Why can't I execute TArray.Contains() with a struct?

Development Programming & Scripting C++
1

I’m trying to work out how the heck to write custom == operators, and I’m really having an incredible amount of trouble figuring out what I’m doing wrong. I know there’s something wrong with what I’m doing, but I can’t figure out exactly what. In my header file, below the #includes and above my UCLASS() dec, I define the struct as per usual, and include the declaration for my operator:


USTRUCT(BlueprintType)
struct FTestStruct{
	GENERATED_USTRUCT_BODY()
		UPROPERTY(BlueprintReadOnly)
		FString name;
	bool operator==(const FTestStruct& lhs);
};

In the .cpp, I add the actual comparing logic:



bool FTestStruct::operator==(FTestStruct& lhs){
	return name == lhs.name; //Return true iff both structs' name string is the same
}

Now at least to my eyes, this looks right, and operations of the format “struct == struct” resolve correctly. However, it fails to compile the instant I try to use this with arrays. The following, for example, gives me an error “binary ‘==’: no operator found which takes a left-hand operand of type ‘const FTestStruct’ (or there is no acceptable conversion)”. Then immediately below that, it gives me a reference to the line in the .cpp my matching logic starts, and says “could be bool FTestStruct::operator ==(const FTestStruct&)”.

FTestStruct tempStruct;
TArray<FTestStruct> testArray;
if (testArray.Contains(tempStruct))
//Do stuff

The problem seems to obviously be that it’s not recognizing my definition to my declaration, but I can’t for the life of me figure out what needs to change in order for the compiler to understand that I’m trying to match the two in my array.

2

You need to use const reference AND you need to mark the operator as const.



bool FTestStruct::operator==(const FTestStruct& arg) const{
...
}

Non-const reference pretty much indicates you’ll write into that value from within the operator. non-const method (operator is a method) indicates that object can change when you call the method. That’s not what container like TArray would want.

Also, those operators can be defined as:



static bool FTestStruct::operator==(const FTestStruct &arg1, const FTestStruct &arg2{
    return arg1.name == arg2.name;
}

Also see Comparison operators - cppreference.com

3

Edited to change: never mind, I was being a moron; your suggestion fixes everything, thank you! The reference on comparison operators was a big help, it never occurred to me to seek a centralized reference. :slight_smile: