	for name, f := range m {
+		v, ok := p.Get(name)
+		if !ok {
+			continue
+		}
+
+		if err := f.Value.Set(v); err != nil {
+			ErrorHandler(err)
+		}
+	}
+}

integrate_test.go

diff --git a/integrate_test.go b/integrate_test.go
new file mode 100644
index 0000000..90daaa2
--- /dev/null
+++ b/integrate_test.go
@@ -0,0 +1,70 @@
+package properties
+
+import (
+	"flag"
+	"fmt"
+	"testing"
+)
+
+// TestFlag verifies Properties.MustFlag without flag.FlagSet.Parse
+func TestFlag(t *testing.T) {
+	f := flag.NewFlagSet("src", flag.PanicOnError)
+	gotS := f.String("s", "?", "string flag")
+	gotI := f.Int("i", -1, "int flag")
+
+	p := NewProperties()
+	p.Set("s", "t")
+	p.Set("i", "9")
+	p.MustFlag(f)
+
+	if want := "t"; *gotS != want {
+		t.Errorf("Got string s=%q, want %q", *gotS, want)
+	}
+	if want := 9; *gotI != want {
+		t.Errorf("Got int i=%d, want %d", *gotI, want)
+	}
+}
+
+// TestFlagOverride verifies Properties.MustFlag with flag.FlagSet.Parse.
+func TestFlagOverride(t *testing.T) {
+	f := flag.NewFlagSet("src", flag.PanicOnError)
+	gotA := f.Int("a", 1, "remain default")
+	gotB := f.Int("b", 2, "customized")
+	gotC := f.Int("c", 3, "overridden")
+
+	f.Parse([]string{"-c", "4"})
+
+	p := NewProperties()
+	p.Set("b", "5")
+	p.Set("c", "6")
+	p.MustFlag(f)
+
+	if want := 1; *gotA != want {
+		t.Errorf("Got remain default a=%d, want %d", *gotA, want)
+	}
+	if want := 5; *gotB != want {
+		t.Errorf("Got customized b=%d, want %d", *gotB, want)
+	}
+	if want := 4; *gotC != want {
+		t.Errorf("Got overriden c=%d, want %d", *gotC, want)
+	}
+}
+
+func ExampleProperties_MustFlag() {
+	x := flag.Int("x", 0, "demo customize")
+	y := flag.Int("y", 0, "demo override")
+
+	// Demo alternative for flag.Parse():
+	flag.CommandLine.Parse([]string{"-y", "10"})
+	fmt.Printf("flagged as x=%d, y=%d\n", *x, *y)
+
+	p := NewProperties()
+	p.Set("x", "7")
+	p.Set("y", "42") // note discard
+	p.MustFlag(flag.CommandLine)
+	fmt.Printf("configured to x=%d, y=%d\n", *x, *y)
+
+	// Output:
+	// flagged as x=0, y=10
+	// configured to x=7, y=10
+}