char* vs std:string vs char[] in C++
In this article, we are going to inspect three different ways of initializing strings in C++ and discuss differences between them.
1. Using char*
Here, str is basically a pointer to the (const)string literal.
Syntax:
char* str = "This is w3wiki";
Pros:
1. Only one pointer is required to refer to whole string. That shows this is memory efficient.
2. No need to declare the size of string beforehand.
CPP
// CPP program to illustrate assigning // *char value to other variable #include <iostream> using namespace std; int main() { // This initialization gives warning in C++. // "deprecated conversion from string constant // to 'char*'" char * str = "Hello" ; const char * str1 = "Hello" ; // No warning // trying to modify const string literal // gives Runtime error str[1] = 'o' ; cout << str << endl; return 0; } |
Output:
This is w3wiki
w3wiki For Everyone
Cons:
1. This works fine in C but writing in this form is a bad idea in C++. That’s why compiler shows warning of “deprecated conversion from string constant to ‘char*'” because in C string literals are arrays of char but in C++ they are constant array of char. Therefore use const keyword before char*.
const char* str = "This is w3wiki";
2. We cannot modify the string at later stage in program. We can change str to point something else but cannot change value present at str. Refer storage-for-strings-in-c for more detail.
CPP
// CPP program to illustrate // std::string functions #include <iostream> using namespace std; int main() { // string assignment string s1 = "Hello" ; string s2 = "World" ; // return length of string cout << s1.size() << endl; // 5 cout << s2.length() << endl; // 5 // concatenate string using + operator. s1 = s1 + s2; cout << s1 << endl; // HelloWorld // append string s1.append( "Beginner" ); cout << s1 << endl; // HelloWorldBeginner string s3 = "HelloWorldBeginner" ; // compare two strings if (s1.compare(s3) == 0) cout << "true" << endl; else cout << "false" << endl; // substring of string s1 // substr(pos, length_of_substring) string sub = s1.substr(0, 5); cout << sub << endl; // Hello // insert into string // insert(pos, string) s1.insert(10, "For" ); cout << s1 << endl; // HelloWorldForBeginner string target = "World" ; // find a target string in s1 size_t pos = s1.find(target); if (pos != std::string::npos) // npos=-1 cout << "Found at Position:" << pos << endl; // pos=5 // replace a portion of string s1 // replace(pos, length_of_portion, string_to_replace) cout << s1.replace(5, 5, "Beginner" ) << endl; // Hellow3wiki return 0; } |
Output:
Segmentation Fault
2. Using std::string
Syntax:
std::string str = "This is w3wiki";
Here str is the object of std::string class which is an instantiation of the basic_string class template that uses char (i.e., bytes) as its character type.
Note: Do not use cstring or string.h functions when you are declaring string with std::string keyword because std::string strings are of basic_string class type and cstring strings are of const char* type. Pros: When dealing exclusively in C++ std:string is the best way to go because of better searching, replacement, and manipulation functions. Some of the useful std:string functions are discussed below.
CPP
// CPP program to illustrate char #include <iostream> using namespace std; int main() { char str[] = "Hello" ; // or char str1[]={'H', 'e', 'l', 'l', 'o', '\0'}; // modify string to "Hollo" str[1] = 'o' ; cout << str << endl; return 0; } |
Output:
5
5
HelloWorld
HelloWorldBeginner
true
Hello
HelloWorldForBeginner
Found at Position:5
Hellow3wiki
Cases where you might prefer char* over std:string
1. When dealing with lower level access like talking to the OS, but usually, if you’re passing the string to the OS then std::string::c_str has it covered.
2. Compatibility with old C code (although std::string’s c_str() method handles most of this).
3. To conserve memory (std::string will likely have more overhead).
3. Using char[]
Syntax:
char str[] = "This is w3wiki";
or
char str[size] = "This is w3wiki";
// Here str is a array of characters denoting the string.
Pros:
1. We can modify the string at later stage in program.
CPP
// CPP program to illustrate char // concatenation using standard functions #include <iostream> #include <cstring> using namespace std; int main() { // take large size of array char str[10] = "Hello" ; cout << "Before Concatenation : " << str << endl; // Hello strcat (str, " World" ); cout << "After Concatenation : " << str; // Hello World return 0; } |
Output:
Hollo
Cons:
1. This is statically allocated sized array which consumes space in the stack.
2. We need to take the large size of array if we want to concatenate or manipulate with other strings since the size of string is fixed. We can use C++ standard library cstring or string.h for that purpose.
CPP
#include
using namespace std;
int main()
{
char s1[10] = “Hello”;
// return length of s1
cout << strlen(s1) << endl;
char s2[50];
// copies s1 into s2
strcpy(s2, s1);
cout << s2 << endl;
char s3[10] = "World";
// concatenates s3 into s2
strcat(s2, s3);
cout << s2 << endl;
char s4[50] = "HelloWorld";
// return 0 if s2 and s4 are equal.
if (strcmp(s2, s4) == 0)
cout << "true" << endl;
else
cout << "false" << endl;
char s5[30];
// copies first 5 chars of s4 into s5
strncpy(s5, s4, 5);
cout << s5 << endl;
char target[10] = "Hello";
// search for target string in s4
if (strstr(s4, target) != NULL)
cout << "true" << endl;
else
cout << "false" << endl;
return 0;
}
Output:
Before Concatenation : Hello
After Concatenation : Hello World
Here are couple of other useful functions of C++ standard library cstring.
CPP
Output :
5
Hello
HelloWorld
true
Hello
true
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