Question

Someone has given me the following C++ code snippet to try out – and now I have lost contact with them (its a long story). Anyway, it won’t compile – I get an error

error C2512: ‘mstream’ : no appropriate default constructor available

Can anyone explain why, and what is needed to fix it.

class mstream : private ostream
{
  public:

  mstream& operator << (char *value)
  {
    printf ("[%s]\n", value);
    return *this;
  }
  mstream& operator << (int value)
  {
    printf ("[%u]\n", value);
    return *this;
  }

};
mstream g_mcout;

EDIT: Oops, missed this out…

ostream& mcout ()
{
  return g_mcout;
}
ostream& SgDebug ()
{
  return g_mcout;
}

FYI: the reason for this strange looking code is all to do with merging a C++ with a C program. The printf()’s will actually be changed to my_printf()’s which do a wide variety of custom things.

Answer 1

ostream has no default constructor; the implicitly created default constructor for mstream is invalid because of it. You need to provide the ostream with a stream buffer:

class mstream : private ostream
{
public:
    mstream() :
    ostream(/* whatever you want */)
    {}

    /* Maybe this is more appropriate:

    mstream(std::streambuf* pBuffer) :
    ostream(pBuffer)
    {}

    */

    // ...
};

So it can construct. What you put there depends on what you’re trying to do.