ENH: refine SpanStream and CharStream methods

- support std::string_view (c++17) or span view (older c++) of stream
  buffer contents. This simplifies formatting + reparsing.

  Example,

      OCharStream os;
      os << ...;

      ISpanStream is(os.view());
      is >> ...;

- additional release() method for ICharStream, OCharStream
  that returns the contents as a DynamicList<char> and resets the stream.

- provide a str() method for API compatibility with older
  std::ostringstream etc.

applications/test/ICharStream1/Test-ICharStream1.C

@@ -37,12 +37,28 @@ Description
 
 using namespace Foam;
 
-Ostream& writeList(Ostream& os, const UList<char>& list)
+Ostream& printString(Ostream& os, const char* first, const char* last)
+{
+    os << '"';
+    for (; first != last; (void)++first)
+    {
+        os << *first;
+    }
+    os << '"';
+
+    return os;
+}
+
+
+Ostream& printView(Ostream& os, const char* first, const char* last)
 {
     char buf[4];
-    os << list.size() << '(';
+    os << label(last-first) << '(';
-    for (const char c : list)
+
+    for (; first != last; (void)++first)
     {
+        const char c = *first;
+
         if (isprint(c))
         {
             os << c;
@@ -67,16 +83,35 @@ Ostream& writeList(Ostream& os, const UList<char>& list)
 }
 
 
+#if __cplusplus >= 201703L
+Ostream& printView(Ostream& os, std::string_view s)
+{
+    return printView(os, s.begin(), s.end());
+}
+#endif
+
+
+Ostream& printView(Ostream& os, stdFoam::span<char> s)
+{
+    return printView(os, s.begin(), s.end());
+}
+
+
+Ostream& printView(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
+Ostream& writeList(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
 Ostream& toString(Ostream& os, const UList<char>& list)
 {
-    os << '"';
+    return printString(os, list.begin(), list.end());
-    for (const char c : list)
-    {
-        os << c;
-    }
-    os << '"';
-
-    return os;
 }
 
 
@@ -85,6 +120,7 @@ void printInfo(const BufType& buf)
 {
     Info<< nl << "=========================" << endl;
     buf.print(Info);
+    Info<< "addr: " << Foam::name(buf.list().cdata()) << nl;
     toString(Info, buf.list());
     Info<< nl << "=========================" << endl;
 }
@@ -136,14 +172,16 @@ int main(int argc, char *argv[])
 
     Info<< "transfer contents to a List" << endl;
 
-    ICharStream ibuf;
-
     // Reclaim data storage from OCharStream -> ICharStream
-    {
+    ICharStream ibuf(std::move(obuf));
-        List<char> data;
+
-        obuf.swap(data);
+    // OLD
-        ibuf.swap(data);
+    // ICharStream ibuf;
-    }
+    // {
+    //     List<char> data;
+    //     obuf.swap(data);
+    //     ibuf.swap(data);
+    // }
 
     Info<< nl;
     Info<< nl << "input string:";

applications/test/OCharStream1/Test-OCharStream1.C

@@ -37,12 +37,28 @@ Description
 
 using namespace Foam;
 
-Ostream& writeList(Ostream& os, const UList<char>& list)
+Ostream& printString(Ostream& os, const char* first, const char* last)
+{
+    os << '"';
+    for (; first != last; (void)++first)
+    {
+        os << *first;
+    }
+    os << '"';
+
+    return os;
+}
+
+
+Ostream& printView(Ostream& os, const char* first, const char* last)
 {
     char buf[4];
-    os << list.size() << '(';
+    os << label(last-first) << '(';
-    for (const char c : list)
+
+    for (; first != last; (void)++first)
     {
+        const char c = *first;
+
         if (isprint(c))
         {
             os << c;
@@ -67,16 +83,35 @@ Ostream& writeList(Ostream& os, const UList<char>& list)
 }
 
 
+#if __cplusplus >= 201703L
+Ostream& printView(Ostream& os, std::string_view s)
+{
+    return printView(os, s.begin(), s.end());
+}
+#endif
+
+
+Ostream& printView(Ostream& os, stdFoam::span<char> s)
+{
+    return printView(os, s.begin(), s.end());
+}
+
+
+Ostream& printView(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
+Ostream& writeList(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list);
+}
+
+
 Ostream& toString(Ostream& os, const UList<char>& list)
 {
-    os << '"';
+    return printString(os, list.begin(), list.end());
-    for (const char c : list)
-    {
-        os << c;
-    }
-    os << '"';
-
-    return os;
 }
 
