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(C) Integarting the Expr Class						[TOC]
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---- VIDEO ------------------------------
https://www.youtube.com/embed/OeiPykbtQTE
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Required Changes
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The parse functions call the constructors of the expression tree class _Expr_.
That means they own the created expression node. When they return the pointer
they transfer ownership to the caller. This means two things:

- When a parse function creates a new node its not only the owner of this node
  but also all its child nodes.
- It therefore transfers the ownership of a complete expression tree not just
  one node.

This needs to be documented:

---- CODE (type=c) -------------------------------------------------------------
// The returned expression tree is owned by the called function:
const struct Expr *parseExpr(void);
const struct Expr *parseTerm(void);
const struct Expr *parsePowerExpr(void);
const struct Expr *parseUnaryExpr(void);
const struct Expr *parseFactor(void);
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Now it is clear that function _parseExprStatement()_ becomes the owner of the
complete expression tree by calling _parseExpr()_. So here we also have to call
the destructor before we return:

---- CODE (type=c) -------------------------------------------------------------
void
parseExprStatement(void)
{
    const struct Expr *expr = parseExpr();
    //printf("> %lf\n", evalExpr(expr));
    printExprTree(expr);
    expected(SEMICOLON);
    getToken();
    deleteExpr(expr);
}
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