C++中string和char[]类型的区别

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.

Answer 1

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.

Answer 2

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.

Answer 3

Well, string type is a completely managed class for character strings, while char[] is still what it was in C, a byte array representing a character string for you.

In terms of API and standard library everything is implemented in terms of strings and not char[], but there are still lots of functions from the libc that receive char[] so you may need to use it for those, apart from that I would always use std::string.

In terms of efficiency of course a raw buffer of unmanaged memory will almost always be faster for lots of things, but take in account comparing strings for example, std::string has always the size to check it first, while with char[] you need to compare character by character.

Answer 4

I personally do not see any reason why one would like to use char* or char[] except for compatibility with old code. std::string's no slower than using a c-string, except that it will handle re-allocation for you. You can set it's size when you create it, and thus avoid re-allocation if you want. It's indexing operator ([]) provides constant time access (and is in every sense of the word the exact same thing as using a c-string indexer). Using the at method gives you bounds checked safety as well, something you don't get with c-strings, unless you write it. Your compiler will most often optimize out the indexer use in release mode. It is easy to mess around with c-strings; things such as delete vs delete[], exception safety, even how to reallocate a c-string.

And when you have to deal with advanced concepts like having COW strings, and non-COW for MT etc, you will need std::string.

If you are worried about copies, as long as you use references, and const references wherever you can, you will not have any overhead due to copies, and it's the same thing as you would be doing with the c-string.

Answer 5

Think of (char *) as string.begin(). The essential difference is that (char *) is an iterator and std::string is a container. If you stick to basic strings a (char *) will give you what std::string::iterator does. You could use (char *) when you want the benefit of an iterator and also compatibility with C, but that's the exception and not the rule. As always, be careful of iterator invalidation. When people say (char *) isn't safe this is what they mean. It's as safe as any other C++ iterator.

Answer 6

Strings have helper functions and manage char arrays automatically. You can concatenate strings, for a char array you would need to copy it to a new array, strings can change their length at runtime. A char array is harder to manage than a string and certain functions may only accept a string as input, requiring you to convert the array to a string. It's better to use strings, they were made so that you don't have to use arrays. If arrays were objectively better we wouldn't have strings.

Answer 7

One of the difference is Null termination (\\0).

In C and C++, char* or char[] will take a pointer to a single char as a parameter and will track along the memory until a 0 memory value is reached (often called the null terminator).

C++ strings can contain embedded \\0 characters, know their length without counting.

#include<stdio.h>
#include<string.h>
#include<iostream>

using namespace std;

void NullTerminatedString(string str){
   int NUll_term = 3;
   str[NUll_term] = '\0';       // specific character is kept as NULL in string
   cout << str << endl <<endl <<endl;
}

void NullTerminatedChar(char *str){
   int NUll_term = 3;
   str[NUll_term] = 0;     // from specific, all the character are removed 
   cout << str << endl;
}

int main(){
  string str = "Feels Happy";
  printf("string = %s\n", str.c_str());
  printf("strlen = %d\n", strlen(str.c_str()));  
  printf("size = %d\n", str.size());  
  printf("sizeof = %d\n", sizeof(str)); // sizeof std::string class  and compiler dependent
  NullTerminatedString(str);


  char str1[12] = "Feels Happy";
  printf("char[] = %s\n", str1);
  printf("strlen = %d\n", strlen(str1));
  printf("sizeof = %d\n", sizeof(str1));    // sizeof char array
  NullTerminatedChar(str1);
  return 0;
}

Output:

strlen = 11
size = 11
sizeof = 32  
Fee s Happy


strlen = 11
sizeof = 12
Fee