Lecture 2 More C++ Jon Macey

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1 Lecture 2 More C++ Jon Macey

2 namespaces namespaces allow us to separate program elements into different named logical units By using namespaces we can declare functions, classes and other code that is only visible within the namespace The following code shows this in action

3 foo1::foo foo2::foo

4 namespace2.cpp:29:3: error: call to 'foo' is ambiguous foo(); ^~~ namespace2.cpp:6:7: note: candidate function void foo() ^ namespace2.cpp:15:7: note: candidate function void foo() ^ namespace2.cpp:30:2: error: call to 'foo' is ambiguous foo(); ^~~ namespace2.cpp:6:7: note: candidate function void foo() ^ namespace2.cpp:15:7: note: candidate function void foo()

5 namespace3.cpp:21:2: error: use of undeclared identifier 'cout'; did you mean 'std::cout'? cout<<"in main\n"; ^~~~ std::cout

7 namespaces It s good to use namespaces makes code more modular can embed namespaces within namespaces can shorten by re-naming them

8 Arrays A Simple data types use a single memory cell block to store a variable. It is sometimes more efficient to group data items together in main memory than to allocate an individual memory cell block for each variable. A simple way to create different fixed amounts of the of the same data type is by the use of an array. An array is a collection of two or more adjacent cells called array elements associated with a specific symbolic name.

9 Array declaration char x[8]; The command above instructs the compiler to allocate 8 memory cells of type char. These cells will be contiguous (next to each other in a continuous block) This mean accessing them can be done in a sequential manner To access the cells we use an index into the cells starting a 0 for the first index.

10 Array Access To access elements of the array we use the subscript operator [ ] This can be used to get or set the array index value as shown sizeof(char)==1 char x[8]; x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] total allocated memory 8 bytes

11 float array sizeof(float)==4 x[0] float x[8]; x[1] x[2] x[3] x[4] x[5] x[6] x[7] total allocated memory 8*sizeof(float)=32 Bytes

12 example #include <iostream> #include <cstdlib> int main() char c='a'; float x[8]=16.0f,12.0f,6.0f,8.0f, 2.5f,12.0f,14.0f,-54.5f}; std::cout<<x[0]<<"\n"; std::cout<<"modify cell 3\n"; std::cout<<x[3]<<"\n"; x[3]=25.0f; std::cout<<x[3]<<"\n"; float sum=x[0]+x[1]; std::cout<<sum<<"\n";; x[3]+=1.0; std::cout<<x[3]<<"\n"; } x[2]=x[0]+x[1]; std::cout<<x[2]<<"\n"; return EXIT_SUCCESS;

13 Array Manipulation Statement Explanation std::cout<<x[0]; Display the value of x[0] which is 16.0 x[3]=25.0; sum=x[0]+x[1]; sum+=x[2]; x[3]+=1.0 x[2]=x[0]+x[1] Store the value 25.0 in x[3] Store the sum of element x[0] and x[1] into the variable sum Adds x[2] to sum Adds 1 to x[3] Stores the sum of x[0] and x[1] into x[2]

14 Array Initialisation #include <iostream> #include <cstdlib> We can initialise an array in a similar way to a normal variable, the code opposite show this int main() // array of vowels char vowels[5]='a','e','i','o','u'}; // the compiler will fill in how many values (25) int primeslt100[]=2,3,5,7,11,13,17,19,23,29,31, 37,41,43,47,53,59,61,67,71,73, 79,83,89,97}; // index to our array int i; // loop and print out the values for(i=0; i<5; ++i) std::cout<<vowels[i]<<" "; } // newline std::cout<<"\n"; } // as we don't know how big the primes array is we need // to figure it out, sizeof will return how big the allocated // space is, however it is in bytes and an int takes up // sizeof(int) (usually 4) bytes so we need to div by this std::cout<<sizeof(primeslt100); int sizeofarray=sizeof(primeslt100)/sizeof(int); std::cout<<" size of array "<<sizeofarray<<"\n"; // loop and print for(i=0; i<sizeofarray; ++i) std::cout<<primeslt100[i]<<" "; } std::cout<<"\n"; return EXIT_SUCCESS;

15 Multi-dimensional arrays We can use multi-dimensional arrays to store data in table / tabular format. Whilst this is quite useful for many applications it is still usually more efficient to use a single dimensional array (more in future lectures) The format for declaration is as follows element-type name[size1] [size2]... [sizen];

16 Example If we wished to create an array to store the data for a game of noughts and crosses (tic tac toe for Americans!) We could do the following char XO[3][3]; Column Row 0 X O X 1 O X O XO[1][2] 2 O X X

$to a - XO[1][2]='O'; int x,y; for(y=0; y<3; ++y) for(x=0; x<3; ++x) std::cout<<xo[y][x]<<" "; } std::cout<<"\n"; } - - - - - O - - - } return$

17 initialising array #include <iostream> #include <cstdlib> int main() char XO[3][3]= }; '-','-','-'}, '-','-','-'}, '-','-','-'}, Init all array elements to a - XO[1][2]='O'; int x,y; for(y=0; y<3; ++y) for(x=0; x<3; ++x) std::cout<<xo[y][x]<<" "; } std::cout<<"\n"; } O } return EXIT_SUCCESS;

18 You can get silly #include <iostream> #include <cstdlib> int main() int array[4][12][23][5][9][2]; array[0][1][2][3][1][0]=10; std::cout<<array[0][1][2][3][1][0]<<"\n"; } return EXIT_SUCCESS; But not recommended!