 
@@ -85,6 +120,7 @@ void printInfo(const BufType& buf)
 {
     Info<< nl << "=========================" << endl;
     buf.print(Info);
+    Info<< "addr: " << Foam::name(buf.list().cdata()) << nl;
     toString(Info, buf.list());
     Info<< nl << "=========================" << endl;
 }
@@ -152,11 +188,6 @@ int main(int argc, char *argv[])
 
     printInfo(obuf);
 
-    obuf.shrink();
-
-    Info<< "after shrink" << nl;
-    printInfo(obuf);
-
     // Add some more
     for (label i=10; i < 15; ++i)
     {
@@ -175,13 +206,16 @@ int main(int argc, char *argv[])
 
     Info<< "transfer contents to a List or ICharStream" << nl;
 
-    ICharStream ibuf;
     // Reclaim data storage from OCharStream -> ICharStream
-    {
+    ICharStream ibuf(std::move(obuf));
-        List<char> data;
+
-        obuf.swap(data);
+    // OLD
-        ibuf.swap(data);
+    // ICharStream ibuf;
-    }
+    // {
+    //     List<char> data;
+    //     obuf.swap(data);
+    //     ibuf.swap(data);
+    // }
 
     Info<<"original:";
     printInfo(obuf);
@@ -253,6 +287,14 @@ int main(int argc, char *argv[])
 
         Info<< "Compact" << nl;
         printInfo(os2);
+
+        Info<< "address: " << Foam::name(os2.list().cdata()) << nl;
+
+        List<char> chars(os2.release());
+        Info<< "chars: " << chars.size() << nl;
+        Info<< "address: " << Foam::name(chars.cdata()) << nl;
+        Info<< "release" << nl;
+        printInfo(os2);
     }
 
 

applications/test/SpanStream1/Test-SpanStream1.C

@@ -39,12 +39,28 @@ Description
 
 using namespace Foam;
 
-Ostream& writeList(Ostream& os, const UList<char>& list)
+Ostream& printString(Ostream& os, const char* first, const char* last)
+{
+    os << '"';
+    for (; first != last; (void)++first)
+    {
+        os << *first;
+    }
+    os << '"';
+
+    return os;
+}
+
+
+Ostream& printView(Ostream& os, const char* first, const char* last)
 {
     char buf[4];
-    os << list.size() << '(';
+    os << label(last-first) << '(';
-    for (const char c : list)
+
+    for (; first != last; (void)++first)
     {
+        const char c = *first;
+
         if (isprint(c))
         {
             os << c;
@@ -69,29 +85,41 @@ Ostream& writeList(Ostream& os, const UList<char>& list)
 }
 
 
+#if __cplusplus >= 201703L
+Ostream& printView(Ostream& os, std::string_view s)
+{
+    return printView(os, s.begin(), s.end());
+}
+#endif
+
+
+Ostream& printView(Ostream& os, stdFoam::span<char> s)
+{
+    return printView(os, s.begin(), s.end());
+}
+
+
+Ostream& printView(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
+Ostream& writeList(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
 Ostream& toString(Ostream& os, const UList<char>& list)
 {
-    os << '"';
+    return printString(os, list.begin(), list.end());
-    for (const char c : list)
-    {
-        os << c;
-    }
-    os << '"';
-
-    return os;
 }
 
 
 Ostream& toString(Ostream& os, const std::vector<char>& list)
 {
-    os << '"';
+    return printString(os, list.data(), list.data() + list.size());
-    for (const char c : list)
-    {
-        os << c;
-    }
-    os << '"';
-
-    return os;
 }
 
 

src/OpenFOAM/db/IOstreams/memory/ICharStream.H

@@ -53,6 +53,7 @@ namespace Foam
 // Forward Declarations
 class icharstream;
 class ICharStream;
+class OCharStream;
 
 // Older names (prior to 2023-08)
 typedef ICharStream IListStream;
@@ -93,14 +94,6 @@ public:
             reset(buffer, nbytes);
         }
 
