Unions Vs Struct.
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I have read in a couple of places that unions should not be used unless its very essential! I do not understand why? sometimes I create unions with many struct in it. Does any one has any idea on why unions should not be prefered ?
-Prakash
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Unions are not "type" safe since the same data element can be represented by two different types.
a programmer traped in a thugs body
ky_rerun wrote:
Unions are not "type" safe since the same data element can be represented by two different types.
Well that is whole purpose of unions, plus there are many other data types which are not type safe.
-Prakash
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I have read in a couple of places that unions should not be used unless its very essential! I do not understand why? sometimes I create unions with many struct in it. Does any one has any idea on why unions should not be prefered ?
-Prakash
Members of unions cannot have an incomplete type, type void, or function type. Therefore members cannot be an instance of the union but can be pointers to the union type being declared. A union type declaration is a template only. Memory is not reserved until the variable is declared. If a union of two types is declared and one value is stored, but the union is accessed with the other type, the results are unreliable. For example, a union of float and int is declared. A float value is stored, but the program later accesses the value as an int. In such a situation, the value would depend on the internal storage of float values. The integer value would not be reliable.
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I have read in a couple of places that unions should not be used unless its very essential! I do not understand why? sometimes I create unions with many struct in it. Does any one has any idea on why unions should not be prefered ?
-Prakash
I general, the purpose of a union is to save space. Say you're storing data for a fruit and you have 3 types: Apples, Oranges and Pears. The representaion of these three fruits is distinct and a fruit can only be 1 of these types. Consider a struct that embodies this design:
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. Apple m_Apple; Orange m_Orange; Pear m_Pear; };If we using this to save our fruit data to disc only one of the three types would actually contain valid data - The space occupied by the others is wasted. We can improve on this by using a union.
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. union { Apple m_Apple; Orange m_Orange; Pear m_Pear; } m_Data };With this format we don't waste as much space - the size of the union
m_Datais the size of its largest member and all members occupy the same memory. As the previous posters indicated - This saving comes at a price - Danger! Steve -
I general, the purpose of a union is to save space. Say you're storing data for a fruit and you have 3 types: Apples, Oranges and Pears. The representaion of these three fruits is distinct and a fruit can only be 1 of these types. Consider a struct that embodies this design:
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. Apple m_Apple; Orange m_Orange; Pear m_Pear; };If we using this to save our fruit data to disc only one of the three types would actually contain valid data - The space occupied by the others is wasted. We can improve on this by using a union.
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. union { Apple m_Apple; Orange m_Orange; Pear m_Pear; } m_Data };With this format we don't waste as much space - the size of the union
m_Datais the size of its largest member and all members occupy the same memory. As the previous posters indicated - This saving comes at a price - Danger! SteveStephen Hewitt wrote:
This saving comes at a price - Danger!
What Danger!
Jesus Loves:rose:
--Owner Drawn:rose: --Nothing special --Defeat is temporary but surrender is permanent --Never say quits --Jesus is Lord:rose:
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Stephen Hewitt wrote:
This saving comes at a price - Danger!
What Danger!
Jesus Loves:rose:
--Owner Drawn:rose: --Nothing special --Defeat is temporary but surrender is permanent --Never say quits --Jesus is Lord:rose:
Hard to spot casts:
union Voodoo { int m_Number; IUnknown* m_pUnk; }; Voodoo v; v.m_Number = 5; v.m_pUnk->AddRef(); // Danger!!!!!What happens here is effectively something like the following:
reinterpret_cast<IUnknown*>(5)->AddRef();In fact this is an old "C" trick used to perform casts which are otherwise illegal. Steve
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Hard to spot casts:
union Voodoo { int m_Number; IUnknown* m_pUnk; }; Voodoo v; v.m_Number = 5; v.m_pUnk->AddRef(); // Danger!!!!!What happens here is effectively something like the following:
reinterpret_cast<IUnknown*>(5)->AddRef();In fact this is an old "C" trick used to perform casts which are otherwise illegal. Steve
Smoking is injurious to health but still some smoke.
Jesus Loves:rose:
--Owner Drawn:rose: --Nothing special --Defeat is temporary but surrender is permanent --Never say quits --Jesus is Lord:rose:
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I general, the purpose of a union is to save space. Say you're storing data for a fruit and you have 3 types: Apples, Oranges and Pears. The representaion of these three fruits is distinct and a fruit can only be 1 of these types. Consider a struct that embodies this design:
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. Apple m_Apple; Orange m_Orange; Pear m_Pear; };If we using this to save our fruit data to disc only one of the three types would actually contain valid data - The space occupied by the others is wasted. We can improve on this by using a union.
struct Fruit { Type m_Type; // Tells us if we're and apple, orange or pear. union { Apple m_Apple; Orange m_Orange; Pear m_Pear; } m_Data };With this format we don't waste as much space - the size of the union
m_Datais the size of its largest member and all members occupy the same memory. As the previous posters indicated - This saving comes at a price - Danger! Stevethe other simple method is u can process the items on the fly...
Stephen Hewitt wrote:
union { Apple m_Apple; Orange m_Orange; Pear m_Pear; } m_Data
for eg as mentioned above u can use 3 types of fruits.... read the quantity of apples for each vendor eg. v1.apple v2.apple and perform the calculations...then v1.orange,v2.orange...(since u don't need v1.apple again)and do the calculations..in that way u can perform computations and re-use the memory this is just one specific instance though hope that helps:)
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Members of unions cannot have an incomplete type, type void, or function type. Therefore members cannot be an instance of the union but can be pointers to the union type being declared. A union type declaration is a template only. Memory is not reserved until the variable is declared. If a union of two types is declared and one value is stored, but the union is accessed with the other type, the results are unreliable. For example, a union of float and int is declared. A float value is stored, but the program later accesses the value as an int. In such a situation, the value would depend on the internal storage of float values. The integer value would not be reliable.
vallikumar wrote:
A union type declaration is a template only. Memory is not reserved until the variable is declared.
? Not sure what you are trying to say. Memory allocation (actually, setting a new high-water mark for the stack) for all stack variables is deferred until the start of the block they are declared in. It sounds like you are trying to say that mem alloc is different for unions than for int's, struct's, ... The size of a union is known at compile time (just like a struct) - it is the size of the largest member. ...cmk Save the whales - collect the whole set
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Unions are not "type" safe since the same data element can be represented by two different types.
a programmer traped in a thugs body
ky_rerun wrote:
Unions are not "type" safe since the same data element can be represented by two different types.
i guess you have a good point check here http://www.weirdrealm.com/prog/cppstds.html[^] it says, Unions Avoid unions, as they defeat strong type-checking and are very difficult to verify in a debugger.
-Prakash