From 2d6fe421413b117fb06faf7c2a26bf6318882f1f Mon Sep 17 00:00:00 2001
From: Pranav Srinivas Kumar <pranav.srinivas.kumar@gmail.com>
Date: Sun, 31 Mar 2019 13:06:55 -0400
Subject: [PATCH] Update README.md

---
 README.md | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/README.md b/README.md
index 9d850c8..28ac092 100644
--- a/README.md
+++ b/README.md
@@ -145,3 +145,23 @@ auto b = program.get<bool>("-b");                               // true
 auto c = program.get<std::vector<float>>("-c");                 // {3.14f, 2.718f}
 auto files = program.get<std::vector<std::string>>("--files");  // {"a.txt", "b.txt", "c.txt"}
 ```
+
+### Restricting the set of values for an argument
+
+```cpp
+argparse::ArgumentParser program("test");
+
+program.add_argument("input")
+  .default_value("baz")
+  .action([=](const std::string& value) {
+    static const std::vector<std::string> choices = { "foo", "bar", "baz" };
+    if (std::find(choices.begin(), choices.end(), value) != choices.end()) {
+      return value;
+    }
+    return std::string{ "baz" };
+  });
+
+program.parse_args({ "./test", "fez" });
+
+auto input = program.get("input"); // baz
+```