Create invalid_handle value and update unmapped shared_mmap handle methods to return it

include/mio/detail/basic_mmap.hpp

@@ -42,11 +42,13 @@ namespace detail {
 enum { map_entire_file = 0 };
 
 #ifdef _WIN32
-    using file_handle_type = HANDLE;
+using file_handle_type = HANDLE;
 #else
-    using file_handle_type = int;
+using file_handle_type = int;
 #endif
 
+constexpr static file_handle_type invalid_handle = INVALID_HANDLE_VALUE;
+
 template<typename ByteT> struct basic_mmap
 {
     using value_type = ByteT;

include/mio/mmap.hpp

@@ -32,6 +32,10 @@ namespace mio {
 // `map`, in which case a memory mapping of the entire file is created.
 using detail::map_entire_file;
 
+// This value represents an invalid file handle type. This can be used to
+// determine whether `basic_mmap::file_handle` is valid, for example.
+using detail::invalid_handle;
+
 template<
     access_mode AccessMode,
     typename ByteT

include/mio/shared_mmap.hpp

@@ -120,8 +120,15 @@ public:
      * however, a mapped region of a file gets its own handle, which is returned by
      * 'mapping_handle'.
      */
-    handle_type file_handle() const noexcept { return pimpl_->file_handle(); }
+    handle_type file_handle() const noexcept
-    handle_type mapping_handle() const noexcept { return pimpl_->mapping_handle(); }
+    {
+        return pimpl_ ? pimpl_->file_handle() : invalid_handle;
+    }
+
+    handle_type mapping_handle() const noexcept
+    {
+        return pimpl_ ? pimpl_->mapping_handle() : invalid_handle;
+    }
 
     /** Returns whether a valid memory mapping has been created. */
     bool is_open() const noexcept { return pimpl_ && pimpl_->is_open(); }
@@ -142,7 +149,9 @@ public:
     size_type size() const noexcept { return pimpl_ ? pimpl_->length() : 0; }
     size_type length() const noexcept { return pimpl_ ? pimpl_->length() : 0; }
     size_type mapped_length() const noexcept
-    { return pimpl_ ? pimpl_->mapped_length() : 0; }
+    {
+        return pimpl_ ? pimpl_->mapped_length() : 0;
+    }
 
     /**
      * Returns the offset, relative to the file's start, at which the mapping was

test/test.cpp

@@ -89,8 +89,7 @@ int main()
         CHECK_INVALID_MMAP(m);
 
         // Invalid handle?
-        m = mio::make_mmap_source(
+        m = mio::make_mmap_source(mio::invalid_handle, 0, 0, error);
-            INVALID_HANDLE_VALUE/*Psst... This is an implementation detail!*/, 0, 0, error);
         CHECK_INVALID_MMAP(m);
 
         // Invalid offset?
@@ -102,6 +101,9 @@ int main()
         // Make sure custom types compile.
         mio::ummap_source _1;
         mio::shared_ummap_source _2;
+        // Make sure shared_mmap mapping compiles as all testing was done on
+        // normal mmaps.
+        mio::shared_mmap_source _3(path, 0, mio::map_entire_file);
     }
 
     std::printf("all tests passed!\n");