Thread: union u_type What it is?

Code:

#include <iostream>
union u_type
{
u_type(short int a);
void showchars();
short int i;
char ch[2];
};
u_type::u_type(short int a)
{
i = a;
}
void u_type::showchars()
{
std::cout << ch[0] << " ";
std::cout << ch[1] << "\n";
}
int main()
{
u_type u(1000);
u.showchars();
return 0;
}

Hello!
What it is? In my head non-selection. The author of the code charges the value in short int i ;. The value is inserted, this desire of the author is seen in the code, and indeed, the number gets where it would be desirable.
But char ch [2]; separately, it simply lies in the union. No number was loaded into it, no symbols were inserted into it at all.
Suddenly we print this to the console. How so? What is in it? Case values? What's going on here?
The console displays a letter or badge, like the Ukrainian letter ш, and a heart, a card suit of hearts. Where from?

Can my eyes today not see the elementary? Or I do not understand important principles?

A union is like a structure, except that all data members occupy the same space in memory. However, your example exhibits undefined behavior, because you may only read from the field that was most recently written to. Most implementations define it to do the thing that you're attempting to demonstrate here, but it would be technically correct, according to the international C++ standard, to do something completely different.

> Suddenly we print this to the console. How so? What is in it? Case values? What's going on here?
Try it with say

u_type u(0x3031);
u_type u(0x4142);
u_type u(0x6670);

Different member union access is implementation defined (in C99 at least).

Originally Posted by c99

With one exception, if the value of a member of a union object is used when the most
recent store to the object was to a different member, the behavior is
implementation-defined. 70) One special guarantee is made in order to simplify the use of
unions: If a union contains several structures that share a common initial sequence (see
below), and if the union object currently contains one of these structures, it is permitted to
inspect the common initial part of any of them anywhere that a declaration of the
completed type of the union is visible. Two structures share a common initial sequence if
corresponding members have compatible types (and, for bit-fields, the same widths) for a
sequence of one or more initial members.

Originally Posted by Salem

Different member union access is implementation defined (in C99 at least).

In C++, according to Union declaration - cppreference.com, it is undefined. I'm looking for the specific section of the standard, but cppreference is usually pretty accurate.

Many thanks! I am very thankful you that you clarify my questions!


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