c++ - How is the memory layout of a class vs. a struct

Question

I come from C programming where the data in a struct is laid out with the top variable first, then the second, third and so on..

I am now programming in C++ and I am using a class instead. I basically want to achieve the same, but I also want get/set methods and also maybe other methods (I also want to try do it in a C++ style and maye learn something new).

Is there a guarantee e.g. that the public variables will be first in memory then the private variable?

Answer 1

Is there a guarantee e.g. that the public variables will be first in memory then the private variable?

No, such a guarantee is not made - C++11 standard, [class.mem]/14:

Nonstatic data members of a (non-union) class with the same access control (Clause 11) are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members with different access control is unspecified (11).

So

struct A
{
    int i, j;
    std::string str;

private:

    float f;

protected:

    double d;
};

It is only guaranteed that, for a given object of type A,

• i has a smaller address than j and
• j has a smaller address than str

Note that the class-keys struct and class have no difference regarding layout whatsoever: Their only difference are access-rights which only exist at compile-time.

---

It only says the order, but not that the first variable actually start at the "first address"? Lets assume a class without inheritance.

Yes, but only for standard-layout classes. There is a row of requirements a class must satisfy to be a standard-layout class, one of them being that all members have the same access-control.
Quoting C++14 (the same applies for C++11, but the wording is more indirect), [class.mem]/19:

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member. Otherwise, its address is the same as the address of its first base classsubobject (if any). [ Note: There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. — end note ]

[class]/7:

A standard-layout class is a class that:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,
• has no virtual functions (10.3) and no virtual base classes (10.1),
• has the same access control (Clause 11) for all non-static data members,
• has no non-standard-layout base classes,
• either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and
• has no base classes of the same type as the first non-static data member. ¹¹⁰

^{110) This ensures that two subobjects that have the same class type and that belong to the same most derived object are not allocated at the same address (5.10).}