void* p = new CString;
<...>
delete p;
p = new CMyCoolClass;
<...>
delete p;

А>void* p = new CString;
<...>>
А>delete p;
А>p = new CMyCoolClass;
<...>>
А>delete p;

5.3.5/3
In the first alternative (delete object), if the static type of the operand is different 
from its dynamic type, the static type shall be a base class of the operand’s dynamic 
type and the static type shall have a virtual destructor or the behavior is undefined.

struct TERM
{
    enum TERM_TYPE { NUM, STR, GRAPH };
    TERM_TYPE Type;

    double*   m_pNum;
    CString*  m_pStr;
    CGraph*   m_pGraph;
};

А>struct TERM
А>{
А>    enum TERM_TYPE { NUM, STR, GRAPH };
А>    TERM_TYPE Type;

А>    double*   m_pNum;
А>    CString*  m_pStr;
А>    CGraph*   m_pGraph;
А>};
А>

 struct TERM
 {
    enum TERM_TYPE { NUM, STR, GRAPH };
    TERM_TYPE Type;
    void *p;
 };

struct TERM
{
    enum TERM_TYPE { NUM, STR, GRAPH };
    TERM_TYPE Type;

    union ptrunion
    {
      double*   pNum;
      CString*  pStr;
      CGraph*   pGraph;
    
      // для удобств :))
      operator double* () { return pNum;}
      operator CString* () { return pStr; }
      operator CGraph* () { return pGraph; }
    }ptr;
};

 > struct TERM
 > {
 >     enum TERM_TYPE { NUM, STR, GRAPH };
 >     TERM_TYPE Type;

 >     double*   m_pNum;
 >     CString*  m_pStr;
 >     CGraph*   m_pGraph;
 > };
 >

class TermData
{
public:
  virtual ~TermData() { }

  . . .
};

class NumData : public TermData
{
  . . .
};

class StrData : public TermData
{
  . . .
};

class GraphData : public TermData
{
  . . .
};

struct Term
{
  . . .

  TermData* term;
};

А>struct TERM
А>{
А>    enum TERM_TYPE { NUM, STR, GRAPH };
А>    TERM_TYPE Type;

А>    double*   m_pNum;
А>    CString*  m_pStr;
А>    CGraph*   m_pGraph;
А>};
А>

CString graph_to_name(const CGraph &);

double sum_double(double,double);