Convert string to char array in C++

Many of us have encountered error ‘cannot convert std::string to char[] or char* data type’. 

Examples: 

Input : string s = "geeksforgeeks" ;
Output : char s[] = { 'g', 'e', 'e', 'k', 's', 'f', 'o',
                     'r', 'g', 'e', 'e', 'k', 's' } ;
Input : string s = "coding" ;
Output : char s[] = { 'c', 'o', 'd', 'i', 'n', 'g' } ;

Method 1: A way to do this is to copy the contents of the string to char array. This can be done with the help of c_str() and strcpy() function of library cstring. 
The c_str() function is used to return a pointer to an array that contains a null terminated sequence of character representing the current value of the string.

Syntax: 

const char* c_str() const ;

If there is an exception thrown then there are no changes in the string. But when we need to find or access the individual elements then we copy it to a char array using strcpy() function. After copying it, we can use it just like a simple array. 
The length of the char array taken should not be less than the length of input string. 

Implementation:

CPP

// CPP program to convert string
// to char array
#include <iostream>
#include <cstring>

using namespace std;

// driver code
int main()
{
    // assigning value to string s
    string s = "geeksforgeeks";

    int n = s.length();

    // declaring character array
    char char_array[n + 1];

    // copying the contents of the
    // string to char array
    strcpy(char_array, s.c_str());

    for (int i = 0; i < n; i++)
        cout << char_array[i];

    return 0;
}

Output

geeksforgeeks

Time complexity: O(n)
Auxiliary Space: O(1)

Method 2:

CPP

// CPP program to convert string
// to char array
#include <iostream>
#include <string>

using namespace std;

// driver code
int main()
{
    // assigning value to string s
    string s("geeksforgeeks");
    // declaring character array : p
    char p[s.length()];

    int i;
    for (i = 0; i < sizeof(p); i++) {
        p[i] = s[i];
        cout << p[i];
    }
    return 0;
}

Output

geeksforgeeks

Time complexity: O(n)
Auxiliary Space: O(1)

Method 3:

This is the simplest and most efficient one. We can directly assign the address of 1st character of the string to a pointer to char. This should be the preferred method unless your logic needs a copy of the string.  

C++14

// CPP program for the above approach
#include <cstring>
#include <iostream>
#include <string>
using namespace std;

// Driver Code
int main()
{
    char* char_arr;
    string str_obj("GeeksForGeeks");
    char_arr = &str_obj[0];
    cout << char_arr;
    return 0;
}

Output

GeeksForGeeks

Time complexity: O(n)
Auxiliary Space: O(1)

Method 4:

An alternate way of Method 1 can be such, without using strcpy() function.

C++14

// CPP program to convert string
// to char array
#include <iostream>
#include <cstring>

using namespace std;

// driver code
int main()
{
    // assigning value to string s
    string st = "GeeksForGeeks";
    //the c_str() function returns a const pointer
    //to null terminated contents.
    const char *str = st.c_str();
    //printing the char array
    cout << str;
    return 0;
}

Output

GeeksForGeeks

Time complexity: O(n)
Auxiliary Space: O(1)