Note: You might want to look at the operator overloading FAQ. Binary operators can either be members of their left-hand argument’s class or free functions. (Some operators, like assignment, must be members.) Since the stream operators’ left-hand argument is a stream, stream operators either have to be members of the stream class or free functions. … Read more
Because I may write Foo and its friend Bar (thus there is a trust relationship). But do I trust the people who write classes that are derived from Bar? Not really. So they should not inherit friendship. Any change in the internal representation of a class will require a modification to anything that is dependent … Read more
There is a very simple pattern, which has retro-actively been dubbed PassKey, and which is very easy in C++11: template <typename T> class Key { friend T; Key() {} Key(Key const&) {} }; And with that: class Foo; class Bar { public: void special(int a, Key<Foo>); }; And the call site, in any Foo method, … Read more
It is explicitly disallowed in the standard, even if some versions of VisualStudio do allow it. C++ Standard 7.1.5.3 Elaborated type specifiers, paragraph 2 3.4.4 describes how name lookup proceeds for the identifier in an elaborated-type-specifier. If the identifier resolves to a class-name or enum-name, the elaborated-type-specifier introduces it into the declaration the same way … Read more
It’s just a warning about a tricky aspect of the language. When you declare a friend function, it is not a member of the class the declaration is in. You can define it there for convenience, but it actually belongs to the namespace. Declaring a friend function which is not a template, inside a class … Read more
Friendship in C++ is not transitive: John is a friend of mine and he can use my wireless connection any time (I trust him). John’s friend Tim though is a waster and though John is my friend I do not include Tim as a friend, and thus I don’t let him use my wireless connection. … Read more
const date& being accepted by the method date_ok means that date_ok takes a reference of type const date. It works similar to pointers, except that the syntax is slightly more .. sugary in your example, int* Y = &x makes Y a pointer of type int * and then assigns it the address of x. … Read more
You need a few forward declarations: template <typename T> class Obj; template <typename T> Obj<T> make_obj(T t); template <typename T> class Obj { private: T & t; Obj (T & t) : t(t) { } Obj() = delete; friend Obj make_obj<T>(T t); }; template <typename T> Obj<T> make_obj(T t) { return Obj<T>(t); } live example … Read more
Here is a highly simplified but hopefully relevant view of what happens when you build your code in C++. C++ splits the load of generating machine executable code in following different phases – Preprocessing – This is where any macros – #defines etc you might be using get expanded. Compiling – Each cpp file along … Read more
After having asked more or less the same question here myself, I wanted to share the (apparently) updated answer for C++11: Quoted from https://stackoverflow.com/a/14759027/1984137: standard $11.7.1 “A nested class is a member and as such has the same access rights as any other member. The members of an enclosing class have no special access to … Read more