Appending BSTR in a LPCSTR

Question

I have a class function which is receving a BSTR. In my class I have a member variable which is LPCSTR. Now I need to append BSTR ins LPCSTR. How I can do that. Here is my function.

void MyClass::MyFunction(BSTR text)
{
    LPCSTR name = "Name: ";
    m_classMember = name + text; // m_classMember is LPCSTR.
}

in my m_classMember I want that after this function value should be "Name: text_received_in_function". How i can do that.

Answer 1

Use the Microsoft specific _bstr_t class, which handles the ANSI/Unicode natively. Something like

#include <comutils.h>
// ...

void MyClass::MyFunction(BSTR text)
{
    _bstr_t name = "Name: " + _bstr_t(text, true);
    m_classMember = (LPCSTR)name;
}

is what you almost want. However, as pointed out by the remarks, you have to manage the lifetime of m_classMember and the concatened string. In the example above, the code is likely to crash.

If you own the MyClass object, you could simply add another member variable:

class MyClass {
private:
  _bstr_t m_concatened;
//...
};

and then use m_classMember as a pointer to the string content of m_concatened.

void MyClass::MyFunction(BSTR text)
{
    m_concatened = "Name: " + _bstr_t(text, true);
    m_classMember = (LPCSTR)m_concatened;
}

Otherwise, prior to the assignment of m_classMember, you should free it in the same way you allocated it (free, delete [], etc), and create a new char* array in which you copy the content of the concatened string. Something like

void MyClass::MyFunction(BSTR text)
{
    _bstr_t name = "Name: " + _bstr_t(text, true);

    // in case it was previously allocated with 'new'
    // should be initialized to 0 in the constructor
    delete [] m_classMember; 
    m_classMember = new char[name.length() + 1];

    strcpy_s(m_classMember, name.length(), (LPCSTR)name);
    m_classMember[name.length()] = 0;
}

should do the work.

Answer 2

First, I suggest you to not use raw char/wchar_t* pointers as data members for strings; in general, it's better (easier, more maintainable, exception-safe, etc.) to use a robust C++ string class.

Since you are writing Windows code, you may want to use ATL::CString, which is well integrated in the context of Win32 programming (e.g.: it offers several conveniences, like loading strings from resources, it works out-of-the-box with the TCHAR model, etc.).

If you want to work with the TCHAR model (and make your code compilable in both ANSI/MBCS and Unicode builds), you may want to use the ATL string conversion helper class CW2T to convert from BSTR (which is Unicode wchar_t*) to char* in ANSI/MBCS build, and leave it as wchar_t* in Unicode builds.

#include <atlstr.h>    // for CString
#include <atlconv.h>   // for CW2T

void MyClass::MyFunction(BSTR text)
{
    // Assume:
    // CString m_classMember;

    m_classMember = _T("Name: ");

    // Concatenate the content of the BSTR.
    // CW2T keeps the BSTR as Unicode in Unicode builds,
    // and converts to char* in ANSI/MBCS builds.
    m_classMember += CW2T(text);
}

Instead, if you just want to compile your code in Unicode (which makes sense in today's world), you can get rid of _T("...") decoration and CW2T, and just use:

void MyClass::MyFunction(BSTR text)
{
    // Assume:
    // CString m_classMember;

    m_classMember = L"Name: ";

    // Concatenate the content of the BSTR.
    m_classMember += text;
}

(Or use STL's std::wstring as others suggested.)