Gunter Don Gunter Don - 3 months ago 9
C++ Question

template constructor for expression node returns wrong value

I have a template class that just seems to fail this on one test.


  • There is a template structure "term" representing the node values of an expression graph.

  • There is a template structure "expression" representing the graph.

  • There is a test procedure "void Test()"



I have beat my head for a few hours now. I would really like some assistance.

void Test()
{
cout << "**** Integer Tests" << endl;
// Default Constructor
integer a;
cout << "integer a = "<< a.Value << endl;
// Copy Constructor
integer b(a);
cout << "integer b(a) = "<< b.Value << endl;
integer c = b;
cout << "integer c = b = "<< c.Value << endl;
// Type Constructor
integer d(5);
cout << "integer d(5) = "<< d.Value << endl;
integer e = 5;
cout << "integer e = 5 = "<< e.Value << endl;
// Type Conversions
int n = e.Value;
cout << "n = e = "<< n << endl;/
n = (int)d.Value + (int)e.Value;
cout << "n = d + e = "<< n << endl;// allowed because a convers to int

cout << "**** Expression Tests" << endl;
expression<int> e1;
expression<int> e2(e1);

expression<int> e3(d);// This line using the copy constructor works
expression<int> e4 = d + e;// this line, using the copy constructor AFTER the operator+ call results in BUM values.

}


The test function "void Test()" fails to give the correct value for the statement :

expression e4 = d + e;

stepping through the call stack...at T+1

1.) calls term::expression& operator+(term& ref)

2.) on return, the left and right values of the expression node are CORRECT!!!

3.) now, the assignment that fails... it calls copy constructor expression(expression& ref)

4.) WTF??? inside this i get bum left and right values.

why, when the copy constructor test succeeds at the line above?

here are the test case templates

template<typename T> struct expression; // Forward Declare expressions, so i can use it in operator+

template<typename T>
struct term
{
T Value;
term() : Value(0)
{
cout << "**** term()" << endl;
} // Default Constructor
term(term& ref) : Value(ref.Value)
{
cout << "**** term(term& ref)" << endl;
} // Copy Constructor
// Type Conversion
term(T value) : Value(value)
{
cout << "**** term(T value)" << endl;
} // Type Constructor
//operator T() const { return Value; } // Type Converter
**expression<T>& operator+(term& ref)
{
return expression<T>(*this, ref);
};**
};

template<typename T>
struct expression
{
term<T> Left;
term<T> Right;
expression()
: Left()
, Right()
{
cout << "**** expression()" << endl;
cout << "left" << Left.Value << endl;
cout << "right" << Right.Value << endl;
}
expression(expression& ref)
: Left(ref.Left)
, Right(ref.Right)
{
cout << "**** expression(expression& ref)" << endl;
cout << "left" << Left.Value << endl;
cout << "right" << Right.Value << endl;
}
expression(term<T>& left)
: Left(left)
, Right()
{
cout << "**** expression(term<T>& left)" << endl;
cout << "left" << Left.Value << endl;
cout << "right" << Right.Value << endl;
}
expression(term<T>& left, term<T>& right)
: Left(left)
, Right(right)
{
cout << "**** expression(term<T>& left, term<T>& right)" << endl;
cout << "left" << Left.Value << endl;
cout << "right" << Right.Value << endl;
}
};
struct integer : public term<int>
{

integer() : term() { }// allows default constructor (not inheritablle)
// tactic to get base class constructors(1 parameter) to compile...
template<class T> integer(T t) : term(t) { }
//template<class T> integer(T& t) : term(t) { }
};
void Test()
{
cout << "**** Integer Tests" << endl;
// Default Constructor
integer a;
cout << "integer a = "<< a.Value << endl;// allowed because integer converts to int
// Copy Constructor
integer b(a);
cout << "integer b(a) = "<< b.Value << endl;
integer c = b;
cout << "integer c = b = "<< c.Value << endl;
// Type Constructor
integer d(5);
cout << "integer d(5) = "<< d.Value << endl;
integer e = 5;
cout << "integer e = 5 = "<< e.Value << endl;
// Type Conversions
int n = e.Value;
cout << "n = e = "<< n << endl;// allowed because a convers to int
// Arithmetic using base type operators... whatever they allow, is allowed.
n = (int)d.Value + (int)e.Value;
cout << "n = d + e = "<< n << endl;// allowed because a convers to int

cout << "**** Expression Tests" << endl;
expression<int> e1;
expression<int> e2(e1);

expression<int> e3(d);// This line using the copy constructor works
expression<int> e4 = d + e;// this line, using the copy constructor AFTER the operator+ call results in BUM values.

}


Output of the above code looks like:

**** Integer Tests
**** term()
integer a = 0
**** term(term& ref)
integer b(a) = 0
**** term(term& ref)
integer c = b = 0
**** term(T value)
integer d(5) = 5
**** term(T value)
integer e = 5 = 5
n = e = 5
n = d + e = 10
**** Expression Tests
**** term()
**** term()
**** expression()
left0
right0
**** term(term& ref)
**** term(term& ref)
**** expression(expression& ref)
left0
right0
**** term(term& ref)
**** term()
**** expression(term<T>& left)
left5
right0
**** term(term& ref)
**** term(term& ref)
**** expression(term<T>& left, term<T>& right)
left5
right5
**** term(term& ref)
**** term(term& ref)
**** expression(expression& ref)
left4061292
right4061296


Edit: as pointed out (very quickly, I might add) the operator+ was returning the reference to a temp on the stack, and not the value as I assumed.

The function was fixed such that the ampersand after expression was removed:

expression<T> operator+(term& ref)
{
return expression<T>(*this, ref);
};


This was an ID10T error. the ampersand was right there in front of me.

Again, Thanks for helping me resolve this.

Answer

Here's your problem:

expression<T>& operator+(term& ref) 

operator+ should return a new object. You're returning a reference to a temporary. That should've even compile, but I'm guessing you're using MSVC which has an extension to allow this for reasons that I don't understand.

There's other problems in your code that should prevent it from compiling to, but that's likely the one that's causing your error. As a first approximation, every use of reference in your code is incorrect - they should all either be references to const or values.