如何判断对象是否存在

[Ice_Wolf]

void CVcQfCtrl::SetQs1(const VARIANT FAR& newValue)
{
// TODO: Add your property handler here
m_Qs1=newValue;

要用newValue传递一个对象，如何判断该对象是否存在？


SetModifiedFlag();
}

[spwnihao]

if(newValue!=NULL)
{
;对象存在
}

[Ice_Wolf]

我定义m_Qs1为variant类型，怎么不能赋值m_Qs1=newValue

听说判断对象是否存在要用IDispatch,但我不知道怎么用

[tleon]

可以用newValue.vt判断，值如下：

VT_EMPTY No value was specified. If an optional argument to an Automation method is left blank, do not pass a VARIANT of type VT_EMPTY. Instead, pass a VARIANT of type VT_ERROR with a value of DISP_E_PARAMNOTFOUND.
VT_EMPTY | VT_BYREF Not valid.
VT_UI1 An unsigned 1-byte character is stored in bVal.
VT_UI1 | VT_BYREF A reference to an unsigned 1-byte character was passed. A pointer to the value is in pbVal.
VT_UI2 An unsigned 2-byte integer value is stored in uiVal.
VT_UI2 | VT_BYREF A reference to an unsigned 2-byte integer was passed. A pointer to the value is in puiVal.
VT_UI4 An unsigned 4-byte integer value is stored in ulVal.
VT_UI4 | VT_BYREF A reference to an unsigend 4-byte integer was passed. A pointer to the value is in pulVal.
VT_UINT An unsigned integer value is stored in uintVal.
VT_UINT | VT_BYREF A reference to an unsigned integer value was passed. A pointer to the value is in puintVal.
VT_INT An integer value is stored in intVal.
VT_INT | VT_BYREF A reference to an integer value was passed. A pointer to the value is in pintVal.
VT_I1 A 1-byte character value is stored in cVal.
VT_I1 | VT_BYREF A reference to a 1-byte character was passed. A pointer the value is in pcVal.
VT_I2 A 2-byte integer value is stored in iVal.
VT_I2 | VT_BYREF A reference to a 2-byte integer was passed. A pointer to the value is in piVal.
VT_I4 A 4-byte integer value is stored in lVal.
VT_I4 | VT_BYREF A reference to a 4-byte integer was passed. A pointer to the value is in plVal.
VT_R4 An IEEE 4-byte real value is stored in fltVal.
VT_R4 | VT_BYREF A reference to an IEEE 4-byte real value was passed. A pointer to the value is in pfltVal.
VT_R8 An 8-byte IEEE real value is stored in dblVal.
VT_R8 | VT_BYREF A reference to an 8-byte IEEE real value was passed. A pointer to its value is in pdblVal.
VT_CY A currency value was specified. A currency number is stored as 64-bit (8-byte), two's complement integer, scaled by 10,000 to give a fixed-point number with 15 digits to the left of the decimal point and 4 digits to the right. The value is in cyVal.
VT_CY | VT_BYREF A reference to a currency value was passed. A pointer to the value is in pcyVal.
VT_BSTR A string was passed; it is stored in bstrVal. This pointer must be obtained and freed by the BSTR functions, which are described in Conversion and Manipulation Functions.
VT_BSTR | VT_BYREF A reference to a string was passed. A BSTR* that points to a BSTR is in pbstrVal. The referenced pointer must be obtained or freed by the BSTR functions.
VT_DECIMAL Decimal variables are stored as 96-bit (12-byte) unsigned integers scaled by a variable power of 10. VT_DECIMAL uses the entire 16 bytes of the Variant.
VT_DECIMAL | VT_BYREF A reference to a decimal value was passed. A pointer to the value is in pdecVal.
VT_NULL A propagating null value was specified. (This should not be confused with the null pointer.) The null value is used for tri-state logic, as with SQL.
VT_NULL | VT_BYREF Not valid.
VT_ERROR An SCODE was specified. The type of the error is specified in scodee. Generally, operations on error values should raise an exception or propagate the error to the return value, as appropriate.
VT_ERROR | VT_BYREF A reference to an SCODE was passed. A pointer to the value is in pscode.
VT_BOOL A 16 bit Boolean (True/False) value was specified. A value of 0xFFFF (all bits 1) indicates True; a value of 0 (all bits 0) indicates False. No other values are valid.
VT_BOOL | VT_BYREF A reference to a Boolean value. A pointer to the Boolean value is in pbool.
VT_DATE A value denoting a date and time was specified. Dates are represented as double-precision numbers, where midnight, January 1, 1900 is 2.0, January 2, 1900 is 3.0, and so on. The value is passed in date.
This is the same numbering system used by most spreadsheet programs, although some specify incorrectly that February 29, 1900 existed, and thus set January 1, 1900 to 1.0. The date can be converted to and from an MS-DOS representation using VariantTimeToDosDateTime, which is discussed in Conversion and Manipulation Functions.

VT_DATE | VT_BYREF A reference to a date was passed. A pointer to the value is in pdate.
VT_DISPATCH A pointer to an object was specified. The pointer is in pdispVal. This object is known only to implement IDispatch. The object can be queried as to whether it supports any other desired interface by calling QueryInterface on the object. Objects that do not implement IDispatch should be passed using VT_UNKNOWN.
VT_DISPATCH | VT_BYREF A pointer to a pointer to an object was specified. The pointer to the object is stored in the location referred to by ppdispVal.
VT_VARIANT Invalid. VARIANTARGs must be passed by reference.
VT_VARIANT | VT_BYREF A pointer to another VARIANTARG is passed in pvarVal. This referenced VARIANTARG, pvarVal, cannot be another VT_VARIANT|VT_BYREF. This value can be used to support languages that allow functions to change the types of variables passed by reference.
VT_UNKNOWN A pointer to an object that implements the IUnknown interface is passed in punkVal.
VT_UNKNOWN | VT_BYREF A pointer to the IUnknown interface is passed in ppunkVal. The pointer to the interface is stored in the location referred to by ppunkVal.
VT_ARRAY | <anything> An array of data type <anything> was passed. (VT_EMPTY and VT_NULL are invalid types to combine with VT_ARRAY.) The pointer in pparray points to an array descriptor, which describes the dimensions, size, and in-memory location of the array. The array descriptor is never accessed directly, but instead is read and modified using the functions described in Conversion and Manipulation Functions.