Difference between string and char[] types in C++


Question

I know a little C and now I'm taking a look at C++. I'm used to char arrays for dealing with C strings, but while I look at C++ code I see there are examples using both string type and char arrays:

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

int main () {
  string mystr;
  cout << "What's your name? ";
  getline (cin, mystr);
  cout << "Hello " << mystr << ".\n";
  cout << "What is your favorite team? ";
  getline (cin, mystr);
  cout << "I like " << mystr << " too!\n";
  return 0;
}

and

#include <iostream>
using namespace std;

int main () {
  char name[256], title[256];

  cout << "Enter your name: ";
  cin.getline (name,256);

  cout << "Enter your favourite movie: ";
  cin.getline (title,256);

  cout << name << "'s favourite movie is " << title;

  return 0;
}

(both examples from http://www.cplusplus.com)

I suppose this is a widely asked and answered (obvious?) question, but it would be nice if someone could tell me what's exactly the difference between that two ways for dealing with strings in C++ (performance, API integration, the way each one is better, ...).

Thank you.

1
115
8/18/2009 10:20:27 PM

Accepted Answer

A char array is just that - an array of characters:

  • If allocated on the stack (like in your example), it will always occupy eg. 256 bytes no matter how long the text it contains is
  • If allocated on the heap (using malloc() or new char[]) you're responsible for releasing the memory afterwards and you will always have the overhead of a heap allocation.
  • If you copy a text of more than 256 chars into the array, it might crash, produce ugly assertion messages or cause unexplainable (mis-)behavior somewhere else in your program.
  • To determine the text's length, the array has to be scanned, character by character, for a \0 character.

A string is a class that contains a char array, but automatically manages it for you. Most string implementations have a built-in array of 16 characters (so short strings don't fragment the heap) and use the heap for longer strings.

You can access a string's char array like this:

std::string myString = "Hello World";
const char *myStringChars = myString.c_str();

C++ strings can contain embedded \0 characters, know their length without counting, are faster than heap-allocated char arrays for short texts and protect you from buffer overruns. Plus they're more readable and easier to use.

-

However, C++ strings are not (very) suitable for usage across DLL boundaries, because this would require any user of such a DLL function to make sure he's using the exact same compiler and C++ runtime implementation, lest he risk his string class behaving differently.

Normally, a string class would also release its heap memory on the calling heap, so it will only be able to free memory again if you're using a shared (.dll or .so) version of the runtime.

In short: use C++ strings in all your internal functions and methods. If you ever write a .dll or .so, use C strings in your public (dll/so-exposed) functions.

166
8/17/2009 11:06:05 AM

Arkaitz is correct that string is a managed type. What this means for you is that you never have to worry about how long the string is, nor do you have to worry about freeing or reallocating the memory of the string.

On the other hand, the char[] notation in the case above has restricted the character buffer to exactly 256 characters. If you tried to write more than 256 characters into that buffer, at best you will overwrite other memory that your program "owns". At worst, you will try to overwrite memory that you do not own, and your OS will kill your program on the spot.

Bottom line? Strings are a lot more programmer friendly, char[]s are a lot more efficient for the computer.


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