CODEWITHSUNDEEP
c++
johnbakers
johnbakers

Reputation: 24771

What is easiest way to force compiler to throw error?

To the folks marking this as duplicate: it is not; the other question addresses enums which are compile-time constants. This is not a constant integral expression thus the solution would be very different. Please see my code below more carefully before suggesting this has already been answered in another question, as it has not in any way. I am checking the value of a member variable on an object, information created at runtime, and I'm curious what I can do with that in this context.

I'm at a point where I need to use something to make the compiler fail if the user of my API does something she should not.

I don't know if that's possible, it is? The options I mention above are primarily run-time, right?

For example, suppose you have a function:

   void doSomethingIncredible(AwesomeClass amazingObject)
    {
     //perform life-changing work here except:
     if (amazingObject.isntAmazing) //a bool property of object
        //uh oh, life sucks, I refuse to compile this

Now calling this function will change how you live your life in all respects, except for occasions in which amazingObject has a particular property switched on, for example, in which case, I want the compiler to not even allow this to pass, i.e. cannot run the program.

Somewhere in the body of the function is a c++ mechanism that forces compiling to fail, which alerts the user that you cannot use this function for such an inferior un-amazing object.

Is this possible?

To clarify, this is something I want to do a compile time based the contents of a variable, as shown in my example above. The suggestion to use static_assert does not apply here.

Upvotes: 7

Views: 14401

Answers (3)

user529758
user529758

Reputation:

You can either static_assert() a condition at compile time (C++11)

static_assert(false, "Hey user! You suck!");

or use

#if (some_erroneous_condition_to_be_avoided)
#error "Hey user! You suck!"
#endif

if you have a GNU-compatible compiler (g++, clang++, etc.)

Upvotes: 21

Richard
Richard

Reputation: 546

It is impossible. The value of the variable is decided at runtime, but you want to throw a compile-time error depending on the runtime value.

Upvotes: 1

Joachim Isaksson
Joachim Isaksson

Reputation: 181077

The only way I can see to get it compile time checked is to subclass AwesomeClass and restrict the new class' creation to only be able to create objects where amazingObject.isntAmazing is never true. Then change the signature to;

void doSomethingIncredible(AwesomeAndAmazingClass amazingObject)

That will prevent the call to the method for objects that are simply awesome but not amazing.

As a maybe more illustrative example (not compiled, so consider pseudo code);

class Thing {
  protected: 
    Color _color;
    Shape _shape;
  public:
    Thing(Color color, Shape shape) {
      _color=color; _shape=shape;
    }
}

class GreenThing : Thing {
  public:
    GreenThing(Shape shape) : Thing(Color.Green, shape) {}
}

void doSomethingIncredible(GreenThing specialThing)
{
  // specialThing here is still a Thing, but also compile time
  // checked to also always be a GreenThing
}

Upvotes: 3

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