C++ Core Guidelines: Rules for Unions

A union is a particular data type where all members start at the same address. A union can hold only one type at a time; therefore, you can save memory. A tagged union is a union that keeps track of its types.

Here are the four rules for unions.

• C.180: Use unions to save memory
• C.181: Avoid “naked” unions
• C.182: Use anonymous unions to implement tagged unions
• C.183: Don’t use a union for type punning

Let's start with the most obvious rule.

C.180: Use unions to save memory

Because a union can hold only one type at one point at a time, you can save memory. The union will be as big as the biggest type.

union Value {
    int i;
    double d;
};

Value v = { 123 };      // now v holds an int
cout << v.i << '\n';    // write 123
v.d = 987.654;          // now v holds a double
cout << v.d << '\n';    // write 987.654

Value is a "naked" union. You should not use it according to the next rule.

C.181: Avoid “naked” unions

"Naked" unions are error-prone because you must keep track of the underlying type.

// nakedUnion.cpp

#include <iostream>

union Value {
    int i;
    double d;
};

int main(){
  
  std::cout << std::endl;

  Value v;
  v.d = 987.654;  // v holds a double
  std::cout << "v.d: " << v.d << std::endl;     
  std::cout << "v.i: " << v.i << std::endl;      // (1)

  std::cout << std::endl;

  v.i = 123;     // v holds an int
  std::cout << "v.i: " << v.i << std::endl;
  std::cout << "v.d: " << v.d << std::endl;      // (2)
  
  std::cout << std::endl;

}

 The union holds a double in the first iteration and an int value in the second iteration. If you read a double as an int (1) or an int as a double (2), you get undefined behavior.

 To overcome this source of errors, you should use a tagged union.

C.182: Use anonymous unions to implement tagged unions

Implementing a tagged union is quite sophisticated. In case you are curious, have a look at rule C.182. I will just make it easy and will write about the new C++ standard.

With C++17, we get a tagged union: std::variant. std::variant is a type-safe union. Here is a first impression.

// variant.cpp

#include <variant>
#include <string>
 
int main(){

  std::variant<int, float> v, w;       // (1)
  v = 12;                              // v contains int
  int i = std::get<int>(v);            // (2)        
                                       
  w = std::get<int>(v);                // (3)
  w = std::get<0>(v);                  // same effect as the previous line
  w = v;                               // same effect as the previous line

                                       // (4)
  //  std::get<double>(v);             // error: no double in [int, float]
  //  std::get<3>(v);                  // error: valid index values are 0 and 1
 
  try{
    std::get<float>(w);                // w contains int, not float: will throw
  }
  catch (std::bad_variant_access&) {}
 
                                       // (5)
  std::variant<std::string> v("abc");  // converting constructors work when unambiguous
  v = "def";                           // converting assignment also works when unambiguous

}

In (2), I define the two variants v and w. Both can have an int and a float value. Their initial value is 0. This is the default value for the first underlying type. v becomes 12. std::get<int>(v) returns the value using the type. Line (3) and the following two lines show three possibilities to assign the variant v the variant w. But you have to keep a few rules in mind. You can ask for a variant's value by type or index. The type must be unique and the index valid (4). If not, you will get a std::bad_variant_access exception. If the constructor or assignment call is unambiguous, a conversion occurs. This is why it's possible to construct a std::variant<std::string> with a C-string or assign a new C-string to the variant (5).

C.183: Don’t use a union for type punning

At first, what is type punning? Type punning is the possibility of a programming language intentionally subverting the type system to treat a type as a different type. One typical way to do type punning in C++ is to read the member of a union with a different type from the one with which it was written.

What is wrong with the following function bad?

union Pun {
    int x;
    unsigned char c[sizeof(int)];
};

void bad(Pun& u)
{
    u.x = 'x';
    cout << u.c[0] << '\n';       // undefined behavior (1)
}

void if_you_must_pun(int& x)
{
    auto p = reinterpret_cast<unsigned char*>(&x);   // (2)

    cout << p[0] << '\n';                            // OK; better 
    // ... 
}

Expression (1) has two issues. First and foremost, it's undefined behavior. Second, the type punning is quite challenging to find. If you have to use type punning, do it with an explicit cast such as reinterpret_cast in (2). With reinterpret_cast you have at least the possibility to spot your type punning afterwards.

What's next?

Admittedly, this final post on rules for classes and class hierarchies was a bit short. with the next post, I will write about the next significant section: enumerations.