Union's
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From: anonymusius on 15 Jul 2006 06:37 I'm sorry if this is an noob qustion but what is the exact point of having two variable's on the same memory adress in an union? Wouldn't it be better/usefuller to just declare one variable? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: simon.rutishauser on 15 Jul 2006 10:37 Hi, On Sat, Jul 15, 2006 at 06:37:20AM -0400, anonymusius wrote: > I'm sorry if this is an noob qustion but what is the exact point of > having two variable's on the same memory adress in an union? > Wouldn't it be better/usefuller to just declare one variable? The point might be that you are sure that you will only use one of these variables (usually having different types) at a time. So it might be a way to save memory for instance when passing data to a function where the type of data is not always the same. Actually that's the only use of unions I have seen in practice so far (and it was *not* me who wrote the code). The problem ist that it might happen that even though you thought you wouldn't use both variables that are in the union at the same time, someone somewhere does. I think this has the potential for lots of bugs that are not easy to spot. Actually I don't think there is much use for unions in C++ nowadays. Maybe there was some in C - but I think you'd much better use derived classes or something else rather than unions. Greetings Simon [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Chris Hills on 15 Jul 2006 10:36 In article <1152894252.318739.207210(a)m79g2000cwm.googlegroups.com>, anonymusius <annoniemo(a)gmail.com> writes >I'm sorry if this is an noob qustion but what is the exact point of >having two variable's on the same memory adress in an union? Wouldn't >it be better/usefuller to just declare one variable? It is a way of converting things. You can put 2 chars (or 8 bit ints) in and get an int (16 bit ) out etc. -- \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ \/\/\/\/\ Chris Hills Staffs England /\/\/\/\/ /\/\/ chris(a)phaedsys.org www.phaedsys.org \/\/\ \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Thomas Richter on 15 Jul 2006 11:45 anonymusius wrote: > what is the exact point of > having two variable's on the same memory adress in an union? Wouldn't > it be better/usefuller to just declare one variable? Unions are more or less a legacy feature of C that still made it into C++, and that is not very often used today. In very memory-critical situations you'd like to be able to re-use the same memory location twice (or even more often) depending on some type descriptor, e.g. union { int i_number; float f_number; }; would be able to store either an integer or a floating point variable describing one quantity in the same memory location(s). Note the "either-or", as this union cannot hold an int and a float simultaneously. As what it holds is up to the interpretation of the program, and this information must be stored elsewhere. Or, another application is very specialized hardware that maps several functions onto the same memory location. Unions are also sometimes (mis?-) used to re-interpret the bit-pattern of a specific internal representation. The above example *could* be used (on some architectures, on some compilers) to re-interpret the bit-pattern describing a floating point variable as an integer. So long, Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: kulkarnivarad on 15 Jul 2006 20:07
anonymusius wrote: > I'm sorry if this is an noob qustion but what is the exact point of > having two variable's on the same memory adress in an union? Wouldn't > it be better/usefuller to just declare one variable? > If you know about the cpu registers, union is the useful way of addressing them. Each register of 16-bit in length (AX,BX,CX etc) can be modified using two 8-bit variables(for AH, AL etc), which is really useful in calling of interrupts, when you want to maintain clarity as well. This is the structure defined in dos.h : struct BYTEREGS { unsigned char al, ah, bl, bh; unsigned char cl, ch, dl, dh; }; struct WORDREGS { unsigned int ax, bx, cx, dx; unsigned int si, di, cflag, flags; }; union REGS { BYTEREGS h; WORDREGS x; }; REGS regs; So you can access entire AX register as regs.x.ax or you can access each high or low byte register AH or AL as regs.h.ah or regs.h.al. e.g. To display a string on text-mode screen using BIOS interrupt 10h using function number 0Ah: #include <dos.h> // set the function number in AH, character to display in AL, // the screen page number in BH and the number of times the character // gets repeated in CX void main (){ REGS in,out; in.h.ah=0x0a; // the interrupt function number in.h.al='a'; // character to display in.h.bh=0; // page number in.x.cx=1; // repeatation int86(0x10,&in,&out); // call interrupt 10h } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |