Add helper to get exactly one size value

Small helper to convert a program argument to
size_t and throw an exception if it doesn't work.

Co-authored-by: Fotis Koutoulakis <[email protected]>
Co-authored-by: Hannes Steffenhagen <[email protected]>

Author: NlightNFotis

src/goto-harness/function_call_harness_generator.cpp

@@ -185,6 +185,15 @@ void function_call_harness_generatort::validate_options()
       "--" FUNCTION_HARNESS_GENERATOR_FUNCTION_OPT};
 }
 
+std::size_t function_call_harness_generatort::require_one_size_value(
+  const std::string &option,
+  const std::list<std::string> &values)
+{
+  static auto const string_value = require_exactly_one_value(option, values);
+  // TODO replace this with string2optional
+  return narrow<std::size_t>(std::stoull(string_value));
+}
+
 const symbolt &
 function_call_harness_generatort::implt::lookup_function_to_call()
 {

src/goto-harness/function_call_harness_generator.h

@@ -36,6 +36,9 @@ class function_call_harness_generatort : public goto_harness_generatort
   void validate_options() override;
 
 private:
+  std::size_t require_one_size_value(
+    const std::string &option,
+    const std::list<std::string> &values);
   struct implt;
   std::unique_ptr<implt> p_impl;
 };