WinHTTP: Add http_error

WinHTTP errors don't get resolved by FormatMessage by default. We need
to format them using WINHTTP.DLL resources.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/WinStd/WinHTTP.h

@@ -141,4 +141,87 @@ namespace winstd
     };
 
     /// @}
+
+    /// \addtogroup WinStdExceptions
+    /// @{
+
+    ///
+    /// WinHTTP error
+    ///
+    class http_error : public num_runtime_error<DWORD>
+    {
+    public:
+        ///
+        /// Constructs an exception
+        ///
+        /// \param[in] num WinHTTP error code
+        ///
+        http_error(_In_ error_type num) : num_runtime_error<DWORD>(num, message(num))
+        {}
+
+        ///
+        /// Constructs an exception
+        ///
+        /// \param[in] num WinHTTP error code
+        /// \param[in] msg Error message
+        ///
+        http_error(_In_ error_type num, _In_ const std::string& msg) : num_runtime_error<DWORD>(num, msg + ": " + message(num))
+        {}
+
+        ///
+        /// Constructs an exception
+        ///
+        /// \param[in] num  WinHTTP error code
+        /// \param[in] msg  Error message
+        ///
+        http_error(_In_ error_type num, _In_z_ const char *msg) : num_runtime_error<DWORD>(num, std::string(msg) + ": " + message(num))
+        {}
+
+        ///
+        /// Constructs an exception using `GetLastError()`
+        ///
+        http_error() : num_runtime_error<DWORD>(GetLastError(), message(GetLastError()))
+        {}
+
+        ///
+        /// Constructs an exception using `GetLastError()`
+        ///
+        /// \param[in] msg  Error message
+        ///
+        http_error(_In_ const std::string& msg) : num_runtime_error<DWORD>(GetLastError(), msg + ": " + message(GetLastError()))
+        {}
+
+        ///
+        /// Constructs an exception using `GetLastError()`
+        ///
+        /// \param[in] msg  Error message
+        ///
+        http_error(_In_z_ const char *msg) : num_runtime_error<DWORD>(GetLastError(), std::string(msg) + ": " + message(GetLastError()))
+        {}
+
+    protected:
+        ///
+        /// Returns a user-readable WinHTTP error message.
+        /// As std::exception messages may only be char*, we use UTF-8 by convention.
+        ///
+        /// \sa [FormatMessage function](https://docs.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-formatmessage)
+        ///
+        static std::string message(_In_ error_type num, _In_opt_ DWORD dwLanguageId = 0)
+        {
+            last_error_saver last_error_save;
+            std::wstring wstr;
+            winstd::library winhttp(LoadLibraryW(L"WINHTTP.DLL"));
+            if (WINHTTP_ERROR_BASE < num && num <= WINHTTP_ERROR_LAST && winhttp && FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_FROM_HMODULE | FORMAT_MESSAGE_IGNORE_INSERTS, (HMODULE)winhttp, num, dwLanguageId, wstr, NULL) ||
+                FormatMessageW(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 0, num, dwLanguageId, wstr, NULL)) {
+                // Stock Windows error messages contain CRLF. Well... Trim all the trailing white space.
+                wstr.erase(wstr.find_last_not_of(L" \t\n\r\f\v") + 1);
+            } else
+                sprintf(wstr, num >= 0x10000 ? L"Error 0x%X" : L"Error %u", num);
+            std::string str;
+            WideCharToMultiByte(CP_UTF8, 0, wstr, str, NULL, NULL);
+            return str;
+        }
+    };
+
+    /// @}
 }