use of string_view in as<T> method (#1187)

Address Issue #881, allowing use of string_view in the as<XX> method on
options.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: phlptp

include/CLI/Option.hpp

@@ -319,7 +319,7 @@ class Option : public OptionBase<Option> {
     /// complete Results of parsing
     results_t results_{};
     /// results after reduction
-    results_t proc_results_{};
+    mutable results_t proc_results_{};
     /// enumeration for the option state machine
     enum class option_state : char {
         parsing = 0,       //!< The option is currently collecting parsed results
@@ -700,7 +700,9 @@ class Option : public OptionBase<Option> {
             } else {
                 res = reduced_results();
             }
-            retval = detail::lexical_conversion<T, T>(res, output);
+            // store the results in a stable location if the output is a view
+            proc_results_ = std::move(res);
+            retval = detail::lexical_conversion<T, T>(proc_results_, output);
         }
         if(!retval) {
             throw ConversionError(get_name(), results_);

tests/OptionTypeTest.cpp

@@ -385,6 +385,40 @@ TEST_CASE_METHOD(TApp, "stringLikeTests", "[optiontype]") {
     CHECK("bca" == m_type.m_value);
 }
 
+#if CLI11_HAS_FILESYSTEM
+#include <string_view>
+// test code from https://github.com/CLIUtils/CLI11/issues/881
+// https://github.com/Jean1995
+TEST_CASE_METHOD(TApp, "AsStringView", "[app]") {
+    app.add_option("--input", "input option")->default_val("optA")->check(CLI::IsMember({"optA", "optB", "optC"}));
+
+    args = {};
+    run();
+    auto inputStr = app["--input"]->as<std::string_view>();
+    CHECK(inputStr == "optA");
+
+    args = {"--input", "optC"};
+    run();
+    inputStr = app["--input"]->as<std::string_view>();
+    CHECK(inputStr == "optC");
+}
+
+#endif
+
+TEST_CASE_METHOD(TApp, "AsStringRef", "[app]") {
+    app.add_option("--input", "input option")->default_val("optA")->check(CLI::IsMember({"optA", "optB", "optC"}));
+
+    args = {};
+    run();
+    const std::string &inputStr = app["--input"]->as<std::string>();
+    CHECK(inputStr == "optA");
+
+    args = {"--input", "optC"};
+    run();
+    const std::string &inputStr2 = app["--input"]->as<std::string>();
+    CHECK(inputStr2 == "optC");
+}
+
 TEST_CASE_METHOD(TApp, "VectorExpectedRange", "[optiontype]") {
     std::vector<std::string> strvec;