High Integrity C++ Coding Standard

11.1 Access Specifiers

11.1.1 Declare all data members private

If direct access to the object state is allowed through public or protected member data, encapsulation is reduced making the code harder to maintain. By implementing a class interface with member functions only, precise control is achieved over modifications to object state as well as allowing for pre and post conditions to be checked when accessing data.

#include <string>
#include <cassert>
               
class C
{
public:
C ();
                 
std::string m_id; // @@- Non-Compliant -@@
};

class D
{
public:
D ();
                 
std::string const & getId () const
{
assert (! m_id.empty () && "Id not yet specified");
return m_id;
}
                 
private:
std::string m_id; // @@+ Compliant +@@
};

Exception

Class types declared with the class key struct in an extern “C” block are intended to be compatible with C, and therefore such definitions are not covered by this rule.

For example:

extern "C"
{
struct S // Compliant
{
int i;
int j;
};
}

References

• HIC++ v3.3 – 3.4.1

View references >

11.2 Friends

11.2.1 Do not use friend declarations

Friend declarations reduce encapsulation, resulting in code that is harder to maintain.

class C
{
public:
C & operator += (C const & other);
                 
friend C const operator + (C const &, C const & lhs); // @@- Non-Compliant -@@
};
               
class D
{
public:
D & operator += (D const & other);
};
               
D const operator + (D const & rhs, D const & lhs) // @@+ Compliant +@@
{
D result (rhs);
result += (lhs);
return result;
}

The following example demonstrates an additional complexity when adding friendship for functions that include universal reference parameters. In such cases an explicit declaration is required for all combinations of the lvalue reference and rvalue reference versions.

template <typename T, typename S>
void foo (T && t, S && s)
{
t.foo ();
}
               
class A
{
private:
void foo ();
               
friend void foo(A &, A &);
friend void foo(A &, A&&);
friend void foo(A&&, A &);
friend void foo(A&&, A&&);
};
               
int main ()
{
A a;
foo(a, a);
foo(a, A());
foo(A(), a);
foo(A(), A());
}

Exception

In some cases friend declarations are part of the class interface, for example:

• serialization via stream input and output operators (operator << and operator >>)
• factory functions requiring access to private constructors
• an iterator class

For example:

# include <cstdint>
class C
{
public :
C (C const &) = default ;
C (C &&) = default ;
private :
C ( int32_t );
friend C createClassC ( int32_t );
};
C createClassC ( int32_t i)
{
// pre condition check on ’i’
return C(i);
}

References

• HIC++ v3.3 – 3.4.4

View references >