19 std::array<t,int size> C++ 11 introduces the std::array this is a c++ version of a normal array It allows static allocation of array data, with a set size. It has the same performance as a normal array but the added advantage that it works like other stl containers (more in a later lecture)

20 #include <iostream> #include <cstdlib> #include <array> int main() std::array<char,5> vowels='a','e','i','o','u'}; for(int i=0; i<vowels.size(); ++i) std::cout<<vowels[i]<<" "; } std::cout<<"\n"; }

21 Type Aliasing In C++ (and C for typedef only) we can alias a typename to use a different identifier. In C++, any valid type can be aliased so that it can be referred to with a different identifier. typedef [existing type] [new type name]; typedef char Byte; typedef unsigned int WORD; typedef float Real;

22 #include <iostream> #include <cstdlib> typedef typedef typedef char Byte; unsigned int Word; float Real; int main() } Byte a='c'; Word i=10; Real x=2.6; std::cout<<a<<" "<<i<<" "<<x<<"\n";

23 type aliasing typedef is a C/C++ construct a second syntax to define type aliases was introduced in the C++ 11 language spec This is not widely used yet and not all compilers support it

24 #include <iostream> #include <cstdlib> using Byte=char; using Word=unsigned int; using Real=float; int main() Byte a='c'; Word i=10; Real x=2.6; std::cout<<a<<" "<<i<<" "<<x<<"\n"; }

25 structures The C/C++ struct keyword allows us to create a group of one or more variables, possibly of different types, grouped together under a single name We can embed structures within other structures And with the use of pointers (more in another lecture) we can make self referential structures.

26 #include <iostream> #include <cstdlib> int main() struct vertex float x; float y; float z; } ; struct vertex p1 = 1.0, 2.5, -1.5}; struct vertex p2 = p1; p2.z=3.0; std::cout<<"point1 "<<p1.x<<" "<<p1.y<<" "<<p1.z<<"\n"; std::cout<<"point2 "<<p2.x<<" "<<p2.y<<" "<<p2.z<<"\n"; }

27 typedef struct We can combine typedef and structs to declare new types. typedef struct int x; int y; }Point2i; typedef struct int r; int g; int b; int a; }Colour4i; typedef struct Point2i p; Colour4i c; }ColourVert;

28 void debug(colourvert p) std::cout<<"["<<p.p.x<<","<<p.p.y<<"] "; std::cout<<"("<<p.c.r<<","<<p.c.g<<","<<p.c.b<<","<<p.c.a<<")\n"; } int main() ColourVert c=23,10},255,0,0,0}}; debug(c); }

29 structs and functions structs can also contain functions, this is the first steps of thinking along the lines of OO and classes typedef struct Point2i p; Colour4i c; void debug() std::cout<<"["<<p.x<<","<<p.y<<"] "; std::cout<<"("<<c.r<<","<<c.g<<","<<c.b<<","<<c.a<<")\n"; } }ColourVert; int main() ColourVert c=23,10},255,0,0,0}}; ColourVert d=99,12},255,0,255,0}}; } c.debug(); d.debug();

30 unions In a structure, the members are allocated separate consecutive chunks of storage. In a union, every member is allocated the same piece of storage 0x100 0x104 0x108 0x10b 0x10f MEMORY

31 unions In a structure, the members are allocated separate consecutive chunks of storage. In a union, every member is allocated the same piece of storage MEMORY [0] [1] [2] [3] 0x100 0x104 0x108 0x10b 0x10f

32 Why use unions? In the previous example we have declared the individual elements of a vector as m_x m_y m_z & m_w This is good to read and easier than using the array syntax [0]=10 etc However as we will see later OpenGL let us use either syntax and there are some advantages (speed) to using the array type syntax. Therefore if we define a union of both we have the advantage of both syntax types.

33 Data structure padding Typically the compiler will align data on byte boundaries, however sometimes in structures we will have data with different size requirements To maintain proper alignment the translator normally inserts additional unnamed data members so that each member is properly aligned. This can cause issues as shown by the following example typedef struct char identifier; char colourmaptype; char imagetype; short cms; short cml; char cmb; } notpacked; std::cout<<"size of char "<<sizeof(char)<<"\n"; std::cout<<"size of short "<<sizeof(short)<<"\n"; size of char 1 So how big notpacked? size of short 2

34 Data Structure Padding In the previous example sizeof(notpacked) will return 10 bytes even though the actual size is 8 bytes. This is because the compiler is padding by two bytes to align to a boundary. In most cases this may not be a problem, however if we are writing to disk a set size of data this would cause issues. The same may also be true when writing data to the GPU. To overcome this problem in g++ we can use a compiler #pragma as shown in the next slide

35 #pragma pack(push, 1) pack to 1 byte boundary typedef struct char identifier; char colourmaptype; char imagetype; short cms; short cml; char cmb; } packed; #pragma pack(pop) return to normal values For more details read this article

From python to C++ part 2. Jon Macey

From python to C++ part 2. Jon Macey From python to C++ part 2 Jon Macey http://nccastaff.bournemouth.ac.uk/jmacey/cppintro jmacey@bournemouth.ac.uk namespaces namespaces allow us to separate program elements into different named logical