About pointers and references

Coming from the managed world (Java, Python and to some extent C#), it was quite a task for me to get grips with managing memory, even now i won’t say i have mastered the art of memory management in C++, but i am reasonably comfortable with it. The point i am trying to make is, watching out for memory isn’t second nature to my programming style….I still need to be on absolute alert when i am writing any C++ code, especially when i am passing pointers around the code. However having said all of that, writing some C++ code in vim, debugging it by writing stuff to the console and compiling it using a makefile does provide an unparalleled amount of geek satisfaction. I cannot believe that i actually sound like i am enjoying coding in C++…..but hey it is what it is.

Ok so now onto the real stuff, while coding C++, i often  need a chart of sorts to keep it all together….so i written some base code below which often helps me remember things in case i forget.

Using pointers and references with C++

About pointers and references

The C++ class below should give an account of some of the possible usages of memory

void changing_with_passby_val(int& value){ //assign a new value value = 42; } /* * Change the value which the pointer is pointing to * within a function **/ void change_value(int* ptr){ int value = 45; /*remember we need to change the value that the pointer * is pointing to and not the value of the pointer itself * hence lets not make a mistake of doing this * ptr = value */ *ptr = value; } int main() { /* * first lets create a simple int pointer * allocating memory with new is the c++ way * if you are doing C, the malloc function * is another way to do this **/ int* ptrVal = new int; /* now that we have a pointer, lets store a value to where we are pointing at*/ *ptrVal = 12; cout<<“initial value:”<<*ptrVal<<endl; /* now lets change the value we are pointing to */ change_value(ptrVal); cout<<“What value are we point to now?:”<<*ptrVal<<endl; /* well there is one other way in which we can change it */ int otherValue = 70; /* the actual value of the pointer i.e. the address is being changed to the address of the variable “otherValue” */ ptrVal = &otherValue; cout<<“Result of our change:”<<*ptrVal<<endl; /* i am still not satisfied, lets try changing it one other way */ changing_with_passby_val(*ptrVal); cout<<“Result of our change:”<<*ptrVal<<endl; /*now before we move on to something else, letstry deleting our * pointers and reinitializing them since we are managing our own * memory, we must and we seriously must remember to clean it up */ //delete ptrVal; /*hmm lets try accessing ptrVal now, the line below should result in a very unpleasent segfault(invalid pointer) error cout<<“Accessing after deletion”<<*ptrVal<<endl;*/ return 0; } ok so now lets do something more fun lets create a 2D int array with a pointer coming from Java, the first thing i would try would be int [][] array2d;  However pointers in C++ opens up another means of doing this. void create_2D_array() { int rows =2; int cols = 2; /*what we are doing here is that we are assigning creating an int * pointer and we are allocating 4 congatious memory locations for * it. */ int* array2D = new int[rows*cols]; /*ok so we have our 2D array now, so lets fill it up with some * random values from 1 to 10 */ for(int i=0;i<rows;i++) { for(int j=0;j<cols;j++) { //cout<<rand()%10<<endl; array2D[rows*i+j] = rand()%10; } } /*great we have populated an array, now lets see what we actually put * into it, now we can either access the elements the same way as we * did above or, we can also do it with just 1 loop instead of 2 loops */ for(int i=0;i<(rows*cols);i++) { //since the values are stored in contagious blocks, we can just //access them individually by their positions in the array cout<<“array value at “<<i<<“:”<<array2D[i]<<endl; } } Cool, so what next? ok i know what, so  if you have a large enough application and you know there is an object that you wont be using for a certain time period, then there is no point of keeping it in memory, especially when memory is limited. Hence in C++ you can delete  it for the time being and re-use it when needed.  Now i my case the application wasn’t large, however the existing objects were fairly large and i had a particular data structure, which i would use once , delete it and after a while reinitialize it with some new data. Sounds simple? well the concept isn’t that complicated but the implementation can be, i mean since i come from the managed world, i often had trouble keeping track of where what was being used/deleted and combine that with async stuff happening in the application that kept getting bigger!….ahh good times!!!! void delete_and_reuse() { /* lets create a pointer to a vector of ints */ vector<int>* nums = new vector<int>; /* lets fill it up with some values */ nums->push_back(40); nums->push_back(2); /* lets check the size and the first element in our vector pointer */ cout<<“size of the vector:”<<nums->size()<<endl; cout<<“first val in our vector*:”<<nums->at(0)<<endl; /* ok now lets destroy what we created */ delete nums; /* and rebuild what we destroyed */ nums = new vector<int>; /* lets populate it with some value and confirm what we added */ nums->push_back(43); cout<<“value in the new vector pointer:”<<nums->at(0)<<endl; }

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