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Published byTrevor Eugene Cole Modified over 9 years ago
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UFS003C3 Lecture 15 Data type in C & C++ Using the STL
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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!
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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];
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Arrays in C 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
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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.
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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.
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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’
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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.
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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.
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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.
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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;
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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.
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