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# size and order of declaration

 P: n/a Hi I read some where about the order variables are declared in a class may help space optimization. find the size of bits a variable takes and declare them from biggest to the lowest. so in my system that would be. double int string char bool is the principle correct? if so, then at the top of the list should be things like vector vector .... and same for map(s) and so on... but what about function declaration? where do then fit, what is the size of a function declaration? thanks Dec 29 '06 #1
5 Replies

 P: n/a Gary Wessle wrote: Hi I read some where about the order variables are declared in a class may help space optimization. find the size of bits a variable takes and declare them from biggest to the lowest. so in my system that would be. double int string char bool bool is often implemented as an int. So it should go next to int. > is the principle correct? No, not quite. It's the alignment of your type that is important, not the size. .... if so, then at the top of the list should be things like vector vector ... and same for map(s) and so on... but what about function declaration? The alignment of a vector. where do then fit, what is the size of a function declaration? I don't think it will ever matter to you what the alignment of a function is. Dec 29 '06 #2

 P: n/a "Gary Wessle" vector That depends on sizeof(vector) vs sizeof(vector). On my system they are the same size, notwithstanding that double is twice as large as int. This is because storage for vectors is allocated on the heap, not in the class object, which only stores a pointer. ... and same for map(s) and so on... but what about function declaration? where do then fit, what is the size of a function declaration? Function declarations/definitions do not add to the size of an object (except that if you have at least one virtual function, then your class object will have a vtable pointer --- or at least that is how it is normally implemented). Functions exist at the level of the class, not at the level of the individual object. I would quote Stroustrup on this whole issue. "However, it is usually best to order members for readability and sort them by size only if there is a demonstrated need to optimize." (TCPL, Special 3rd ed., p. 103). -- John Carson Dec 29 '06 #3

 P: n/a Gianni Mariani

 P: n/a "Gary Wessle" Gary Wessle wrote: >>HiI read some where about the order variables are declared in a classmay help space optimization.find the size of bits a variable takes and declare them from biggestto the lowest.so in my system that would be.doubleintstringcharbool bool is often implemented as an int. So it should go next to int. >>is the principle correct? No, not quite. It's the alignment of your type that is important,not the size. Stroustrup 3rd ed. top of p. 103 "You can minimize wasted space by simply ordering members by size (largest member first)." is this statement correct only inside the structure and not for general use? thanks It is not clear what you mean by "general use". However, an implementation is only required to store members in the order in which they are declared when there are no intervening access specifiers (section 9.2/12 of the Standard). Thus given: class Test { private: double d; int i; public: bool b; }; d must precede i, but the access specifier that occurs between d and i, on the one hand, and b on the other hand, means that the placement of b relative to d and i could be different from the order of declaration. Specifically, b might come first. -- John Carson Dec 29 '06 #5

 P: n/a "Gary Wessle" vector ... and same for map(s) and so on... but what about function declaration? where do then fit, what is the size of a function declaration? thanks Yes and no. Consider a structure (or class) such as: class MyClass { char Char1; int Int1; char Char2; }; What is the size of that class? (on my system with sizeof int being 4) Well, the compiler will allocate one byte for the char, then skip 3 bytes so the int is aligned properly, then 4 bytes for the int, then 1 byte for the second char, then skip 3 more bytes so an array of this class will be aligned properly. So 12 bytes, when only 6 are actually used. So we can arrange it and make the largest first so it would become: class MyClass { int Int1; char Char1; char Char2; }; What's the size of this class? 4 bytes for the int, 1 byte for the first char, 1 byte for the 2nd char, skip 2 bytes for alignment. 8 bytes total. But we get the same result with: class MyClass { char Char1; char Char2; int Int1; }; 1 byte, 1 byte, skip 2, 4 bytes. Generally, the compler will align a type on it's size. If a variable is 4 bytes, it will align it every 4 bytes. 2 bytes, 2, etc... I said generally, there may be exceptions with OS/compilers/CPUs, etc... As long as you understand what's going on it should be relatively easy to figure out how to get rid of wasted space. I.E. This is extremly wasteful: class MyClass { char Type1; int Val1; char Type2; int Val2; char Type3; int Val3; }; 24 bytes. A little rearranging: class MyClass { char Type1; char Type2; char Type3; int Val1; int Val2; int Val3; }; 16 bytes. I could of put the ints first. However, I tend to arrange my class variables on their usage, grouping like with like. Just use a little common sense and things should be fine. Dec 29 '06 #6

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