add custom parse example

Author: phlptp

README.md

@@ -800,7 +800,7 @@ Every `add_` option you have seen so far depends on one method that takes a lamb
 
 Other values can be added as long as they support `operator>>` (and defaults can be printed if they support `operator<<`). To add a new type, for example, provide a custom `operator>>` with an `istream` (inside the CLI namespace is fine if you don't want to interfere with an existing `operator>>`).
 
-If you wanted to extend this to support a completely new type, use a lambda or add a specialization of the `lexical_cast` function template in the namespace `CLI::detail` with the type you need to convert to. Some examples of some new parsers for `complex<double>` that support all of the features of a standard `add_options` call are in [one of the tests](./tests/NewParseTest.cpp). A simpler example is shown below:
+If you wanted to extend this to support a completely new type, use a lambda or add a specialization of the `lexical_cast` function template in the namespace of the type you need to convert to. Some examples of some new parsers for `complex<double>` that support all of the features of a standard `add_options` call are in [one of the tests](./tests/NewParseTest.cpp). A simpler example is shown below:
 
 #### Example
 
@@ -872,6 +872,7 @@ The API is [documented here][api-docs]. Also see the [CLI11 tutorial GitBook][gi
 Several short examples of different features are included in the repository. A brief description of each is included here
 
  - [callback_passthrough](https://github.com/CLIUtils/CLI11/blob/master/examples/callback_passthrough.cpp): Example of directly passing remaining arguments through to a callback function which generates a CLI11 application based on existing arguments.
+ - [custom_parse](https://github.com/CLIUtils/CLI11/blob/master/examples/custom_parse.cpp):  Based on [Issue #566](https://github.com/CLIUtils/CLI11/issues/566), example of custom parser
  - [digit_args](https://github.com/CLIUtils/CLI11/blob/master/examples/digit_args.cpp):  Based on [Issue #123](https://github.com/CLIUtils/CLI11/issues/123), uses digit flags to pass a value
  - [enum](https://github.com/CLIUtils/CLI11/blob/master/examples/enum.cpp):  Using enumerations in an option, and the use of [CheckedTransformer](#transforming-validators)
  - [enum_ostream](https://github.com/CLIUtils/CLI11/blob/master/examples/enum_ostream.cpp):  In addition to the contents of example enum.cpp, this example shows how a custom ostream operator overrides CLI11's enum streaming.

examples/CMakeLists.txt

@@ -226,3 +226,8 @@ set_property(TEST retired_retired_test2 PROPERTY PASS_REGULAR_EXPRESSION "WARNIN
 set_property(TEST retired_retired_test3 PROPERTY PASS_REGULAR_EXPRESSION "WARNING.*retired")
 
 set_property(TEST retired_deprecated PROPERTY PASS_REGULAR_EXPRESSION "deprecated.*not_deprecated")
+
+#--------------------------------------------
+add_cli_exe(custom_parse custom_parse.cpp)
+add_test(NAME cp_test COMMAND custom_parse --dv 1.7)
+set_property(TEST cp_test PROPERTY PASS_REGULAR_EXPRESSION "called correct")

examples/custom_parse.cpp

@@ -0,0 +1,42 @@
+/** from Issue #566 on github https://github.com/CLIUtils/CLI11/issues/566 */
+
+#include <CLI/CLI.hpp>
+#include <iostream>
+#include <sstream>
+/** example file to demonstrate a custom lexical cast function*/
+
+template <class T = int> struct Values {
+    T a;
+    T b;
+    T c;
+};
+
+/** in C++20 this is constructible from a double due to the new aggregate initialization in C++20.*/
+using DoubleValues = Values<double>;
+
+auto &&operator>>(std::istringstream &in, Values<double> &v) {
+    std::string input;
+    in >> input;
+    std::cout << "called correct function " __FUNCTION__ " ! "
+                                                         "val: "
+              << input << std::endl;
+    return in;
+}
+// the lexical cast operator should be in the same namespace as the type for ADL to work properly
+bool lexical_cast(const std::string &input, Values<double> &v) {
+    std::cout << "called correct function " __FUNCTION__ " ! "
+                                                         "val: "
+              << input << std::endl;
+    return true;
+}
+
+DoubleValues doubles;
+void argparse(CLI::Option_group *group) { group->add_option("--dv", doubles)->default_str("0"); }
+
+int main(int argc, char **argv) {
+    CLI::App app;
+
+    argparse(app.add_option_group("param"));
+    CLI11_PARSE(app, argc, argv);
+    return 0;
+}