c/c++ questions on pointers (double pointers)

问题:

So its been a while since I took courses on c and c++ and I'm curious on c pointers (im going to use the new keyword in my examples even thought I know malloc is the C way). I always recall my teacher always forcing us to use pointer, she would never take assignments with arrays, she proved to us that there are less commands needed in assembly language when you used pointers rather then used arrays. I want to continue this good practice but I seem to be struggling to use pointers, specifically double pounter.

Lets say I want to create a word bank without using the c++ string datatype and I have a double pointer of type char.

int main()
{
   string fileName = "file.txt";
   char** wordBank = null;
   int wordCount = countWords(fileName); //somefunction to get word count
}

Now I need to allocate a memory space big enough for the entire bank But im unsure on how to do this I believe it is somthing like this?

wordBank = new char*[wordCount];

now I need to allocate space specifilly for the size of each word, which i am still unsure aobut.

for(int i = 0; i < wordCount; i++)
{
   wordLength = getWordLength(fileName, i); // some function to get word length of each...
   //... word in the bank
   (*wordBank) = new char[wordLength];
}

Okay the last part I'm confused about is passing double pointers through functions. lets say I have a function that lets me manipulate a an entire word, lets say I want to just pass the word, what would I pass through the function call and what would the function definition have. And lets say I want to pass the entire bank and a number that will move the pointer, what would I pass through the function call and what would the function definition have. Sorry for all the questions usually If I try to answer these things by myself by writing short programs but I'm having difficulty just getting it to compile. I appreciate the responses I receive.

回答1:

To allocate the bank:

wordBank = malloc(wordCount * sizeof(char *));

To allocate individual words:

char *addWord(char **wordBank, size_t idx, char *word) {
  wordBank[idx] = malloc(strlen(word) + 1);
  // this is how you pass a word
  strcpy(wordBank[idx], word);
  return wordBank[idx];
}

// how you pass the wordBank:
addWord(wordBand, index, someWord);

But having more instructions in the assembly is not necessarily bad. A constant overhead is generally not a problem in programming. I would use std::string and std::vector<string> and spend my time on real problems. At least not on debugging mallocs and frees, assigning and testing NULL pointers.

回答2:

My take:

wordBank = new char*[wordCount];

This is fine: you're allocating an array of wordCount elements of char*

for(int i = 0; i < wordCount; i++)
{
   wordLength = getWordLength(fileName, i); // some function to get word length of each...
   //... word in the bank
   wordBank[i] = new char[wordLength];
}

As it has been indicated in the comments (@angdev), for each item in the array (of char *) we need to allocate memory.

One word os caution is that you must remember to deallocate that memory when done... and that you should deallocate with the right operation for the allocation you did:

• if you used malloc() ---> use free()
• if you used new ---> use delete`
• if you used new [] ---> use delete []`

Finally, if you need to modify a pointer inside a function, use a reference to a pointer:

char *myPointer=0;

fillIt(myPointer);

So... if you want fillIt() to actually modify where myPointer points, the function should be:

void fillIt(char * & p) { ... }

Alternatively you can define it as:

void fillIt(char ** p) { ... }

and invoked it as

char *myPointer=0;

fillIt(&myPointer);