G52CPP C++ Programming Lecture 8. Dr Jason Atkin

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1 G52CPP C++ Programming Lecture 8 Dr Jason Atkin 1

2 Last lecture Dynamic memory allocation Memory re-allocation to grow arrays Linked lists Use -> rather than. pcurrent = pcurrent -> pnext; 2

3 Aside: do not use variable sized arrays Variable length arrays are NOT valid in C++ Sadly, gcc on avon, bann etc will allow them in C++ E.g.: int myfunc( int isize ) { } char array[isize]; Size of array is not a constant, it depends upon the value of variable You must use a numeric literal or a constant for a size You can use a #define to set it to a literal If you need variable size arrays, use malloc() or new Use: g++ -pedantic myfile.cpp to get a warning 3

4 This lecture More pre-processor And macros #ifdef Volatile and register keywords Unions and bitwise structs Sizes of structs and unions 4

5 #define and macros 5

6 #define A semi-intelligent find and replace facility Often considered bad in C++ code (useful in C) const is used more often, especially for members Template functions are better than macros Example: define a constant : #define MAX_ENTRIES 100 Replace occurrences of MAX_ENTRIES by the text 100 (without quotes), e.g. in: if ( entry_num < MAX_ENTRIES ) { } Remember: Done by the pre-processor! E.g. NOT actually a definition of a constant Constant #defines usually written in CAPITALS 6

7 #define and macro definitions You can use #define to define a macro: #define max(a,b) (((a)>(b))? (a) : (b)) int v1 = max( 40, 234 ); int v1 = (((40)>(234))? (40) : (234)) int v2 = max( v1, 99 ); int v2 = (((v1)>(99))? (v1) : (99)) int v3 = max ( v1, v2 ); int v3 = (((v1)>(v2))? (v1) : (v2)) Remember: done by the pre-processor! NOT a function call 7

8 MyHeader.h What is the output here? #ifndef MY_HEADER_H #define MY_HEADER_H #define max(a,b) (((a)>(b))? (a) : (b)) #endif MyTest.cpp #include <cstdio> #include MyHeader.h int main( int argc, char* argv[] ) { int a = 1, b = 1; while ( a < 10 ) { printf( a = %d, b = %d, a, b ); printf( max = %d\n, max(a++,b++) ); } } 8

9 The (surprise?) output printf( a = %d, b = %d, a, b ); printf( max = %d\n, max(a++,b++) ); The output is: a = 1, b = 1 max = 2 a = 2, b = 3 max = 4 a = 3, b = 5 max = 6 a = 4, b = 7 max = 8 a = 5, b = 9 max = 10 a = 6, b = 11 max = 12 a = 7, b = 13 max = 14 a = 8, b = 15 max = 16 a = 9, b = 17 max = 18 Why? 9

10 The (surprise?) output printf( a = %d, b = %d, a, b ); printf( max = %d\n, max(a++,b++) ); The output is: a = 1, b = 1 max = 2 a = 2, b = 3 max = 4 a = 3, b = 5 max = 6 a = 4, b = 7 max = 8 a = 5, b = 9 max = 10 a = 6, b = 11 max = 12 a = 7, b = 13 max = 14 a = 8, b = 15 max = 16 a = 9, b = 17 max = 18 Why? max( a++, b++ ) expands to: ((a++)>(b++))? (a++) : (b++) So, whichever number is greater will get incremented twice, and the lesser number only once 10

11 Warning about macros Do not use a macro where the evaluation of the parameters may have a side-effect E.g. max(a++,b++) Evaluating these parameters alters a value A side-effect inline template functions are better 11

12 #define and #ifdef 12

13 Conditional compilation You can remove parts of the source code if desired Done by the pre-processor (not even compiled) E.g. Only include code if some name has been defined earlier (in the code or included header file) #ifdef <NAME_OF_DEFINE> <Include this code if there was a matching #define> #else <Include this code if there was NOT a matching #define> #endif To include only if not defined use #ifndef There is also a #if <condition> 13

14 Conditional compilation Platform-dependent code can be included e.g. Include only if on a specific machine: #ifdef WINDOWS windows code here #elif SYS5UNIX System 5 code here #endif Often used for cross-platform code The correct #define has to be made somewhere to specify the current platform Know that this can be done, recognise it 14

15 Avoiding multiple inclusion Code to include the contents of a file only once: #ifndef UNIQUE_DEFINE_NAME_FOR_FILE #define UNIQUE_DEFINE_NAME_FOR_FILE include the rest of the file here #endif To work, the name in the #define has to be unique throughout the program E.g. you probably should include the path of the header file, not just the filename Example: mycode/game/graphics/screen.h could be called MYCODE_GAME_GRAPHICS_SCREEN_H By convention, #defines are in upper case 15

16 Example coursework file #ifndef DISPLAYABLEOBJECT_H #define DISPLAYABLEOBJECT_H #include "BaseEngine.h" If not already marked as included Mark it as included now, by setting the #define Includes the header files it needs class DisplayableObject { public: // Constructor DisplayableObject(BaseEngine* pengine); // Destructor virtual ~DisplayableObject(void); private: // True if item is visible bool m_bvisible; }; #endif End of the #ifdef around the contents 16

17 Three rules for header files 1. Ensure that the header file #includes everything that it needs itself i.e. #include any headers it depends upon 2. Ensure that it doesn t matter if the header file is included multiple times See previous slides 3. Ensure that header files can be included in any order A consequence of the first two rules 17

