Add environment variable processing to the configuration pointer. (#891)

Fixes #890 

Add parsing of environmental variables when supplied for the config file
option.

---------

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

include/CLI/StringTools.hpp

@@ -223,6 +223,8 @@ CLI11_INLINE std::size_t escape_detect(std::string &str, std::size_t offset);
 /// Add quotes if the string contains spaces
 CLI11_INLINE std::string &add_quotes_if_needed(std::string &str);
 
+/// get the value of an environmental variable or empty string if empty
+CLI11_INLINE std::string get_environment_value(const std::string &env_name);
 }  // namespace detail
 
 // [CLI11:string_tools_hpp:end]

include/CLI/TypeTools.hpp

@@ -1333,6 +1333,9 @@ template <class AssignTo,
                       detail::enabler> = detail::dummy>
 bool lexical_conversion(const std::vector<std ::string> &strings, AssignTo &output) {
     output.erase(output.begin(), output.end());
+    if(strings.empty()) {
+        return true;
+    }
     if(strings.size() == 1 && strings[0] == "{}") {
         return true;
     }

include/CLI/impl/App_inl.hpp

@@ -315,8 +315,11 @@ CLI11_INLINE Option *App::set_config(std::string option_name,
         }
         if(!default_filename.empty()) {
             config_ptr_->default_str(std::move(default_filename));
+            config_ptr_->force_callback_ = true;
         }
         config_ptr_->configurable(false);
+        // set the option to take the last value given by default
+        config_ptr_->take_last();
     }
 
     return config_ptr_;
@@ -1014,10 +1017,18 @@ CLI11_INLINE void App::_process_config_file() {
     if(config_ptr_ != nullptr) {
         bool config_required = config_ptr_->get_required();
         auto file_given = config_ptr_->count() > 0;
+        if(!(file_given || config_ptr_->envname_.empty())) {
+            std::string ename_string = detail::get_environment_value(config_ptr_->envname_);
+            if(!ename_string.empty()) {
+                config_ptr_->add_result(ename_string);
+            }
+        }
+        config_ptr_->run_callback();
+
         auto config_files = config_ptr_->as<std::vector<std::string>>();
         if(config_files.empty() || config_files.front().empty()) {
             if(config_required) {
-                throw FileError::Missing("no specified config file");
+                throw FileError("config file is required but none was given");
             }
             return;
         }
@@ -1028,9 +1039,6 @@ CLI11_INLINE void App::_process_config_file() {
                 try {
                     std::vector<ConfigItem> values = config_formatter_->from_file(config_file);
                     _parse_config(values);
-                    if(!file_given) {
-                        config_ptr_->add_result(config_file);
-                    }
                 } catch(const FileError &) {
                     if(config_required || file_given)
                         throw;
@@ -1045,23 +1053,7 @@ CLI11_INLINE void App::_process_config_file() {
 CLI11_INLINE void App::_process_env() {
     for(const Option_p &opt : options_) {
         if(opt->count() == 0 && !opt->envname_.empty()) {
-            char *buffer = nullptr;
-            std::string ename_string;
-
-#ifdef _MSC_VER
-            // Windows version
-            std::size_t sz = 0;
-            if(_dupenv_s(&buffer, &sz, opt->envname_.c_str()) == 0 && buffer != nullptr) {
-                ename_string = std::string(buffer);
-                free(buffer);
-            }
-#else
-            // This also works on Windows, but gives a warning
-            buffer = std::getenv(opt->envname_.c_str());
-            if(buffer != nullptr)
-                ename_string = std::string(buffer);
-#endif
-
+            std::string ename_string = detail::get_environment_value(opt->envname_);
             if(!ename_string.empty()) {
                 opt->add_result(ename_string);
             }

include/CLI/impl/StringTools_inl.hpp

@@ -255,6 +255,27 @@ CLI11_INLINE std::string &add_quotes_if_needed(std::string &str) {
     return str;
 }
 
+std::string get_environment_value(const std::string &env_name) {
+    char *buffer = nullptr;
+    std::string ename_string;
+
+#ifdef _MSC_VER
+    // Windows version
+    std::size_t sz = 0;
+    if(_dupenv_s(&buffer, &sz, env_name.c_str()) == 0 && buffer != nullptr) {
+        ename_string = std::string(buffer);
+        free(buffer);
+    }
+#else
+    // This also works on Windows, but gives a warning
+    buffer = std::getenv(env_name.c_str());
+    if(buffer != nullptr) {
+        ename_string = std::string(buffer);
+    }
+#endif
+    return ename_string;
+}
+
 }  // namespace detail
 // [CLI11:string_tools_inl_hpp:end]
 }  // namespace CLI

tests/ConfigFileTest.cpp

@@ -673,6 +673,36 @@ TEST_CASE_METHOD(TApp, "IniNotRequiredNotDefault", "[config]") {
     CHECK(tmpini2.c_str() == app.get_config_ptr()->as<std::string>());
 }
 
+TEST_CASE_METHOD(TApp, "IniEnvironmentalFileName", "[config]") {
+
+    TempFile tmpini{"TestIniTmp.ini"};
+
+    app.set_config("--config", "")->envname("CONFIG")->required();
+
+    {
+        std::ofstream out{tmpini};
+        out << "[default]" << std::endl;
+        out << "two=99" << std::endl;
+        out << "three=3" << std::endl;
+    }
+
+    int one{0}, two{0}, three{0};
+    app.add_option("--one", one);
+    app.add_option("--two", two);
+    app.add_option("--three", three);
+
+    put_env("CONFIG", tmpini);
+
+    CHECK_NOTHROW(run());
+
+    CHECK(two == 99);
+    CHECK(three == 3);
+
+    unset_env("CONFIG");
+
+    CHECK_THROWS_AS(run(), CLI::FileError);
+}
+
 TEST_CASE_METHOD(TApp, "MultiConfig", "[config]") {
 
     TempFile tmpini{"TestIniTmp.ini"};

tests/HelpersTest.cpp

@@ -1354,3 +1354,14 @@ TEST_CASE("FixNewLines: EdgesCheck", "[helpers]") {
     std::string result = CLI::detail::fix_newlines("; ", input);
     CHECK(output == result);
 }
+
+TEST_CASE("String: environment", "[helpers]") {
+    put_env("TEST1", "TESTS");
+
+    auto value = CLI::detail::get_environment_value("TEST1");
+    CHECK(value == "TESTS");
+    unset_env("TEST1");
+
+    value = CLI::detail::get_environment_value("TEST2");
+    CHECK(value.empty());
+}