Improve Win32Error, library handling code.

1 files changed, 28 insertions, 16 deletions

diff --git a/c/common.h b/c/common.h
index 709a258..3f27ba4 100644
--- a/c/common.h
+++ b/c/common.h
@@ -14,19 +14,45 @@
 template <typename T> std::basic_string<T> BstrFromSz(const char* sz, int iCp = CP_UTF8);
 int EBMessageBox(const TCHAR* tszText, const TCHAR* tszCaption, unsigned uType);
 
+/* Conditionally choose between two values. */
+template <bool B, typename X, typename Y>
+constexpr std::enable_if_t<B == true, X> choose(X x, Y) { return x; }
+template <bool B, typename X, typename Y>
+constexpr std::enable_if_t<B == false, Y> choose(X, Y y) { return y; }
+
+/* Conditionally choose between ANSI and wide string literal. */
+#define AWTEXT(t, s) choose<std::is_same_v<t, wchar_t>>(L"" s, s)
+
+/* Conditionally choose between ANSI and wide Windows API function. */
+#define AWFUN(t, f) choose<std::is_same_v<t, wchar_t>>(f##W, f##A)
+
 struct Win32Error : public std::exception
 {
 	Win32Error();
 	Win32Error(DWORD dwErr);
 	~Win32Error();
 	template <typename T = char> const T* what() const noexcept;
-
+	DWORD dwErr;
 private:
-	DWORD m_dwErr;
 	char* m_szMsg = NULL;
 	wchar_t* m_wszMsg = NULL;
 };
 
+template <typename T>
+const T* Win32Error::what() const noexcept
+{
+	TCHAR* tszMsg = choose<std::is_same_v<T, wchar_t>>(m_wszMsg, m_szMsg);
+	if (!tszMsg)
+		AWFUN(T, FormatMessage)(
+		    FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM|FORMAT_MESSAGE_IGNORE_INSERTS,
+		    NULL,
+		    dwErr,
+		    MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US),
+		    (TCHAR*)&tszMsg,
+		    0, NULL);
+	return tszMsg;
+}
+
 struct Library
 {
 	Library(const TCHAR* tszLibrary);
@@ -79,20 +105,6 @@ inline T prefer(const T x)
 	return x;
 }
 
-/* Conditionally choose between two values. This template is necessary
- * because the ternary conditional operator can choose only between
- * values of the same type. */
-template <bool B, typename X, typename Y>
-constexpr std::enable_if_t<B == true, X> choose(X x, Y) { return x; }
-template <bool B, typename X, typename Y>
-constexpr std::enable_if_t<B == false, Y> choose(X, Y y) { return y; }
-
-/* Conditionally choose between ANSI and wide string literal. */
-#define AWTEXT(t, s) choose<std::is_same_v<t, wchar_t>>(L"" s, s)
-
-/* Conditionally choose between ANSI and wide Windows API function. */
-#define AWFUN(t, f) choose<std::is_same_v<t, wchar_t>>(f##W, f##A)
-
 /* Return integer scaled for current DPI. */
 inline int Dpi(const int i)
 {