How to handle a syntactically valid but logically invalid argument passed to the constructor?

Question

I need to make a class Expr having public interface like this:

class Expr{
    //...
public:
   Expr(const char*);
   int eval();           //Evaluates the expression and gives the result
   void print();
};

In the design, if user enters an invalid string to construct an Expr object like "123++233+23/45", would it be right to construct the Object initially and notify the error when eval() is called on that object.

Or the error should be checked at that point itself and en exception be thrown, though that would lead to serious increase in runtime. And the user may write code furthur with assumption that Object is created and will discover the error at runtime only..

Such problems arise always in creating a class, is there a rather standard way to handle such errors made at user's part????

See [here](http://stackoverflow.com/questions/1158410/how-to-handle-incorrect-values-in-a-constructor). Also, consider marking your constructor `explicit`. — CB Bailey, Mar 14 '12 at 08:51
Or rather using some other efficient code as a static function for that purpose only, I mean the either way... — bhuwansahni, Mar 14 '12 at 08:52

score 5 · Accepted Answer · answered Mar 14 '12 at 08:53

5

The only standard part about how you do this is thorough documentation.

I prefer throwing the errors as early as possible, or using a factory for objects of this type - objects that require specific arguments to be initialized. If you use a factory, you can return a NULL or a nullptr or whatever.

I don't see the point in constructing the object and returning an error only when eval() is called. What's the point? The object is invalid anyway, why wait until you use it?

and an exception be thrown, though that would lead to serious increase in runtime.

Have you profiled this? Don't not use exceptions because you assume increase in runtime.

answered Mar 14 '12 at 08:53

Luchian Grigore

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"When you must fail, fail noisily and as soon as possible." – bkconrad Mar 14 '12 at 08:58
I mean checking would lead to increase in runtime – bhuwansahni Mar 14 '12 at 08:59
2

@bhuwansahni: Why? The expression has to be checked for validity at _some_ stage. – Mankarse Mar 14 '12 at 09:03
Also, throwing exceptions during construction would lead to serious problems in inheritance, won't it??? – bhuwansahni Mar 14 '12 at 09:06
@bhuwansahni not if you handle it correctly. You should definitely read up on the subject of throwing exceptions from constructors. There are some good threads here on SO on the topic. – Luchian Grigore Mar 14 '12 at 09:17

score 5 · Answer 2 · answered Mar 14 '12 at 08:53

5

class illogical_expression_exception : public virtual exception {};

class Expr{
    //...
    int result; // store evaluated result.
public:
   explicit Expr(const char*);
   int getResult();           // Evaluate & Parse in the Constructor. 
   void print();
};

/* in constructor */

if ( ! checkExpression(expr) ) throw illogical_expression_exception();

/* in main() */
try{ Expr my_expr("2+2*2"); }
catch(const illogical_expression_exception& e){ 
   cout << "Illogical Expression." << endl; 
}

answered Mar 14 '12 at 08:53

ApprenticeHacker

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Of course, one would assume that the `illogical_expression_exception` reports the position in the expression that lead to this diagnosis as well as an explanation of why this is considered illogical. And one would of course call `e.what()` within the `catch` clause to print that diagnosis. – Matthieu M. Mar 14 '12 at 09:09
@MatthieuM. yes. But I didn't mention (or to be honest, forgot to mention ) that for brevity. :) – ApprenticeHacker Mar 14 '12 at 10:26

How to handle a syntactically valid but logically invalid argument passed to the constructor?

2 Answers2