-        // //- Construct (deep copy) from span character content
-        // explicit icharstream(stdFoam::span<char> s)
-        // :
-        //     icharstream()
-        // {
-        //     reset(buffer, nbytes);
-        // }
-
         //- Move construct from List
         icharstream(List<char>&& buffer)
         :
@@ -143,7 +136,28 @@ public:
         //- Span of the input characters (is modifiable!)
         UList<char> list() const
         {
-            return buffer_type::span_list();
+            return UList<char>
+            (
+                buffer_type::data_bytes(),
+                label(buffer_type::size_bytes())
+            );
+        }
+
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(buffer_type::view())
+        {
+            return buffer_type::view();
+        }
+
+        //- For istringstream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        std::string str() const
+        {
+            return std::string
+            (
+                buffer_type::data_bytes(),
+                buffer_type::size_bytes()
+            );
         }
 
         //- Rewind the stream, clearing any old errors
@@ -153,21 +167,21 @@ public:
             stream_type::clear();  // Clear old errors
         }
 
-        //- Reset content (copy)
+        //- Reset stream content (copy), reset positions
         void reset(const char* buffer, size_t nbytes)
         {
             buffer_type::reset(buffer, nbytes);
             stream_type::clear();  // Clear old errors
         }
 
-        //- Transfer list contents to List buffer
+        //- Exchange stream content and parameter contents, reset positions
         void swap(List<char>& other)
         {
             buffer_type::swap(other);
             stream_type::clear();  // Clear old errors
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange stream content and parameter contents, reset positions
         template<int SizeMin>
         void swap(DynamicList<char,SizeMin>& other)
         {
@@ -175,11 +189,18 @@ public:
             stream_type::clear();  // Clear old errors
         }
 
+        //- Reset stream and return contents as a List
+        DynamicList<char> release()
+        {
+            DynamicList<char> chars(buffer_type::release());
+            stream_type::clear();  // Clear old errors
+            return chars;
+        }
+
         //- Some information about the input buffer position/capacity
         void debug_info(Ostream& os) const
         {
-            os  << "get="
+            os  << "get=" << input_pos() << '/' << capacity();
-                << input_pos() << '/' << capacity();
         }
 };
 
@@ -252,7 +273,7 @@ public:
         //- Move construct from List
         explicit ICharStream
         (
-            ::Foam::List<char>&& buffer,  // Fully qualify (issue #1521)
+            List<char>&& buffer,
             IOstreamOption streamOpt = IOstreamOption()
         )
         :
@@ -296,6 +317,19 @@ public:
         //- Span of the input characters (is modifiable!)
         UList<char> list() const { return stream_.list(); }
 
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(stream_.view())
+        {
+            return stream_.view();
+        }
+
+        //- For IStringStream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        auto str() const -> decltype(stream_.str())
+        {
+            return stream_.str();
+        }
+
         //- Reset content (copy)
         void reset(const char* buffer, size_t nbytes)
         {
@@ -303,14 +337,14 @@ public:
             syncState();
         }
 
-        //- Transfer list contents to List buffer
+        //- Exchange stream content and parameter contents, reset positions
         void swap(List<char>& other)
         {
             stream_.swap(other);
             syncState();
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange stream content and parameter contents, reset positions
         template<int SizeMin>
         void swap(DynamicList<char,SizeMin>& other)
         {
@@ -318,6 +352,14 @@ public:
             syncState();
         }
 
+        //- Reset stream and return contents as a List
+        DynamicList<char> release()
+        {
+            DynamicList<char> chars(stream_.release());
+            syncState();
+            return chars;
+        }
+
         //- Rewind the stream, clearing any old errors
         virtual void rewind() override
         {

src/OpenFOAM/db/IOstreams/memory/ISpanStream.H

@@ -64,9 +64,9 @@ See Also
 #ifndef Foam_ISpanStream_H
 #define Foam_ISpanStream_H
 
+#include "memoryStreamBuffer.H"
 #include "UList.H"
 #include "ISstream.H"
-#include "memoryStreamBuffer.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -117,12 +117,28 @@ public:
             stream_type(static_cast<buffer_type*>(this))
         {}
 
