c++ - Can't get exact position of BSTR data in VARIANT struct -


i have strange problem reading bstr value variant struct.

i use following javascript code send data c++ code:

external.cppcall("zhttshow", 1, "y");

in invoke function in c++ code, tried access data:

hresult stdmethodcalltype webbrowser::invoke(_in_  dispid dispidmember, _in_  refiid  riid, _in_  lcid lcid, _in_  word wflags, _in_  dispparams *pp, _out_opt_ variant *pvarresult, _out_opt_ excepinfo *pexcepinfo, _out_opt_ uint *puargerr) {     if (dispidmember == 100)     {         unsigned int k = pp->cargs;         if (k > 0)         {             variant *vvariant = &pp->rgvarg[k - 1];             if ((vvariant->vt & vt_bstr) == vt_bstr && vvariant->pvarval->bstrval)             {                 wchar_t *wide = (wchar_t*)(vvariant->pvarval->bstrval);                 // other code             }         }     } }

in above code, wide point undefined position. if pointer of vvariant->pvarval->bstrval point position near bstr value:

variant *vvariant = &pp->rgvarg[k - 1]; if ((vvariant->vt & vt_bstr) == vt_bstr && vvariant->pvarval->bstrval) {     wchar_t *wide = (wchar_t*)(&vvariant->pvarval->bstrval);     // other code }

wide point position behide bstr data 8 bytes, instead "zhttshow" string, "show". if subtract wide 4 desired data.

bellow debug screen second c++ code:

enter image description here

question:

why wide doesn't point bstr's exact position? error in code , how fix it?

a bstr not wchar_t*. may similar, not. , more importanlty, wchar_t* not bstr. bstrs have structure, header sits in front of pointed-to value, real bstr starts few bytes before address. see bstr data type:

a bstr composite data type consists of length prefix, data string, , terminator.

this why should use appropriate bstr specific apis. best, use compiler support _bstr_t. extract bstr variant correctly use _variant_t extractor. let compiler deal details.


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