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name Punditsdkoslkdosdkoskdo

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

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 << ".
";
  cout << "What is your favorite team? ";
  getline (cin, mystr);
  cout << "I like " << mystr << " too!
";
  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.

 

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 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 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.

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