Remove static data from Configurator. (#696)

# Remove static data from `Configurator`.

Fixes #614 (partially)

---
# Change Description

Made the static data into singletonoid functions. The data in question
are:

- The list of configuration sources
- The true command line
- The pointer to the `AdditionalConfiguration` object

The command line values were combined into a struct to avoid passing
more than one return value.
The pointer to the additional configuration was set to a `nullptr`
initially, with null checks used when the pointer value is used.

---
# Test Description

The `Configurator_test` still passes.

core/src/Configurator.cpp

@@ -11,12 +11,6 @@
 
 namespace Nextsim {
 
-std::vector<std::unique_ptr<std::istream>> Configurator::sources;
-int Configurator::m_argc;
-char** Configurator::m_argv;
-static NoAdditionalConfiguration noAddConf;
-Configurator::AdditionalConfiguration* Configurator::p_addConf = &noAddConf;
-
 boost::program_options::variables_map Configurator::parse(
     const boost::program_options::options_description& opt)
 {
@@ -25,9 +19,9 @@ boost::program_options::variables_map Configurator::parse(
     // Parse the command file for any overrides
     int use_argc;
     char** use_argv;
-    if (m_argc && m_argv) {
-        use_argc = m_argc;
-        use_argv = m_argv;
+    if (commandLine().m_argc && commandLine().m_argv) {
+        use_argc = commandLine().m_argc;
+        use_argv = commandLine().m_argv;
     } else {
         // Use a fake command line to ensure at least one parse happens in all cases
         use_argc = 1;
@@ -44,7 +38,7 @@ boost::program_options::variables_map Configurator::parse(
     boost::program_options::store(parsed, vm);
 
     // Parse the named streams for configuration
-    for (auto iter = sources.begin(); iter != sources.end(); ++iter) {
+    for (auto iter = sources().begin(); iter != sources().end(); ++iter) {
         try {
             boost::program_options::store(
                 boost::program_options::parse_config_file(**iter, opt, true), vm);
@@ -66,12 +60,12 @@ void Configurator::addSStream(const std::stringstream& sstream)
     addStream(std::move(std::unique_ptr<std::istream>(new std::stringstream(sstream.str()))));
 }
 
-void Configurator::setAdditionalConfiguration(AdditionalConfiguration* pAC) { p_addConf = pAC; }
+void Configurator::setAdditionalConfiguration(AdditionalConfiguration* pAC) { addConf() = pAC; }
 
 void Configurator::getAdditionalConfiguration(const std::string& source)
 {
-    if (p_addConf) {
-        addSStream(p_addConf->read(source));
+    if (addConf()) {
+        addSStream(addConf()->read(source));
     }
 }
 

core/src/include/Configurator.hpp

@@ -67,7 +67,7 @@ class Configurator {
      */
     inline static void addStream(std::unique_ptr<std::istream> pis)
     {
-        sources.push_back(std::move(pis));
+        sources().push_back(std::move(pis));
     }
     /*!
      * @brief Adds several istream sources of configuration data.
@@ -94,7 +94,7 @@ class Configurator {
     /*!
      * Removes previously assigned stream data sources, both files and istreams.
      */
-    inline static void clearStreams() { sources.clear(); }
+    inline static void clearStreams() { sources().clear(); }
 
     /*!
      * Removes all data sources, both streams and command line.
@@ -117,8 +117,8 @@ class Configurator {
      */
     inline static void setCommandLine(int argc, char* argv[])
     {
-        m_argc = argc;
-        m_argv = argv;
+        commandLine().m_argc = argc;
+        commandLine().m_argv = argv;
     }
 
     /*!
@@ -145,10 +145,21 @@ class Configurator {
         const boost::program_options::options_description& opt);
 
 private:
-    static std::vector<std::unique_ptr<std::istream>> sources;
+    static std::vector<std::unique_ptr<std::istream>>& sources()
+    {
+        static std::vector<std::unique_ptr<std::istream>> sources;
+        return sources;
+    }
 
-    static int m_argc;
-    static char** m_argv;
+    struct CommandLine {
+        int m_argc;
+        char** m_argv;
+    };
+    static CommandLine& commandLine()
+    {
+        static CommandLine cmdln;
+        return cmdln;
+    }
 
 public:
     class AdditionalConfiguration {
@@ -171,12 +182,11 @@ class Configurator {
     static void setAdditionalConfiguration(AdditionalConfiguration* pAC);
 
 private:
-    static AdditionalConfiguration* p_addConf;
-};
-
-//! A default implementation of Configurator::AdditionalConfiguration
-class NoAdditionalConfiguration : public Configurator::AdditionalConfiguration {
-    std::stringstream read(const std::string& source) override { return std::stringstream(); }
+    static AdditionalConfiguration*& addConf()
+    {
+        static AdditionalConfiguration* p_addConf = nullptr;
+        return p_addConf;
+    }
 };
 
 } /* namespace Nextsim */