collect2: ld returned 1 exit status

[learning c++]

Hi, everyone:
I compiled a program and the result is:

Expr::~Expr [in-charge]()
collect2: ld returned 1 exit status

In my program, I defined the a class "Expr" and constructor "Expr"

class Expr
{
  int val1;
  int val2;
  char op;

  static int value(char c);

public:
  Expr(string const&);
  int operator () () const;
  void print(ostream& useStream) const;
  ~Expr();
};

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