collect2: ld returned 1 exit status

["learning c++" <learning_c@xxxxxxxxxxx>]

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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