The object has type qualifiers that are not compatible

[AlekK]

hello,in BattleTank game i’m having problem with this line of code:
GetControlledTank()->AimAt(GetPlayerTank()->GetActorLocation());
i have an error on GetControlledTank() that says:
the object has type qualifiers that are not compatible with the member function ATank::AimAt().

i tried googling it and finally found out that const objects can only invoke const member functions and then i tried to declare AimAt() method definition as const,but it’s still no use and i still get the same error.only when i remove const keyword in GetControlledTank() definition the code compiles with no error.anyway, i haven’t seen Ben having such an issue when he was implementing this.any idea what the problem is?
Thanks in advance!

[DanM]

Could you provide more information.
Where are you calling this function? What were your declarations with const?

[AlekK]

thanks for your response.i have a c++ class called ATank which is a pawn. here is the declaration in Tank.h:
public:
const void AimAt(FVector HitLocation);
and in .cpp file i implemented it like this:
const void ATank::AimAt(FVector HitLocation){
auto TankName=GetName();
UE_LOG(LogTemp, Warning, TEXT("%s is at%s),*TankName,HitLocation.ToString());
}
and there is another c++ class called TankPlayerController at which i have GetControllledTank function here is the header file and .cpp:
public:
const ATank GetControlledTank();

.cpp file:
cont ATank* GetControlledTank(){
cast(ATank)(GetPawn()); //Atank is actually in <> it wouldn’t show it if i wrote it in <> that’s why i used simple parentheses.
}
and i’m calling in tick function of TankPlayerController like so:
void ATankPlayerController::Tick(float DeltaTime)
{
Super::Tick(DeltaTime);
GetControlledTank()->AimAt(GetPlayerTank()->GetActorLocation());
}
i hope these are sufficient.

[DanM]

You have const on the wrong side

const ATank* GetControlledTank();

This means you’re returning a const actor that is const not a const member function to do that you would write

ATank* GetControlledTank() const;

[AlekK]

thanks its working.but i dunno the use of second one i mean how it works can you explain a little more ? i used to think whole my life that it doesn’t matter where you put the const keyword and both are the same.

[DanM]

As in the value that is returned is const.

const AActor* Actor = /*something*/;
//vs
AActor* Actor = /*something*/;

[AlekK]

look in this function: const ATank* GetControlledTank();
it means the value that it returns is const and cannot be changed right ? and you are saying the second one is also the same. then it means there is no difference. but u said they are different.

[DanM]

They are different.

const ATank* GetControlledTank();

This returns a const object. It is not a const member function.

ATank* GetControlledTank() const;

This on the other hand returns a non-const object but is a const member function.

---

A const member function means it can’t modify data members.

class Example
{
    int Value = 10;
    void Foo() const
    {
        Value = 2; //ERROR trying to modify data member in const member function
    }
};

[AlekK]

oh i see thanks a lot so if its just a void function like AimAt() can we use const on both sides ? i mean are these two the same:
1_const AimAt()
2_AimAt() const

[DanM]

No that’s the same thing as above although missing the type. const at the end and only the end of a function means it’s a const member function.

[AlekK]

then something like this :const void AimAt() is meaningless right ? but i declared it like this and got no errors.although i still would get the error as you said cuz it wouldn’t recognize it as a const member function.it means the compiler just ignores the word const here right ?
and the last thing.sorry that i’m asking a lot.does it make sense to declare a normal function as const too ? i mean a function outside a class.thanks so much!

[DanM]

then something like this :const void AimAt() is meaningless right ?

Yes.

does it make sense to declare a normal function as const too ? i mean a function outside a class.

There’s no such thing as a const non-member function - what would the meaning of that be? - so that would fail to compile.

[AlekK]

thank you for your perfect and exact answers!.