-        // //- Construct (shallow copy) from span character content
+        #if __cplusplus >= 201703L
-        // ispanstream(stdFoam::span<char> s)
+        //- Construct (shallow copy) from std::string_view content
-        // :
+        explicit ispanstream(std::string_view s)
-        //     buffer_type(const_cast<char*>(s.data()), s.size()),
+        {
-        //     stream_type(static_cast<buffer_type*>(this))
+            buffer_type(const_cast<char*>(s.data()), s.size()),
-        // {}
+            stream_type(static_cast<buffer_type*>(this))
+        }
+        #endif
+
+        //- Construct (shallow copy) from span character content
+        explicit ispanstream(stdFoam::span<char> s)
+        :
+            buffer_type(const_cast<char*>(s.data()), s.size()),
+            stream_type(static_cast<buffer_type*>(this))
+        {}
+
+        //- Construct (shallow copy) from span character content
+        explicit ispanstream(stdFoam::span<const char> s)
+        :
+            buffer_type(const_cast<char*>(s.data()), s.size()),
+            stream_type(static_cast<buffer_type*>(this))
+        {}
 
 
     // Member Functions
@@ -149,7 +165,28 @@ public:
         //- Span of the input characters (is modifiable!)
         UList<char> list() const
         {
-            return buffer_type::span_list();
+            return UList<char>
+            (
+                buffer_type::data_bytes(),
+                label(buffer_type::size_bytes())
+            );
+        }
+
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(buffer_type::view())
+        {
+            return buffer_type::view();
+        }
+
+        //- For istringstream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        std::string str() const
+        {
+            return std::string
+            (
+                buffer_type::data_bytes(),
+                buffer_type::size_bytes()
+            );
         }
 
         //- Rewind the stream, clearing any old errors
@@ -259,17 +296,6 @@ public:
             reset(buffer);
         }
 
-        //- Construct using data area from a List and number of bytes
-        ISpanStream
-        (
-            const ::Foam::UList<char>& buffer,
-            size_t nbytes,
-            IOstreamOption streamOpt = IOstreamOption()
-        )
-        :
-            ISpanStream(buffer.cdata(), nbytes, streamOpt)
-        {}
-
         //- Construct using data area from a List and its inherent storage size
         //  Uses addressed size, thus no special treatment for a DynamicList
         explicit ISpanStream
@@ -281,6 +307,38 @@ public:
             ISpanStream(buffer.cdata(), buffer.size(), streamOpt)
         {}
 
+        #if __cplusplus >= 201703L
+        //- Construct (shallow copy) from std::string_view content
+        explicit ISpanStream
+        (
+            std::string_view s,
+            IOstreamOption streamOpt = IOstreamOption()
+        )
+        :
+            ISpanStream(s.data(), s.size(), streamOpt)
+        {}
+        #endif
+
+        //- Construct (shallow copy) from span character content
+        explicit ISpanStream
+        (
+            stdFoam::span<const char> s,
+            IOstreamOption streamOpt = IOstreamOption()
+        )
+        :
+            ISpanStream(s.data(), s.size(), streamOpt)
+        {}
+
+        //- Construct (shallow copy) from span character content
+        explicit ISpanStream
+        (
+            stdFoam::span<char> s,
+            IOstreamOption streamOpt = IOstreamOption()
+        )
+        :
+            ISpanStream(s.data(), s.size(), streamOpt)
+        {}
+
 
     // Member Functions
 
@@ -300,10 +358,22 @@ public:
         //- Same as (capacity() - input_pos())
         std::streamsize remaining() const { return stream_.remaining(); }
 
-
         //- Span of the current input characters (is modifiable!)
         UList<char> list() const { return stream_.list(); }
 
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(stream_.view())
+        {
+            return stream_.view();
+        }
+
+        //- For IStringStream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        auto str() const -> decltype(stream_.str())
+        {
+            return stream_.str();
+        }
+
         //- Reset input area, position to buffer start and clear errors
         void reset(const char* buffer, size_t nbytes)
         {
@@ -318,6 +388,29 @@ public:
             syncState();
         }
 
