Original version from @lczech

examples/CMakeLists.txt

@@ -20,3 +20,5 @@ add_cli_exe(groups groups.cpp)
 add_cli_exe(inter_argument_order inter_argument_order.cpp)
 add_cli_exe(prefix_command prefix_command.cpp)
 add_cli_exe(enum enum.cpp)
+
+add_subdirectory(subcom_in_files)

examples/subcom_in_files/CMakeLists.txt

@@ -0,0 +1 @@
+add_cli_exe(main main.cpp subcommand_a.cpp subcommand_a.hpp)

examples/subcom_in_files/main.cpp

@@ -0,0 +1,20 @@
+// ===================================================================
+//   main.cpp
+// ===================================================================
+
+#include "subcommand_a.hpp"
+
+int main( int argc, char** argv ) {
+    CLI::App app{"..."};
+
+    // Call the setup functions for the subcommands.
+    // They return their option structs, so that the variables to which the options bind
+    // are still alive when `app.parse()` is called.
+    auto subcommand_a_opt = setup_subcommand_a( app );
+
+    // More setup if needed, i.e., other subcommands etc.
+
+    CLI11_PARSE(app, argc, argv);
+
+    return 0;
+}

examples/subcom_in_files/subcommand_a.cpp

@@ -0,0 +1,39 @@
+// ===================================================================
+//   subcommand_a.cpp
+// ===================================================================
+
+#include "subcommand_a.hpp"
+
+/// Set up a subcommand and return a unique ptr to a struct that holds all its options.
+/// The variables of the struct are bound to the CLI options.
+/// We use a unique ptr so that the addresses of the variables are stable for binding,
+/// and return it to the caller (main), so that the object itself stays alive.
+std::unique_ptr<SubcommandAOptions> setup_subcommand_a( CLI::App& app )
+{
+    // Create the option and subcommand objects.
+    auto opt = std::unique_ptr<SubcommandAOptions>(new SubcommandAOptions());
+    auto sub = app.add_subcommand( "subcommand_a", "performs subcommand a", true );
+
+    // Add options to sub, binding them to opt.
+    sub->add_option("-f,--file", opt->file, "File name");
+    sub->add_flag("--with-foo", opt->with_foo, "Counter");
+
+    // Set the run function as callback to be called when this subcommand is issued.
+    sub->set_callback( [&]() {
+        run_subcommand_a( *opt );
+    });
+
+    return opt;
+}
+
+/// The function that runs our code.
+/// This could also simply be in the callback lambda itself,
+/// but having a separate function is cleaner.
+void run_subcommand_a( SubcommandAOptions const& opt )
+{
+    // Do stuff...
+    std::cout << "Working on file: " << opt.file << std::endl;
+    if( opt.with_foo ) {
+        std::cout << "Using foo!" << std::endl;
+    }
+}

examples/subcom_in_files/subcommand_a.hpp

@@ -0,0 +1,18 @@
+// ===================================================================
+//   subcommand_a.hpp
+// ===================================================================
+
+#include "CLI/CLI.hpp"
+#include <memory>
+#include <string>
+
+/// Collection of all options of Subcommand A.
+struct SubcommandAOptions
+{
+    std::string file;
+    bool        with_foo;
+};
+
+// Function declarations.
+std::unique_ptr<SubcommandAOptions> setup_subcommand_a( CLI::App& app );
+void run_subcommand_a( SubcommandAOptions const& opt );