BCGControlBar Pro for MFC
void CBCGPRibbonComboBox::EnableCalculator ( BOOL  bEnable = TRUE,
const CStringList *  plstAdditionalCommands = NULL,
const CList< UINT, UINT > *  plstExtCommands = NULL,
LPCTSTR  lpszDisplayFormat = NULL 

Puts a Ribbon combo box control in "Calculator" mode.

Use this function to enable or disable "Calculator" mode. In this mode a combo box control displays Calculator control in drop down part. The edit part of combo box displays calculation results.

You can use plstAdditionalCommands to provide a list of additional calculator command, such as sin, cos etc. You can use plstExtCommands to provide a list of custom commands.

bEnableTRUE - enable "Calculator" mode, FALSE - enable regular combo box mode.
plstAdditionalCommandsA list of additional built-in commands to be displayed by Calculator control.
plstExtCommandsA list of custom (user-defined) commands.
lpszDisplayFormatA display format. The default format is: "%.16lf"