_bstr_t objects

A _bstr_t object encapsulates the BSTR data type. The class manages resource allocation and de-allocation through function calls to SysAllocString and SysFreeString.

It isn’t necessary to de-allocate or free a string by calling SysFreeString that has been allocated by _bstr_t. Doing this might lead to double free of the allocated string when the _bstr_t class is destructed. This could lead
to access violation.

_bstr_t is defined in comutil.h as follows:

 

class _bstr_t { public:     // Constructors     //     _bstr_t() throw();     _bstr_t(const _bstr_t& s) throw();     _bstr_t(const char* s) ;     _bstr_t(const wchar_t* s) ;     _bstr_t(const _variant_t& var) ;     _bstr_t(BSTR bstr, bool fCopy) ;       // Destructor     //     ~_bstr_t() throw();       // Assignment operators     //     _bstr_t& operator=(const _bstr_t& s) throw();     _bstr_t& operator=(const char* s);     _bstr_t& operator=(const wchar_t* s);     _bstr_t& operator=(const _variant_t& var);       // Operators     //     _bstr_t& operator+=(const _bstr_t& s);     _bstr_t operator+(const _bstr_t& s) const;      // Friend operators     //     friend _bstr_t operator+(const char* s1, const _bstr_t& s2);     friend _bstr_t operator+(const wchar_t* s1, const _bstr_t& s2);       // Extractors     operator const wchar_t*() const throw();     operator wchar_t*() const throw();     operator const char*() const ;     operator char*() const ;       // Comparison operators     //     bool operator!() const throw();     bool operator==(const _bstr_t& str) const throw();     bool operator!=(const _bstr_t& str) const throw();     bool operator<(const _bstr_t& str) const throw();     bool operator>(const _bstr_t& str) const throw();     bool operator<=(const _bstr_t& str) const throw();     bool operator>=(const _bstr_t& str) const throw();       // Low-level helper functions     //     BSTR copy(bool fCopy = true) const;     unsigned int length() const throw();      // Binary string assign     //     void Assign(BSTR s);      // Get the physical BSTR     //     BSTR& GetBSTR();     BSTR* GetAddress();       // Attach to the internal BSTR w/o copying     //     void Attach(BSTR s) ;       // Detach the internal BSTR     //     BSTR Detach() throw();   private:     // Referenced counted wrapper     //     class  Data_t {     public:          // Constructors         //         Data_t(const char* s);         Data_t(const wchar_t* s);         Data_t(BSTR bstr, bool fCopy);         Data_t(const _bstr_t& s1, const _bstr_t& s2) ;           // Reference counting routines         //         unsigned long AddRef() throw();         unsigned long Release() throw();         unsigned long RefCount() const throw();          // Extractors         //         operator const wchar_t*() const throw();         operator const char*() const ;          // Low-level helper functions         //         const wchar_t* GetWString() const throw();         wchar_t*& GetWString() throw();         const char* GetString() const;          BSTR Copy() const;         void Assign(BSTR s) ;         void Attach(BSTR s) throw();         unsigned int Length() const throw();         int Compare(const Data_t& str) const throw();           //  Exception agnostic wrapper for new         //         void*  operator new(size_t sz);                private:         BSTR m_wstr;         mutable char* m_str;         unsigned long m_RefCount;           // Never allow default construction         //         Data_t() throw();           // Never allow copy         //         Data_t(const Data_t& s) throw();           // Prevent deletes from outside. Release() must be used.         //         ~Data_t() throw();          void _Free() throw();     };   private:     // Reference counted representation     //     Data_t* m_Data;   private:     // Low-level utilities     //     void _AddRef() throw();     void _Free() throw();     int _Compare(const _bstr_t& str) const throw(); };

 

The destructor does a free as follows:

 

// Destructor // inline _bstr_t::~_bstr_t() throw() {     _Free(); }   // Free the BSTR // inline void _bstr_t::_Free() throw() {     if(m_Data != NULL) {         m_Data->Release();         m_Data = NULL;     } }

  

m_Data is a member of the Data_t class. m_Data->Release() decrements the reference count of the object and deletes the object, which actually calls Data_t::_Free() to free the string by calling SysFreeString() .
Please see the code for Data_t::_Free() below.

 

inline void _bstr_t::Data_t::_Free() throw() {    if (m_wstr != NULL)    {        ::SysFreeString(m_wstr);    }    if (m_str != NULL)   {       delete [] m_str;   } }

 

Here are the Data_t functions that allocate the BSTR.

 

// Construct a Data_t from a const char* // inline _bstr_t::Data_t::Data_t(const  char* s)     : m_str(NULL), m_RefCount(1) {     m_wstr = _com_util::ConvertStringToBSTR(s); }   // Construct a Data_t from a const wchar_t* // inline _bstr_t::Data_t::Data_t(const  wchar_t* s)     : m_str(NULL), m_RefCount(1) {     m_wstr = ::SysAllocString(s);       if (m_wstr == NULL && s != NULL) {         _com_issue_error(E_OUTOFMEMORY);     } }

 

Reference: msdn.microsoft.com/en-us/library/zthfhkd6(v=vs.80).ASPX