[DOC] Copy over the cookbook entry about the custom validator (#62)

* [DOC] Copy over the cookbook entry about "how to build your own validator"

Signed-off-by: Lydia Buntrock <[email protected]>

* Apply suggestions from code review

Co-authored-by: Enrico Seiler <[email protected]>

doc/cookbook/index.md

@@ -17,3 +17,9 @@ please feel free to contact us.
 # Constructing a subcommand argument parser
 
 \include doc/howto/subcommand_argument_parser/subcommand_arg_parse.cpp
+
+# Write a custom validator {#cookbook_custom_validator}
+This recipe implements a validator that checks whether a numeric argument is an integral square (i.e. 0, 1, 4, 9...).
+Invalid values throw a sharg::validation_error.
+
+\snippet doc/tutorial/concepts/custom_validator_solution.cpp validator

doc/tutorial/argument_parser/index.md

@@ -418,7 +418,7 @@ input when parsing the command line. The sharg::argument_parser provides **valid
 A *validator* is a [functor](https://stackoverflow.com/questions/356950/what-are-c-functors-and-their-uses) that is
 called within the argument parser after retrieving and converting a command line argument. We provide several validators,
 which we hope cover most of the use cases, but you can always create your own validator
-(see section [Create your own validator](#section_create_your_own_validator)).
+(see section [Create your own validator](#cookbook_custom_validator)).
 
 \attention You can pass a validator to the sharg::argument_parser::add_option function only after passing the
 sharg::option_spec parameter. Pass the sharg::option_spec::standard tag if there are no further restrictions on your option.
@@ -432,8 +432,8 @@ The following validators are provided in the SeqAn library and can be included w
 All the validators below work on single values or a container of values. In case the variable is a container,
 the validator is called **on each element** separately.
 
-\note If the validators below do not suit your needs, you can always create your own validator. See the concept tutorial
-for an example of how to create your own validator.
+\note If the validators below do not suit your needs, you can always create your own validator
+(see section [Create your own validator](#cookbook_custom_validator)).
 
 ### The sharg::arithmetic_range_validator
 

doc/tutorial/concepts/custom_validator_solution.cpp

@@ -0,0 +1,54 @@
+#include <sharg/argument_parser.hpp>
+//![validator]
+#include <cmath>
+
+struct custom_validator
+{
+    using option_value_type = double; // used for all arithmetic types
+
+    void operator() (option_value_type const & val) const
+    {
+        if ((std::round(val)                         != val) ||  // not an integer
+            (std::pow(std::round(std::sqrt(val)), 2) != val))    // not a square
+        {
+            throw sharg::validation_error{"The provided number is not an arithmetic square."};
+        }
+    }
+
+    std::string get_help_page_message () const
+    {
+        return "Value must be the square of an integral number.";
+    }
+};
+//![validator]
+
+static_assert(sharg::validator<custom_validator>);
+
+//![main]
+int main(int argc, char ** argv)
+{
+    sharg::argument_parser myparser("Test-Parser", argc, argv);
+
+    int32_t variable{};
+    int16_t variable2{};
+
+    myparser.add_option(variable, 'i', "", "An int that is a square", sharg::option_spec::standard,
+                        custom_validator{}); // ← your validator is used!
+
+    myparser.add_option(variable2, 'j', "", "An int that is a square and within [0,20].", sharg::option_spec::standard,
+                        custom_validator{} | sharg::arithmetic_range_validator{0, 20}); // ← now it's chained
+
+    try
+    {
+         myparser.parse(); // trigger command line parsing
+    }
+    catch (sharg::argument_parser_error const & ext)
+    {
+        std::cout << ext.what() << '\n';
+        return -1;
+    }
+    std::cout << "Yeah!\n";
+
+    return 0;
+}
+//![main]

doc/tutorial/concepts/custom_validator_solution.out

@@ -0,0 +1,3 @@
+Test-Parser
+===========
+    Try -h or --help for more information.