ParmParse: Type Hints (#4364)

## Summary

Today, we use the `ParmParse` object not only anymore to read text-based
key-value pairs from input files and CLI options, but additionally and
sometimes exclusively populate it heavily from the C++ and Python APIs
directly.

This proposes a concept to store the type of the latest `add` with each
entry, which can help to expose `ParmParse` options, e.g., when
exporting them to Python dictionaries and other file formats (YAML,
TOML, JSON, XML, etc.). When a typehint is found, we can cast to the
correct type automatically for the value, instead of returning a string.

## Additional background

AMReX-Codes/pyamrex#417 (comment)

## Checklist

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [x] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Author: ax3l

Src/Base/AMReX_ParmParse.H

@@ -3,8 +3,10 @@
 #include <AMReX_Config.H>
 
 #include <AMReX_BLassert.H>
+#include <AMReX_Box.H>
 #include <AMReX_Enum.H>
 #include <AMReX_INT.H>
+#include <AMReX_IntVect.H>
 #include <AMReX_IParser.H>
 #include <AMReX_Parser.H>
 #include <AMReX_TypeTraits.H>
@@ -15,7 +17,9 @@
 #include <set>
 #include <string>
 #include <string_view>
+#include <type_traits>
 #include <unordered_map>
+#include <variant>
 #include <vector>
 
 namespace amrex {
@@ -1633,6 +1637,17 @@ public:
         // vector<vector<std::string>>.
         std::vector<std::vector<std::string>> m_vals;
         mutable Long                          m_count = 0;
+        std::variant<
+            std::string*,
+            bool*,
+            int*,
+            long*,
+            long long*,
+            amrex::IntVect*,
+            amrex::Box*,
+            float*,
+            double*
+        > m_typehint = static_cast<std::string*>(nullptr);
     };
     using Table = std::unordered_map<std::string, PP_entry>;
 

Src/Base/AMReX_ParmParse.cpp

@@ -864,6 +864,8 @@ saddval (const std::string& name, const T& ref)
     auto& entry = g_table[name];
     entry.m_vals.emplace_back(std::vector<std::string>{val.str()});
     ++entry.m_count;
+    using T_ptr = std::decay_t<T>*;
+    entry.m_typehint = static_cast<T_ptr>(nullptr);
 }
 
 template <class T>
@@ -881,6 +883,8 @@ saddarr (const std::string& name, const std::vector<T>& ref)
     auto& entry = g_table[name];
     entry.m_vals.emplace_back(std::move(arr));
     ++entry.m_count;
+    using T_ptr = std::decay_t<T>*;
+    entry.m_typehint = static_cast<T_ptr>(nullptr);
 }
 
 // Initialize ParmParse.