C++ Strings

In C++, string is an object of std::string class that represents sequence of characters. We can perform many operations on strings such as concatenation, comparison, conversion etc.

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C++ supports a wide range of functions that manipulate null-terminated strings −

Sr.No	Function & Purpose
1	strcpy(s1, s2); Copies string s2 into string s1.
2	strcat(s1, s2); Concatenates string s2 onto the end of string s1.
3	strlen(s1); Returns the length of string s1.
4	strcmp(s1, s2); Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2.
5	strchr(s1, ch); Returns a pointer to the first occurrence of character ch in string s1.
6	strstr(s1, s2); Returns a pointer to the first occurrence of string s2 in string s1.

Following example makes use of few of the above-mentioned functions −

#include <iostream>
#include <cstring>

using namespace std;

int main () {

   char str1[10] = "Hello";
   char str2[10] = "World";
   char str3[10];
   int  len ;

   // copy str1 into str3
   strcpy( str3, str1);
   cout << "strcpy( str3, str1) : " << str3 << endl;

   // concatenates str1 and str2
   strcat( str1, str2);
   cout << "strcat( str1, str2): " << str1 << endl;

   // total lenghth of str1 after concatenation
   len = strlen(str1);
   cout << "strlen(str1) : " << len << endl;

   return 0;
}

When the above code is compiled and executed, it produces result something as follows −

strcpy( str3, str1) : Hello
strcat( str1, str2): HelloWorld
strlen(str1) : 10

The String Class in C++

The standard C++ library provides a string class type that supports all the operations mentioned above, additionally much more functionality. Let us check the following example −

#include <iostream>
#include <string>

using namespace std;

int main () {

   string str1 = "Hello";
   string str2 = "World";
   string str3;
   int  len ;

   // copy str1 into str3
   str3 = str1;
   cout << "str3 : " << str3 << endl;

   // concatenates str1 and str2
   str3 = str1 + str2;
   cout << "str1 + str2 : " << str3 << endl;

   // total length of str3 after concatenation
   len = str3.size();
   cout << "str3.size() :  " << len << endl;

   return 0;
}

When the above code is compiled and executed, it produces result something as follows −

str3 : Hello
str1 + str2 : HelloWorld
str3.size() :  10

Difference b/w char[] and String in C++

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.

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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.