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UFS003C3 Lecture 15. Data type in C & C++ Using the STL. Arrays in C. Arrays are a very simple form of storage in C int list[20]; char word[30]; struct thing mylist[10]; Access is simple via an index x = list[9]; printf(“%c”,word[29]); mylist[2].element=x;
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UFS003C3 Lecture 15 Data type in C & C++ Using the STL
Arrays in C • Arrays are a very simple form of storage in C • int list[20]; • char word[30]; • struct thing mylist[10]; • Access is simple via an index • x = list[9]; • printf(“%c”,word[29]); • mylist[2].element=x; • Always remember C arrays start at 0 and go to N-1!
Pointers and arrays • Pointers can easily be used with C arrays • Char *cp, word[20]; • strcpy(word,”hello borld”); • cp=word; //or &word[0] • *(cp+7)=‘w’; //same as word[7]=‘w’ • Always remember • char *cp • is not the same as • char word[20];
Positive points Arrays are a very convenient & fast access method of data storage They can be randomly accessed Very easy to use data structure Negative points They are of fixed size at compilation time .i.e they can’t grow/shrink It is difficult to insert/delete data Arrays in C
Dynamic memory access in C • Frequently a program will require memory to be dynamically created and deleted during the program’s execution time. • The data must be requested through a library called malloc and released by free. • These routines use pointers to memory and will create or free a requested size of memory.
Using malloc() • malloc is used to request memory from a system area called the ‘heap’ • char *cp; • cp = (char *) malloc(80); • if ( cp != NULL ) ……. • The pointer cp now points to a block of 80 characters. • NULL can be returned if there is no or not enough memory.
Using free() • Free is used to free up a block of memory previously allocated by malloc(). • char *cp; • cp = (char *) malloc(80); • ……..doing something with the memory • free(cp) • Data in freed memory will be inaccessible and overwritten. • Can only free memory that has been ‘malloced’
Arrays and pointers in C++ • C++ allows the full use of arrays and pointer access in the same way that C does. • So any access in C is permissible in C++ • However C++ has better ways of doing some more complex data structure access controls and more clearly defined memory allocation.
Memory allocation in C++ • Dynamic memory allocation is simpler in C++ and often automatic • With pre-created classes the memory allocation is dealt with by the constructor and other methods • Explicit requests for heap storage can be made with the new keyword.
Implicit memory allocation • When using a library class, such as string, for example, the constructors in that class will automatically do the memory allocation. • string s(“hello”), t, u; • t = “world”; • U = s + t; • All the memory allocation is dealt with either by the constructor or the member functions.
Explicit memory allocation • C++ has the new operator. This allocates heap memory for specified objects or classes of objects • int *p = new int; • The memory can be released using the delete operator • delete p;
Explicit memory allocation • The new operator can actually be used to create arrays which are dimensioned at run time. • int n; • cout << “enter an array length “ ; • cin >>n; • char * cp = new char[n]; • The array is called with new and the dimension inputted.
