The Abstract specifier makes a class uninstantiable in the Editor and for most other purposes. This has several implications, including that the class (if it is an Actor) may not be placed in a Level. Nor can Abstract components be added to Actors. It also means that non-Actor classes cannot be instanced in Blueprints (e.g. by using a Construct node). This specifier prevents instantiation of the class in code as well. For example, the UMeshComponent class is marked as Abstract. This means that the following constructor code, which attempts to instantiate a Mesh Component, will fail to compile:
AActor::AActor()
{
MeshComp = CreateDefaultSubobject<UMeshComponent>(TEXT("Mesh"));
}The Unreal Header Tool will emit the following (or a similar) error message: "Class which was marked abstract was trying to be loaded. It will be nulled out on save." However, subclasses of an Abstract class can be instantiated. For example, the following construction using USkeletalMeshComponent, a subclass of UMeshComponent, compiles without issue:
AActor::AActor()
{
MeshComp = CreateDefaultSubobject<USkeletalMeshComponent>(TEXT("Mesh"));
}On a related note, a Blueprint class can be marked as Abstract from its Class Settings panel. This appears to have the same behavior as marking a native class with the Abstract specifier.