How to use qsort for string in C++ - c++

Question

I want to use qsort function to sort the characters in the strings using C++.
#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
int compare_str(void const *a,void const *b){
char const *aa=(char const *)a;
char const *bb=(char const *)b;
if(*aa==*bb) return 0;
else if(*aa>*bb) return 1;
else return -1;
}
int main(){
string str="cake";
int len=str.length();
qsort(str,len,sizeof(str[0]),compare_str);
cout<<str;
return 0;
}
But it throws :
20 42 [Error] cannot convert 'std::string {aka std::basic_string<char>}' to 'void*' for argument '1' to 'void qsort(void*, size_t, size_t, int (*)(const void*, const void*))'
It would be great if anyone could provide an efficient way to do this.

Answer 1

I strongly recommend the modern method of
#include <algorithm>
#include <iostream>
#include <string>
int main()
{
std::string s("cake");
std::sort(s.begin(), s.end());
std::cout << s << std::endl; // Prints "acek".
return 0;
}
Plus, using std::sort over qsort allows the compiler to optimize better, so it's a win-win...

Answer 2

Your comparator for qsort expects C strings, not C++ std::strings. You should either declare str to be char str[]
char str[] = "cake";
qsort(str, strlen(cake), sizeof(char), compare_str); // Consider renaming to compare_char
or (better) use std::sort:
string str = "cake";
sort(str.begin(), str.end());

Answer 3

If you really want to do this, just pass a pointer to the string's contents:
qsort(str.c_str(),len,sizeof(str[0]),compare_str);
That said, you really should consider using the functions provided in the STL rather than those from the old C library...

Answer 4

You should use the function sort() under the header <algorithm>. This function is very flexible and you can use it in different manner. For sorting as you wish in question you can just write:
#include <algorithm>
#include <iostream>
#include <string>
using namespace std;
int main()
{
string s="cake";
sort(s.begin(), s.end());
cout << s << endl;
return 0;
}
//output: acek
again by using sort() we can implement it in a range. If you want to sort first two element , the code will be
sort(s.begin(), s.begin()+2);
for above code the output will be
//output: acke
so if we want to sort first n element then we can write
sort(s.begin,s.begin()+n);
we can also modify the sort function. In that case we have to pass three parameter instead of two. The third parameter will be a functions which returns a bool value.For example , if we want to sort in descending order then our code will be like this
#include <algorithm>
#include <iostream>
#include <string>
using namespace std;
bool desc(char i, char j)
{
return i>j;
}
int main()
{
string s="cake";
sort(s.begin(), s.end(),desc);
cout << s << endl;
return 0;
}
//output: keca

Answer 5

#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
int compare_str(void const *a,void const *b){
char const *aa=(char const *)a;
char const *bb=(char const *)b;
if(*aa==*bb) return 0;
else if(*aa>*bb) return 1;
else return -1;
}
int main(){
string str="cake";
int len=str.length();
qsort(const_cast<char*>(str.c_str()),len,sizeof(str[0]),compare_str);
cout<<str<<endl;
return 0;
}