It’s explicitly legal. The rule is that the types used to instantiate a template in the standard library must be complete, unless otherwise specified. In the case of unique_ptr, §20.7.1/5 says “[…] The template parameter T of unique_ptr may be an incomplete type.” There are certain operations on the pointer which require a complete type; … Read more
Wrap the invocation into a function of its own so that foo() def foo(): print “Hi!” will break, but def bar(): foo() def foo(): print “Hi!” bar() will work properly. The general rule in Python is that a function should be defined before its usage, which does not necessarily mean it needs to be higher … Read more
The forward-declaration method is almost always better. (I can’t think of a situation where including a file where you can use a forward declaration is better, but I’m not gonna say it’s always better just in case). There are no downsides to forward-declaring classes, but I can think of some downsides for including headers unnecessarily: … Read more
What you can do is to wrap the invocation into a function of its own. So that foo() def foo(): print “Hi!” will break, but def bar(): foo() def foo(): print “Hi!” bar() will be working properly. General rule in Python is not that function should be defined higher in the code (as in Pascal), … Read more
It basically tells the compiler that the class RootViewController exists, without specifying what exactly it looks like (ie: its methods, properties, etc). You can use this to write code that includes RootViewController member variables without having to include the full class declaration. This is particularly useful in resolving circular dependencies – for example, where say … Read more
You can do forward typedef. But to do typedef A B; you must first forward declare A: class A; typedef A B;
This is simply not possible. You cannot forward declare a nested structure outside the container. You can only forward declare it within the container. You’ll need to do one of the following Make the class non-nested Change your declaration order so that the nested class is fully defined first Create a common base class that … Read more
You can’t do it, it’s a hole in the C++ language. You’ll have to un-nest at least one of the nested classes.
Why forward-declare is necessary in C++ The compiler wants to ensure you haven’t made spelling mistakes or passed the wrong number of arguments to the function. So, it insists that it first sees a declaration of ‘add’ (or any other types, classes, or functions) before it is used. This really just allows the compiler to … Read more
Put yourself in the compiler’s position: when you forward declare a type, all the compiler knows is that this type exists; it knows nothing about its size, members, or methods. This is why it’s called an incomplete type. Therefore, you cannot use the type to declare a member, or a base class, since the compiler … Read more