What’s the difference between VARIANT and VARIANTARG?

Raymond Chen

One of my colleagues asked me, “What’s the difference between VARIANT and VARIANTARG?”

If you look at the definitions in the oaidl.h header file, you’ll see that VARIANTARG is just an alias for VARIANT.



“Why have two names for the same thing?”

The two structures are physically identical, but the rules surrounding them are different.

This is mentioned rather opaquely in the documentation for VARIANT:

VARIANTARG describes arguments passed within DISPPARAMS, and VARIANT to specify variant data that cannot be passed by reference.

When a variant refers to another variant by using the VT_VARIANT | VT_BYREF vartype, the variant being referred to cannot also be of type VT_VARIANT | VT_BYREF. VARIANTs can be passed by value, even if VARIANTARGs cannot.

The first sentence says that you use VARIANTARG as part of a DISPPARAMS, which is the structure used to pass parameters (also known as “arguments”) to methods of dispatch interfaces.

The second sentence is not relevant to the discussion. It says that only one level of pointer chasing is allowed. You can’t send the method on a wild goose chase where you pass a variant that says “The real data is over there, in that other variant”, and then have the second variant say, “Ha ha, fooled, you. The real data is over there in that other other variant.”

The third sentence starts to hint at the underlying issue. It says that VARIANTs can be passed by value, but VARIANTARGs cannot.

Interesting, but no real insight as to why you can pass VARIANT by value but not VARIANTARG.

There’s another MSDN page titled VARIANT and VARIANTARG. Maybe that’ll help us get to the bottom of the mystery.

The VARIANT type cannot have the VT_BYREF bit set.

Aha, that’s the difference. The VARIANTARG structure is allowed to say, “Hey, I don’t contain the data you want, but you can look over there for it.” For example, it could set its variant type to VT_BYREF | VT_I4 to say, “There is an integer, but it’s not stored in the lVal member. Instead, you have to go to the plVal member, which is a pointer to the integer you want.”

This explains why VARIANT can be copied, but VARIANTARG cannot: If you try to copy a VARIANTARG that uses VT_BYREF, you are just copying the raw pointer to the data, not the data itself. You have no control over the memory being pointed to, so you have no way to prevent it from being freed.

Using VT_BYREF is allowed in a DISPPARAMS because the caller assumes the responsibility of keeping the pointed-to data valid for the duration of the method call. That’s just one of the basic ground rules of programming, specifically the stability requirement. The caller has to wait for the method call to return before it can free the memory pointed to by the VARIANTARG.

Okay, so what if you’re implementing a method and you want to make a copy of the VARIANTARG? How do you deal with the VT_BYREF?

This is where the Variant­Copy­Ind function comes into play. This function takes a VARIANTARG, possibly with VT_BYREF, and converts it into a VARIANT, with all VT_BYREF removed. It does this by chasing the pointer one level and copying the value back into the VARIANT. For example, if the VARIANTARG were a VT_BYREF | VT_I4, then the Variant­Copy­Ind function would follow the plVal pointer, read the integer stored there, and copy it to the output VARIANT‘s lVal member, resulting in a simple VT_I4.

The “Ind” therefore stands for “Indirect”. The Variant­Copy­Ind function indirects through the pointer hiding inside the VT_BYREF.

Well, that was a strange bit of spelunking.


Discussion is closed.

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