How to return a char array created in function?

Question

I've been programming badly for quite a while and I only really just realised. I have previously created many functions that return character strings as char arrays (or at least pointers to them).

The other day someone pointed out that when my functions return the char arrays pointed to by my functions have gone out of scope and I'm essentially now pointing to a random bit of memory (A nasty dangling pointer).

I didn't really notice this for a while because the char arrays when outputted to the console didn't appear to be corrupt (probably because there wasn't time for that data to be overwritten). I did however notice this when I was returning a string buffer (char array) generated by reading the serial port which was frequently corrupt.

So, how best should I do it?

My bad code is as follows:

#include <cstdlib>
#include <iostream>

using namespace std;

char* myBadFunction(){
    char charArray[] = "Some string\n";
    char* charPointer = charArray;
    return charPointer;
}


int main(int argc, char** argv) {

    cout << myBadFunction();

    return 0;
}

I understand that I should perhaps allocate memory in the program before calling the function or create a global variable to put the returned string in, but if my called function is used by many different programs when how should it know the size of the buffer being passed into it in advance and when should this memory be deleted?

The following code also doesn't do what I want it to properly:

#include <cstdlib>
#include <iostream>

using namespace std;

void fillArray(char* charPointer){
    char charArray[] = "Some string\n"; // Create string
    charPointer = charArray; // Not correct, want to fill predefined array with created string
    return;
}


int main(int argc, char** argv) {

    char predefinedArray[50] = {0};
    fillArray(predefinedArray);
    cout << predefinedArray;

    return 0;
}

I want to fill the array that the pointer parsed points to but this doesnt' happen in the code above.

Also, when should I use the new[] command to create my array? is it needed? and when should I call delete[] on it.

Many thanks for this, its obviously very fundamental but something I've been doing wrong for a while.

Answer 1

You have two options for returning an array in C++. You can fill in pre-allocated memory (good), or allocate your own within the function and return it (bad). The reason that the first is preferred is because it re-enforces proper disposal of allocated memory.

A basic example would look like this:

void fillArray(char* buffer, int sz) {
  char text[] = "hello there!";
  if (sizeof(text)>sz) {
    // overflow! Buffer is too small!
    return;
  }
  for (int n=0;n<sizeof(text);n++) {
    buffer[n] = text[n];
  }
}

int main() {
  char* buffer = new char[30]; // allocates a buffer of 30 bytes.
  fillArray(buffer,30);
  cout << buffer;
  delete [] buffer;
}

/* note that it would be easier to use static memory in this example */

It isn't hard when you think about the problem.

Answer 2

"Some string\n" is a string literal and will therefore exist for the lifetime of the program, so the following would be valid:

#include <cstdlib>
#include <iostream>

using namespace std;

char* myGoodFunction(){
    char* charPointer = "Some string\n";
    return charPointer;
}

int main(int argc, char** argv) {
    cout << myGoodFunction();

    return 0;
}

Of course this is only useful if the function always returns the same string. If the returned string can vary (generally the case) then you can declare the char array in your function as static and return it's address (as has already been suggested).

Answer 3

Declare the array as "static" varible and return with its address. This code works, but causes a warning :

#include <cstdlib>
#include <iostream>
using namespace std;

char* myBadFunction(){
    static char charArray[] = "Some string\n";    // insert "static"
    // char* charPointer = charArray;  
    return charArray;             // charArray is a pointer to the static array
}                                 // after returning static varibles stay safe
int main(int argc, char** argv) {
    cout << myBadFunction();
    return 0;
}

Answer 4

The simplest way would be to return a std::string, and if you needed access to the internal char array use std::string::c_str().

#include <iostream>
#include <string>

using namespace std;

string myGoodFunction(){
    char charArray[] = "Some string\n";
    return string(charArray);
}


int main(int argc, char** argv) {  
    cout << myGoodFunction();
    return 0;
}

If you need to return something other than a char array, remember that pointers can be used as iterators. This allows you to encapsulate an array in a vector or a similar structure:

vector<int> returnInts() {
    int someNums[] = { 1, 2, 3, 4 };
    return vector<int>(someNums, someNums + 4);
}