定义仅某个类所需的结构的最佳方法是什么？

Question

Given the following simplified class which contains a vector of MyStruct objects, where should I define MyStruct assuming that it's only being used inside of Foo and should not be seen or used from outside?

class Foo
{
  std::vector<MyStruct> array;
};

Two possibilities I've come across are putting the definition of MyStruct right inside of Foo or using an anonymous namespace.

Answer 1

If array is in any way a public part of Foo interface, declare MyStruct like this:

class Foo
{
public:
    struct MyStruct { /* stuff */ };
    /* ... */
};

Otherwise: declare it in the private section of Foo .

Answer 2

In this case, you have to define it in Foo , since it has to be completely defined before you use it to instantiate std::vector . Otherwise, it depends; I'd tend to define it in a private namespace: an unnamed namespace in the implementation file if all of the functions are in a single source file, otherwise in a private, implementation header file in a private namespace (eg FooPrivate ) if they aren't.

Answer 3

If you do not want it to be used outside Foo, then the best way is to declare it in the private section of Foo

class Foo
{
    struct My_Struct;
}