Conversion function for error checking considered

问题:

I'd like to have a simple way of checking for an object to be valid. I thought of a simple conversion function, something like this:

operator bool() const { return is_valid; }

Checking for it to be valid would be very simple now

// is my object invalid?
if (!my_object) std::cerr << "my_object isn't valid" << std::endl;

Is this considered a good practise?

回答1:

In C++03, you need to use the safe bool idiom to avoid evil things:

int x = my_object; // this works

In C++11 you can use an explicit conversion:

explicit operator bool() const
{
    // verify if valid
    return is_valid;
}

This way you need to be explicit about the conversion to bool, so you can no longer do crazy things by accident (in C++ you can always do crazy things on purpose):

int x = my_object; // does not compile because there's no explicit conversion
bool y = bool(my_object); // an explicit conversion does the trick

This still works as normal in places like if and while that require a boolean expression, because the condition of those statements is contextually converted to bool:

// this uses the explicit conversion "implicitly"
if (my_object)
{
    ...
}

This is documented in §4[conv]:

An expression e can be implicitly converted to a type T if and only if the declaration T t=e; is well-formed, for some invented temporary variable t (§8.5). Certain language constructs require that an expression be converted to a Boolean value. An expression e appearing in such a context is said to be contextually converted to bool and is well-formed if and only if the declaration bool t(e); is well-formed, for some invented temporary variable t (§8.5). The effect of either implicit conversion is the same as performing the declaration and initialization and then using the temporary variable as the result of the conversion.

(What makes the difference is the use of bool t(e); instead of bool t = e;.)

The places were this contextual conversion to bool happens are:

• the conditions of if, while, and for statements;
• the operators of logical negation !, logical conjunction &&, and logical disjunction ||;
• the conditional operator ?:;
• the condition of static_assert;
• the optional constant expression of the noexcept exception specifier;

回答2:

No, a simple bool conversion operator is not, as you can now make evil comparisions between unrelated types. Generally, yes, a conversion function is a-okay. Just use the right one (safe-bool idiom). I can't explain it any better than the given links.

回答3:

The original question was

Is this considered a good practise?

The issue with the safe bool conversion was very relevant in practice, but fortunately has now been addressed by the standards.

But the judgement if this approach is appropriate, is a question of design.

By introducing such a "validity check", effectively you are stating that your objects can be in an invalid state. That is, speaking in the terms of computer science, you added a new separate value to the value domain represented by your objects. A so called bottom value

The most prominent example for a value domain with that propery is the pointer. A pointer can refer to various memory locations, but it also can be NULL (invalid).

Thus we need do ask ourselves: does such a bottom value really reflect the nature of the things we want to model with our classes -- and -- do we really need to cover this aspect of the nature within our model?

Experience shows that bottom values tend to be error prone, easy to forget and generally more of a liability than an asset. If you're able to arrange your code in a way that your objects can not be invalid, your code gets simpler, easier to read, to understand and to maintain..