+        #if __cplusplus >= 201703L
+        //- Reset input area to use data from a std::string_view
+        void reset(std::string_view s)
+        {
+            stream_.reset(s.data(), s.size());
+            syncState();
+        }
+        #endif
+
+        //- Reset input area to use data from span character content
+        void reset(stdFoam::span<char> s)
+        {
+            stream_.reset(s.data(), s.size());
+            syncState();
+        }
+
+        //- Reset input area to use data from span character content
+        void reset(stdFoam::span<const char> s)
+        {
+            stream_.reset(s.data(), s.size());
+            syncState();
+        }
+
         //- Rewind the stream, clearing any old errors
         virtual void rewind() override
         {

src/OpenFOAM/db/IOstreams/memory/OCharStream.H

@@ -124,12 +124,6 @@ public:
             buffer_type::reserve(n);
         }
 
-        //- Span of the current output characters (is modifiable!)
-        UList<char> list() const
-        {
-            return buffer_type::span_list();
-        }
-
         //- Rewind the stream, clearing any old errors
         void rewind()
         {
@@ -137,14 +131,41 @@ public:
             stream_type::clear();  // Clear old errors
         }
 
-        //- Transfer list contents to List buffer
+        //- Span of the current output characters (is modifiable!)
+        UList<char> list() const
+        {
+            return UList<char>
+            (
+                buffer_type::data_bytes(),
+                label(buffer_type::size_bytes())
+            );
+        }
+
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(buffer_type::view())
+        {
+            return buffer_type::view();
+        }
+
+        //- For ostringstream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        std::string str() const
+        {
+            return std::string
+            (
+                buffer_type::data_bytes(),
+                buffer_type::size_bytes()
+            );
+        }
+
+        //- Exchange stream content and parameter contents, reset positions
         void swap(List<char>& other)
         {
             buffer_type::swap(other);
             stream_type::clear();  // Clear old errors
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange stream content and parameter contents, reset positions
         template<int SizeMin>
         void swap(DynamicList<char,SizeMin>& other)
         {
@@ -152,11 +173,18 @@ public:
             stream_type::clear();  // Clear old errors
         }
 
+        //- Reset buffer and return contents
+        DynamicList<char> release()
+        {
+            DynamicList<char> chars(buffer_type::release());
+            stream_type::clear();  // Clear old errors
+            return chars;
+        }
+
         //- Some information about the output buffer position/capacity
         void debug_info(Ostream& os) const
         {
-            os  << "put="
+            os  << "put=" << output_pos() << '/' << capacity();
-                << output_pos() << '/' << capacity();
         }
 };
 
@@ -214,19 +242,7 @@ public:
             OSstream(stream_, "output", streamOpt.format(), streamOpt.version())
         {}
 
-        //- Construct with initial reserved number of bytes
+        //- Move construct from a List of initial storage
-        explicit OCharStream
-        (
-            size_t nbytes,
-            IOstreamOption streamOpt = IOstreamOption()
-        )
-        :
-            OCharStream(streamOpt)
-        {
-            stream_.reserve(nbytes);
-        }
-
-        //- Move construct from a List
         explicit OCharStream
         (
             ::Foam::List<char>&& buffer,
@@ -238,7 +254,8 @@ public:
             stream_.swap(buffer);
         }
 
-        //- Move construct from a DynamicList (uses entire capacity)
+        //- Move construct from a DynamicList of initial storage
+        //- (uses entire capacity)
         template<int SizeMin>
         explicit OCharStream
         (
@@ -272,14 +289,27 @@ public:
         //- Span of the current output characters (is modifiable!)
         UList<char> list() const { return stream_.list(); }
 
-        //- Transfer list contents to List buffer
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(stream_.view())
+        {
+            return stream_.view();
+        }
+
+        //- For OStringStream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        auto str() const -> decltype(stream_.str())
+        {
+            return stream_.str();
+        }
+
+        //- Exchange stream content and parameter contents, reset positions
         void swap(List<char>& other)
         {
             stream_.swap(other);
             syncState();
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange stream content and parameter contents, reset positions
         template<int SizeMin>
         void swap(DynamicList<char,SizeMin>& other)
         {
@@ -287,6 +317,14 @@ public:
             syncState();
         }
 
+        //- Reset buffer and return contents
+        DynamicList<char> release()
+        {
+            DynamicList<char> chars(stream_.release());
+            syncState();
+            return chars;
+        }
+
         //- Rewind the stream, clearing any old errors
         virtual void rewind()
         {

src/OpenFOAM/db/IOstreams/memory/OSpanStream.H

@@ -132,13 +132,6 @@ public:
             stream_type(static_cast<buffer_type*>(this))
         {}
 
