Incompatible operand types when using ternary conditional operator

Andrey Chernukha

this code:

  bool contains = std::find(indexes.begin(), indexes.end(), i) != indexes.end();
  CardAbility* cardAbility = contains ? new CardAbilityBurn(i) : new CardAbilityEmpty;

gives me the following error:

Incompatible operand types CardAbilityBurn and CardAbilityEmpty

However, if I write the code like this:

 if (contains)
 {
    cardAbility = new CardAbilityBurn(i);
 }
 else
 {
    cardAbility = new CardAbilityEmpty;
 }

Then the compiler doesn't care. why is it like this? I want to use the ternary conditional operator because it's only one line. What's up

I need to point out (I think you might need this info), CardAbilityEmptyandCardAbilityBurn both stem from CardAbilitysiblings.

thanks

Quentin

C++'s type system determines the type of an expression from the inside out [1] . This means that the type of the conditional expression is determined before assignment to to , and the compiler must choose between only andCardAbility*CardAbilityBurn*CardAbilityEmpty* .

Since C++ has multiple inheritance and more possible conversion paths, since all types are not superclasses of other types, the compilation ends here.

In order to compile successfully, you need to provide the missing part: cast one or both operands to a base class type, so the entire conditional expression can take that type.

auto* cardAbility = contains
    ? static_cast<CardAbility*>(new CardAbilityBurn(i))
    : static_cast<CardAbility*>(new CardAbilityEmpty  );

(Note the use of auto, since you already provided the target type in the right-hand side expression.)

It 's but a bit puzzling, so in the end if-else the structure is better suited in this case.

[1] There is one exception: overloaded function names do not have a definite type until they are (implicitly or explicitly) converted to one of their versions.

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