Support parsing value from ENV variable

README.md

@@ -153,6 +153,14 @@ completely replaced by calling `options.custom_help`. Note that you might
 also want to override the positional help by calling `options.positional_help`.
 
 
+## Value from ENV variable
+
+When parameter is not set, value will be fetched from an environment variable (if such variable is defined).
+
+```cpp
+cxxopts::value<int>()->env("MY_VAR")
+```
+
 ## Example
 
 Putting all together:

include/cxxopts.hpp

@@ -314,6 +314,15 @@ namespace cxxopts
     virtual std::shared_ptr<Value>
     implicit_value(const std::string& value) = 0;
 
+    virtual std::shared_ptr<Value>
+    env(const std::string& var) = 0;
+
+    virtual bool
+    has_env() const = 0;
+
+    virtual std::string
+    get_env_var() const = 0;
+
     virtual std::shared_ptr<Value>
     no_implicit_value() = 0;
 
@@ -888,6 +897,26 @@ namespace cxxopts
         return m_implicit_value;
       }
 
+      std::shared_ptr<Value>
+      env(const std::string& var)
+      {
+        m_env = true;
+        m_env_var = var;
+        return shared_from_this();
+      }
+
+      bool
+      has_env() const
+      {
+        return m_env;
+      }
+
+      std::string
+      get_env_var() const
+      {
+        return m_env_var;
+      }
+
       bool
       is_boolean() const
       {
@@ -913,9 +942,11 @@ namespace cxxopts
 
       bool m_default = false;
       bool m_implicit = false;
+      bool m_env = false;
 
       std::string m_default_value;
       std::string m_implicit_value;
+      std::string m_env_var;
     };
 
     template <typename T>
@@ -1961,6 +1992,15 @@ ParseResult::parse(int& argc, char**& argv)
     if(value.has_default() && !store.count() && !store.has_default()){
       parse_default(detail);
     }
+
+    if(value.has_env() && store.count() < 1){
+      const char* env = std::getenv(value.get_env_var().c_str());
+      if(env){
+        std::string str(env);
+        store.parse(detail, str);
+      }
+    }
+
   }
 
   if (consume_remaining)

test/options.cpp

@@ -774,3 +774,39 @@ TEST_CASE("Option add with add_option(string, Option)", "[options]") {
   CHECK(result["aggregate"].as<int>() == 4);
   CHECK(result["test"].as<int>() == 5);
 }
+
+TEST_CASE("Value from ENV variable", "[options]") {
+  cxxopts::Options options("Reads value from ENV", " - if no parameter value is passed");
+
+  options.add_options("", {
+    {"b,bar", "bar option", cxxopts::value<int>()->env("CXXOPTS_BAR")},
+    {"f,foo", "foo option", cxxopts::value<int>()->env("CXXOPTS_FOO")},
+    {"z,baz", "baz option", cxxopts::value<int>()->env("CXXOPTS_BAZ")->default_value("99")},
+    {"e,empty", "empty option", cxxopts::value<int>()->env("CXXOPTS_EMPTY")->default_value("1")},
+  });
+
+  putenv((char*)"CXXOPTS_FOO=7");
+  putenv((char*)"CXXOPTS_BAR=8");
+  putenv((char*)"CXXOPTS_BAZ=9");
+
+  Argv argv_({
+       "test",
+       "--foo",
+       "5"
+     });
+  auto argc = argv_.argc();
+  char** argv = argv_.argv();
+  auto result = options.parse(argc, argv);
+
+  CHECK(result.arguments().size()==1);
+  CHECK(options.groups().size() == 1);
+  CHECK(result.count("foo") == 1);
+  // value passed as an argument should take precedence
+  CHECK(result["foo"].as<int>() == 5);
+  // from ENV variable
+  CHECK(result["bar"].as<int>() == 8);
+    // from ENV variable
+  CHECK(result["baz"].as<int>() == 9);
+  // env not defined, should fallback to default value
+  CHECK(result["empty"].as<int>() == 1);
+}