C++: Can a struct inherit from a class?

Question

I am looking at the implementation of an API that I am using.

I noticed that a struct is inheriting from a class and I paused to ponder on it...

First, I d

Answer 1

In C++

struct A { /* some fields/methods ... */ };

is equivalent to:

class A { public: /* some fields/methods ... */ };

And

class A { /* some fields/methods ... */ };

is equivalent to:

struct A { private: /* some fields/methods ... */ };

That means that the members of a struct/class are by default public/private.

Using struct also changes the default inheritance to public, i.e.

struct A { }; // or: class A { };
class B : A { };

is equivalent to

struct A { }; // or: class  A { };
struct B : private A { };

And the other way around, this

struct A { }; // or: class A { };
struct B : A { };

is equivalent to:

struct A { }; // or: class A { };
class B : public A { };

Summary: Yes, a struct can inherit from a class. The difference between the class and struct keywords is just a change in the default private/public specifiers.

Answer 2

A class will not publicly inherit from a struct. A struct will publicly inherit from a class or a struct.

class A
{
public:
    int a;
};
struct B : A
{};

B b; b.a=5; //OK. a is accessible

class A
{
public:
    int a;
};
struct B : public A
{};

It means the same. B b; b.a=5; //OK. a is accessible

struct A
{int a;};
class B : A
{};

B b; b.a=5; //NOT OK. a is NOT accessible

struct A
{int a;};
class B : public A
{};

B b; b.a=5; //OK. a is accessible

Finally:

class A
{int a;};
class B : A
{};

B b; b.a=5; //NOT OK. a is NOT accessible

class A
{int a;};
class B : public A
{};

B b; b.a=5; //NOT OK. a is NOT accessible

Answer 3

Yes. Struct can inherit from a class and vice versa. The accessibility rule is

$11.2/2- "In the absence of an access-specifier for a base class, public is assumed when the derived class is declared struct and private is assumed when the class is declared class."

EDIT 2: So you can change your class as:. Note that it is a bad idea to have public data members usually.

class user_messages {  // i also changed the name when OP was modified :)
public:   
   std::list<std::string> messages;  
};