lightatdawn
#include <stdio.h> #include <string.h> //------------------------------------ // A Standard Hello World Function //------------------------------------ void HelloWorld1() { //Here we simply use printf to display the text "1 - Hello World!" printf("1 - Hello World!\n"); } //------------------------------------ // Displaying Hello World from a String //------------------------------------ void HelloWorld2() { //First we make an array of type char, 14 elements //long. Why 14? Since the \n is only a single //character (a carriage return character) shouldn't //we only need 13? Nope. Strings are terminated by //a NULL character. Without this we would run into //problems when manipulating them (such as reading //past the end into memory we dont own... Very bad. char String[14]; //Now we use strcpy to copy the text to String strcpy(String, "Hello World!\n"); //And print away printf("2 - %s", String); } //------------------------------------ // Allocating Memory for Hello World //------------------------------------ void HelloWorld3() { //Our variable is a pointer of type char. This means //it will store a value that points to a location in //memory. We assign pString to NULL by default even //though we dont really need to. Since we'll be //assigning pString a new value on the next line, //this is pointless, but its a good habit to always //set pointers to NULL before possible use. char * pString = NULL; //Now a pointer is useless unless it points to //something (other than NULL that is). So we want //it to point to a section of memory that we can //use to store our Hello World string. What we need //is 14 elements the size of a char. We use the 'new' //operator to acomplish this. Notice that 'new' returns //a pointer to the memory its giving us, so we say //say 'pString = new...' so that pString points there. pString = new char[14]; //Now check this out. We can copy text to our new //memory using the pointer. Looks a lot like the //previous functions use doesnt it? strcpy(pString, "Hello World!\n"); //Yup, its easy to use. printf("3 - %s", pString); //Now comes a very important part. We need to give back //the memory that we got by calling 'new'. If we forget //to delete allocated memory, we get whats called a //memory leak. If this function was called lots of times, //or it had allocated a lot of memory each time, our //program would quickly slow down and eventually crash. //Memory keeps getting allocated, and your system isnt //allowed to use it for anything else until its deleted. delete [] pString; } //------------------------------------ // Passing Hello World to a function //------------------------------------ void HelloWorld4(char * pString) { //This demonstrates passing a pointer to a function as an //argument. If you look in main() you will notice that this //function is called differently from the others: //HelloWorld4("4 - Hello World!\n"); //Notice that we have to pass in something for pString to //point to. If you dont pass anything to this function when //its called, you will recieve an error. //Now we can just use this variable like it was declared here printf("4 - %s", pString); } //------------------------------------ // Combining some previous concepts and more //------------------------------------ void HelloWorld5(char * pString) { //We passed in a pointer to a string just like the last //example. But we also need a pointer to a string that //is going to contain everything we put together in a //minute. char * pAlteredString = NULL; //Allocate memory for pAlteredString. Its going to hold //the contents of what pString points to as well as 8 more //characters. But we dont know how big pString is! Thats //where we use strlen. It gets the length of a string. //strlen doesnt count the terminating NULL character so we //have to make room for that in our calculations. Hence the //+9 instead of the expected 8. pAlteredString = new char[strlen(pString) + 9]; //Copy what pString points to over to the memory that //pAlteredString points to. strcpy(pAlteredString, pString); //Very simular to strcpy, we now use strcat to copy some //more text on the end of pAlteredString. strcat(pAlteredString, " World!\n"); //Now print. printf("5 - %s", pAlteredString); //Of course, since we allocated memory, we have to delete it, //just like before. delete [] pAlteredString; } //------------------------------------ // Returning Hello World //------------------------------------ char * HelloWorld6() { //Notice that this function isnt of type void. Its return //value is a pointer of type char. In main we see this: //printf( "6 - %s", HelloWorld6() ); //Since HelloWorld6 returns a pointer to char back to the //caller, we can use the function just like a char * variable //in this instance. //Return this text return "Hello World!