-        // //- Construct (shallow copy) from span character content
-        // ospanstream(stdFoam::span<char> s)
-        // :
-        //     buffer_type(const_cast<char*>(s.data()), s.size()),
-        //     stream_type(static_cast<buffer_type*>(this))
-        // {}
-
 
     // Member Functions
 
@@ -157,7 +150,28 @@ public:
         //- Span of the current output characters (is modifiable!)
         UList<char> list() const
         {
-            return buffer_type::span_list();
+            return UList<char>
+            (
+                buffer_type::data_bytes(),
+                label(buffer_type::size_bytes())
+            );
+        }
+
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(buffer_type::view())
+        {
+            return buffer_type::view();
+        }
+
+        //- For ostringstream compatibility, return the buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        std::string str() const
+        {
+            return std::string
+            (
+                buffer_type::data_bytes(),
+                buffer_type::size_bytes()
+            );
         }
 
         //- Rewind the stream, clearing any old errors
@@ -222,7 +236,6 @@ protected:
                          Class OSpanStream Declaration
 \*---------------------------------------------------------------------------*/
 
-//- An OSstream attached to an unallocated external buffer
 class OSpanStream
 :
     public Detail::OSpanStreamAllocator,
@@ -269,17 +282,6 @@ public:
             stream_.reset(buffer, nbytes);
         }
 
-        //- Construct using data area from a List and number of bytes
-        OSpanStream
-        (
-            ::Foam::UList<char>& buffer,
-            size_t nbytes,
-            IOstreamOption streamOpt = IOstreamOption()
-        )
-        :
-            OSpanStream(buffer.data(), nbytes, streamOpt)
-        {}
-
         //- Construct using data area from a List and its inherent storage size
         explicit OSpanStream
         (
@@ -321,6 +323,19 @@ public:
         //- Span of the current output characters (is modifiable!)
         UList<char> list() const { return stream_.list(); }
 
+        //- A string_view (c++17) or span view (older c++) of buffer contents
+        auto view() const -> decltype(stream_.view())
+        {
+            return stream_.view();
+        }
+
+        //- For OStringStream compatibility, return buffer as string copy.
+        //  Use sparingly - it creates a full copy!!
+        auto str() const -> decltype(stream_.str())
+        {
+            return stream_.str();
+        }
+
         //- Reset the put area
         void reset(char* buffer, size_t nbytes)
         {

src/OpenFOAM/db/IOstreams/memory/memoryStreamBuffer.H

@@ -36,10 +36,9 @@ Description
 #define Foam_memoryStreamBuffer_H
 
 #include "stdFoam.H"  // For span
-#include "UList.H"
+#include "DynamicList.H"
 
 #include <memory>
-#include <sstream>    // Possibly want stringstream too...
 #include <type_traits>
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
@@ -208,17 +207,23 @@ public:
             return (gptr() < egptr()) ? (egptr() - gptr()) : 0;
         }
 
-        //- Span of the input characters (is modifiable!)
+        //- The span data (start of input characters)
-        UList<char> span_list() const
+        char* data_bytes() const { return eback(); }
-        {
-            return UList<char>(eback(), (egptr() - eback()));
-        }
 
-        // //- The span of input characters (is modifiable!)
+        //- The span size (number of input characters)
-        // stdFoam::span<char> span() const
+        std::streamsize size_bytes() const { return (egptr() - eback()); }
-        // {
+
-        //     return stdFoam::span<char>(eback(), (egptr() - eback()));
+        #if __cplusplus >= 201703L
-        // }
+        std::string_view view() const
+        {
+            return std::string_view(data_bytes(), size_bytes());
+        }
+        #else
+        stdFoam::span<const char> view() const
+        {
+            return stdFoam::span<const char>(data_bytes(), size_bytes());
+        }
+        #endif
 
         //- Some information about the input buffer position/capacity
         void info(Ostream& os) const
@@ -273,7 +278,7 @@ public:
         template<int SizeMin>
         in_dynamic(::Foam::DynamicList<char,SizeMin>&& buffer)
         {
-            storage_.transfer(buffer);
+            storage_.transfer(buffer);  // Implies shrink_to_fit
             sync_gbuffer();
         }
 