18 Volatile and register (so that you know that they exist) 18

19 volatile and register keywords The volatile keyword is important if other threads or processes may access the data (e.g. shared memory) Know that it exists and when you should use it Tells the compiler that data may change outside this thread or program (similar meaning in Java) Will turn off some potential optimisations/caching Value must be checked every time it is needed Compiler cannot assume it is unchanged Example: volatile int v = 4; volatile float f = 16.7f; Another one to know: the register keyword Request to store value in a register not a variable Again, know that it exists and what it does Not usually needed with modern optimising compilers 19

20 unions Treating something as one thing OR another Very rarely used compared with structs usually for low-level (e.g. o/s) code 20

21 Unions unions are very similar to structs except that the data members are in the same place In structs data members are one after another in memory (possibly with gaps) In unions data members all have the same address i.e. data is of one type OR another, not both 21

22 Unions Elements of unions are in the SAME place Elements of unions may be different sizes A union is as big as the biggest thing in it (plus any packing) Unions are a way of providing different ways of looking at the same memory union charorlong { }; unsigned long ul; char ar[8]; Size 4? Size 8 Addr: ul ar 1000 [0] 1001 ul [1] 1002 [2] 1003 [3] 1004 [4] 1005 [5] 1006 [6] [7]

23 Bitfields 23

24 Bit fields Within structs you can specify fields with size less than a byte struct position { unsigned char x : 3; /* 3 bits */ unsigned char y : 3; /* 3 bits */ unsigned char z : 2; /* 2 bits */ }; Which order the bits appear in the bytes is undefined (i.e. it could be high bits first, but could be low bits first, so bit order is implementation dependent) For clarity only: no faster at runtime that using a char/int and the bitwise operators (&,, etc ) 24

25 typedef 25

26 typedef Declare a new type name using typedef Usage: typedef <old_type> <new_name> E.g. typedef struct DATE { int d, m, y; } Date; Even C code can then use type Date instead of struct DATE In C++ (not C) you can omit the keywords struct, enum, union anyway Similar to an automatic typedef 26

27 Sizes and packing 27

28 structs struct DateTime { int time; char day; char month; short year; }; int main( int argc, char* argv[] ) { DateTime dt = { 80000, 01, 04, 1990 }; printf( "DOB: %5d %02d/%02d/%04d\n", dt.time, dt.day, dt.month, dt.year ); } return 0; 28

29 struct content positions struct DateTime { int time; char day; char month; short year; }; printf( "Address of dt = %p, size %d\n", &dt, sizeof(dt) ); printf( "Address of dt.time = %p, size %d\n", &(dt.time), sizeof(dt.time) ); printf( "Address of dt.day = %p, size %d\n", &(dt.day), sizeof(dt.day) ); printf( "Address of dt.month = %p, size %d\n", &(dt.month), sizeof(dt.month) ); printf( "Address of dt.year = %p, size %d\n", &(dt.year), sizeof(dt.year) ); 29

30 Positions in memory Time first time day month year Address Size dt 0x7fffaab dt.time 0x7fffaab dt.day 0x7fffaab dt.month 0x7fffaab dt.year 0x7fffaab

31 Gaps when day is first Day first day 0x7fff69becaf0 Size of structure: 12 time 0x7fff69becaf4 month 0x7fff69becaf8 year 0x7fff69becafa 31

32 May have gaps at the end Month last day 0x7fff69becaf0 Size of structure: 12 time 0x7fff69becaf4 year 0x7fff69becaf8 month 0x7fff69becafa 32

33 Tell it to pack on 1 byte boundaries #pragma pack(1) day time Address Size dt 0x7fff7e dt.day 0x7fff7e dt.time 0x7fff7e dt.month 0x7fff7e dt.year 0x7fff7e month year 33

34 Positions in memory Time first Day first #pragma pack(1) time day day time day month year time month year month year 34

35 #pragma structs may get empty space in them To align members for maximum speed You can usually tell compiler to pack structs e.g. with gcc can use the command: #pragma pack(1) #pragma means a compiler/operating system specific pre-processor directive 35

36 #pragma pack(1) #include <cstdio> struct A { int i; char c; }; union B { int i; char c; }; #pragma pack(1) struct C { int i; char c; }; union D { int i; char c; }; Example: char : 1 int : 4 struct A :? union B :? struct C :? union D :? int main( int argc, char** argv ) { printf( "sizeof(char): %d\n", sizeof(char) ); printf( "sizeof(int): %d\n", sizeof(int) ); printf( "sizeof(struct A): %d\n", sizeof(struct A) ); printf( "sizeof(union B): %d\n", sizeof(union B) ); printf( "sizeof(struct C): %d\n", sizeof(struct C) ); printf( "sizeof(union D): %d\n", sizeof(union D) ); return 0; } 36

37 #pragma pack(1) #include <cstdio> struct A { int i; char c; }; union B { int i; char c; }; #pragma pack(1) struct C { int i; char c; }; union D { int i; char c; }; int main( int argc, char** argv ) { printf( "sizeof(char): %d\n", sizeof(char) ); printf( "sizeof(int): %d\n", sizeof(int) ); printf( "sizeof(struct A): %d\n", sizeof(a) ); printf( "sizeof(union B): %d\n", sizeof(b) ); printf( "sizeof(struct C): %d\n", sizeof(c) ); printf( "sizeof(union D): %d\n", sizeof(d) ); return 0; } Example: char : 1 int : 4 struct A : 8 union B : 4 struct C : 5 union D : 4 37

38 Sizes of unions and structs If there is no excess space for packing: sizeof(struct) is total of the size of the members (i.e. sum of member sizes) Members are one after another in memory Bitfield structs use minimum number of bytes necessary sizeof(union) is size of the largest member (i.e. maximum of member sizes) All members are in the same place Largest member determines size 38

39 Next lecture classes (and C++ structs) Member functions inline functions 39

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