\n"; } //------------------------------------ // Error checking Hello World //------------------------------------ int HelloWorld7(char * pString) { //Here, we see a lot of the same things we saw before, used //in a slightly different manner. We're passed in a pointer //to char, and the return type is of type int. Previous //examples showed us how that works. //Here we check to make sure that something actually got //passed in. Remember that pString is just a pointer and in //a more complicated program, its possible that pString might //not be pointing to anything yet, which would make it very //evil for us to be using. This displays one important reason //for setting pointers to NULL when they're declared. It makes //it easy to check if theres any memory where its pointing. if (pString != NULL) { //If pString does point to something, then we print it printf("7 - %s", pString); } else { //If pString doesnt point to anything then we print this //error message. printf("Invalid parameter! pString is not an initialised pointer!\n"); //Then we exit the function by calling return. We're going //to return the value 1. Go have a look in main to see how //we're going to handle what happens to this value. return 1; } //Just for kicks we're going to check to see if the text is //what we expect it to be. We use strcmp to compare pString //to what we're expecting. if (strcmp(pString, "Hello World!\n") != 0) { //If the text wasn't exactly "Hello World!\n", then we're //going to print an error about it and return 1 to the //calling function to report our error. printf("Invalid parameter! pString contains invalid data!\n"); return 1; } //It looks like everything worked out fine. We're going to //return 0 to tell the calling function that everything is //going smoothly. return 0; } //------------------------------------ // Disecting Hello World //------------------------------------ void HelloWorld8(char * pString, unsigned int len) { //We pass two arguments to this function. The first is a //pointer to type char as we've seem a million times before. //The second is an integer (int) which is going to tell us //the length of the text that pString points to. //Looking in main we see: //HelloWorld8("Hello World! - With Garbage!!!\n", 12); //Notice that we're passing in a really long string, but that //only the "Hello World!" part gets displayed. That because //we're passing in the len value as 12, and this function will //only display that many characters of the string. Try changing //that value to see it in action... //We need to keep track of what element we're printing so we //need a variable for that. unsigned int i; //By default display which function number we're on first: printf("8 - "); //If someone set len up to be longer than the string they passed //in, that would get messy. We would be accessing memory that we //dont own, and thats something we never want to do. So we truncate //len if its longer than the total character in pString. if (len > strlen(pString)) len = strlen(pString); //Now we loop through the text displaying 'len' # of characters. for(i = 0; i < len; i ++) { //Since pString is a pointer to n elements of type char, we //can use the [] operator to move through the elements in //memory. Its exactly like the elements in an array, we're //just seeing it at a more naked level. printf("%c", pString[i]); } //Put a carriage return at the end of everything. printf("\n"); } //------------------------------------ // Figuring out Hello World //------------------------------------ void HelloWorld9(char * pString) { //This is a lot like the previous function, but notice that //we have no idea how much we're supposed to display. We're //going to keep reading the memory pointed to by pString //until we reach the end. But how do we know when we get to //the end? NULL terminating character to the resuce! //We need to set our counting variable to 0 here since we're //not using a for loop to do it. unsigned int i = 0; //By default display which function number we're on first: printf("9 - "); //A do...while loop. This will keep looping until the //condition in the 'while' enclosing brackets is true. //In this case that means when the element of memory we //just moved to is NULL. Everything else works just like //in the previous example. Print each character then move //to the next element and repeat. do { printf("%c", pString[i]); i ++; }while(pString[i] != NULL); } //------------------------------------ // MAIN //------------------------------------ int main (void) { HelloWorld1(); HelloWorld2(); HelloWorld3(); HelloWorld4("Hello World!\n"); HelloWorld5("Hello"); printf( "6 - %s", HelloWorld6() ); if (HelloWorld7("Hello World!\n") != 0) { printf("HelloWorld7 function failed!\n"); return 1; } HelloWorld8("Hello World! - With Garbage!!!\n", 12); HelloWorld9("Hello World!\n"); return 0; }