Use `union`s to save memory

Reason

A union allows a single piece of memory to be used for different types of objects at different times. Consequently, it can be used to save memory when we have several objects that are never used at the same time.

Example

union Value {
    int x;
    double d;
};

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

But heed the warning: Avoid "naked" unions

Example

// Short-string optimization

constexpr size_t buffer_size = 16; // Slightly larger than the size of a pointer

class Immutable_string {
public:
    Immutable_string(const char* str) :
        size(strlen(str))
    {
        if (size < buffer_size)
            strcpy_s(string_buffer, buffer_size, str);
        else {
            string_ptr = new char[size + 1];
            strcpy_s(string_ptr, size + 1, str);
        }
    }

    ~Immutable_string()
    {
        if (size >= buffer_size)
            delete[] string_ptr;
    }

    const char* get_str() const
    {
        return (size < buffer_size) ? string_buffer : string_ptr;
    }

private:
    // If the string is short enough, we store the string itself
    // instead of a pointer to the string.
    union {
        char* string_ptr;
        char string_buffer[buffer_size];
    };

    const size_t size;
};

Enforcement

???