error: invalid conversion from ‘const char*’ to ‘char*’ [duplicate]

Question

Possible Duplicate:
Using strtok with a std::string

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

using namespace std;

int main()
{
    string s("hello hi here whola");
    string background;
    char *strval;

    char* tok = strtok_r(s.c_str()," ",&strval);
    while(tok !=NULL)
    {
    cout << tok <<"\n";
    if (tok == "&")
        background = tok;
    else
    {
        statements1;
        statement2.. ;
    }
    tok = strtok_r(NULL, " ",&strval);
    }

    return 0;
}

output:

new.cpp: In function ‘int main()’:
new.cpp:13:47: error: invalid conversion from ‘const char*’ to ‘char*’ [-fpermissive]
/usr/include/string.h:359:14: error:   initializing argument 1 of ‘char* strtok_r(char*, const char*, char**)’ [-fpermissive]

Answer 1

The s.c_str() returns a pointer to the const char to prevent you from modifying the backing up memory. You need to make a writable copy of this constant string say with strdup() function as strtok() really modifies the string that you are scanning for tokens.