Mixing Class and Struct

Mixing class and struct

It looks to me like it's defined behavior. In particular, §9.1/2 says:

A declaration consisting solely of class-key identifier ; is either a redeclaration of the
name in the current scope or a forward declaration of the identifier as a class name. It introduces the class name into the current scope.

The standard distinguishes between using class, struct or union when defining a class, but here, talking about a declaration, no such distinction is made -- using one class-key is equivalent to any other.

When should you use a class vs a struct in C++?

The differences between a class and a struct in C++ is:

struct members and base classes/structs are public by default.
class members and base classes/struts are private by default.

Both classes and structs can have a mixture of public, protected and private members, can use inheritance and can have member functions.

I would recommend you:

use struct for plain-old-data structures without any class-like features;
use class when you make use of features such as private or protected members, non-default constructors and operators, etc.

What are the differences between struct and class in C++?

You forget the tricky 2nd difference between classes and structs.

Quoth the standard (§11.2.2 in C++98 through C++11):

In 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.

And just for completeness' sake, the more widely known difference between class and struct is defined in (11.2):

Member of a class defined with the
keyword class are private by
default. Members of a class defined
with the keywords struct or union
are public by default.

Additional difference: the keyword class can be used to declare template parameters, while the struct keyword cannot be so used.

C++ Class or Struct compatiblity with C struct

Yes.

Use the same types in the same order in both languages
Make sure the class doesn't have anything virtual in it (so you don't get a vtable pointer stuck on the front)
Depending on the compilers used you may need to adjust the structure packing (usually with pragmas) to ensure compatibility.

(edit)

Also, you must take care to check the sizeof() the types with your compilers. For example, I've encountered a compiler that stored shorts as 32 bit values (when most will use 16). A more common case is that an int will usually be 32 bits on a 32-bit architecture and 64 bits on a 64-bit architecture.

Forward declaration as struct vs class

struct and class are completely interchangeable as far as forward declarations are concerned. Even for definitions, they only affect the default access specifier of the objects members, everything else is equivalent. You always define "classes" of objects.

The only place where struct must be used over class, is when forward declaring opaque data for c bindings.

Regarding your edit:

I know the differences between class/struct regarding default public/private but I'm asking specifically about the forward declarations and possible consequences of swapping them.

Visual C++ produces warning C4099. It does it, because the name decoration for its functions incorporates the keyword you used. Therefore, programs may fail to link properly. So perfectly standard compliant code may not link when using VC++ (A bonehead move on Microsoft's part, AFAIC).

A discussion of this warning, as well as why it can be ignored if you are disciplined, can be found here

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

Yes, struct can inherit from class in C++.

In C++, classes and struct are the same except for their default behaviour with regards to inheritance and access levels of members.

C++ class

Default Inheritance = private
Default Access Level for Member Variables and Functions = private

C++ struct

Default Inheritance = public
Default Access Level for Member Variables and Functions = public

Mix of template and struct

Just remove typedef:

template <class T1, class T2>
class MyClass {
    struct myStruct{
        T1 templateMember;
        // rest of members
    } ;
};

Is it possible to have C struct as a member of C++ class?

No, you don't need to use the constructor to initialize.
Compiler knows it's a "struct" data type because you defined it that way.
Plain C structures also valid c++ structures so there's nothing called as plain c structure.
Learn more about compilers here: https://www.freecodecamp.org/news/c-compiler-explained-what-is-the-compiler-and-how-do-you-use-it/

Mixing Class and Struct