@@ -302,24 +307,29 @@ public:
             sync_gbuffer();
         }
 
-        //- Transfer list contents to List buffer
+        //- Exchange buffer content and parameter contents, reset positions
-        void swap(::Foam::List<char>& other)
+        void swap(List<char>& other)
         {
-            storage_.swap(other);  // Swap contents
+            other.swap(storage_);  // Swap contents
             sync_gbuffer();
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange buffer content and parameter contents, reset positions
         template<int SizeMin>
         void swap(DynamicList<char,SizeMin>& other)
         {
-            List<char> tmp(std::move(storage_));
+            // NB: not storage_.swap(other)! - incorrect slicing
-
+            other.swap(storage_);  // Swap contents: implies shrink_to_fit
-            other.shrink();             // Use addressed length only
-            storage_.transfer(other);
-            other.transfer(tmp);
             sync_gbuffer();
         }
+
+        //- Reset buffer and return contents
+        DynamicList<char> release()
+        {
+            DynamicList<char> chars(std::move(storage_));
+            sync_gbuffer();
+            return chars;
+        }
 };
 
 
@@ -387,17 +397,23 @@ public:
         //- The put buffer capacity
         std::streamsize span_capacity() const { return (epptr() - pbase()); }
 
-        //- Span of the output characters (is modifiable!)
+        //- The span data (start of output characters)
-        UList<char> span_list() const
+        char* data_bytes() const { return pbase(); }
-        {
-            return UList<char>(pbase(), (pptr() - pbase()));
-        }
 
-        //- The span of output characters (is modifiable!)
+        //- The span size (size of output buffer)
-        // stdFoam::span<char> span() const
+        std::streamsize size_bytes() const { return (pptr() - pbase()); }
-        // {
+
-        //     return stdFoam::span<char>(pbase(), (pptr() - pbase()));
+        #if __cplusplus >= 201703L
-        // }
+        std::string_view view() const
+        {
+            return std::string_view(data_bytes(), size_bytes());
+        }
+        #else
+        stdFoam::span<const char> view() const
+        {
+            return stdFoam::span<const char>(data_bytes(), size_bytes());
+        }
+        #endif
 
         //- Some information about the output buffer position/capacity
         void info(Ostream& os) const
@@ -532,38 +548,48 @@ public:
         }
 
         //- Shrink storage to addressed storage
-        inline void shrink()
+        void shrink()
         {
-            const auto cur = span_tellp();   // Addressed length
+            const auto cur = span_tellp();  // Addressed length
 
             storage_.resize(cur);
             sync_pbuffer();
             pbump(cur);
         }
 
-        //- Transfer list contents to List buffer
+        //- Exchange buffer content and parameter contents, reset positions
-        inline void swap(List<char>& other)
+        void swap(List<char>& other)
         {
-            const auto cur = span_tellp(); // Addressed length
+            const auto cur = span_tellp();  // Output length
-
+            other.swap(storage_);
-            storage_.swap(other);       // Swap contents
+            other.resize(cur);              // Truncate to output length
-            other.resize(cur);          // Adjust to addressed length
             sync_pbuffer();
         }
 
-        //- Transfer list contents to a DynamicList buffer
+        //- Exchange buffer content and parameter contents, reset positions
         template<int SizeMin>
-        inline void swap(DynamicList<char,SizeMin>& other)
+        void swap(DynamicList<char,SizeMin>& other)
         {
-            const auto cur = span_tellp();   // Addressed length
+            const auto cur = span_tellp();  // Output length
-            List<char> tmp(std::move(storage_));
 
-            other.resize(other.capacity());  // Use entire space
+            other.resize(other.capacity()); // Use entire space
-            storage_.transfer(other);
+            other.swap(storage_);  // NB: not storage_.swap(other)
-            other.transfer(tmp);
+            other.resize(cur);              // Restrict to output length
-            other.resize(cur);          // Adjust to addressed length
             sync_pbuffer();
         }
+
+        //- Reset buffer and return contents as a DynamicList.
+        //- The list size corresponds to the region of output.
+        DynamicList<char> release()
+        {
+            const auto cur = span_tellp();  // Output length
+            DynamicList<char> chars(std::move(storage_));
+            chars.resize(cur);              // Restrict to output length
+
+            if (chars.empty()) chars.clearStorage();  // Can destroy now
+            sync_pbuffer();
+            return chars;
+        }
 };