Recursively reversing a string without parameters - c++

Question

I've been given a class smartReverse which contains one member data which is a string called str. I have to implement a member method (without any sort of helper function which takes no parameters and returns the reversed version of str.
This is my attempt so far but this does nothing but send the first character to the end of the string. And from this point I'm pretty clueless. I know how to do this using a helper function but am not allowed to use them here.
string smartReverse::rev_recursive() const
{
if (str.length() <= 1)
return str;
char first_char = str[0];
smartReverse* remainder = new smartReverse(str.substr(1));
remainder->rev_recursive();
return remainder->getString() + first_char;
}

Answer 1

With memleak removed, and using rev_recursive result, the fixed version might be:
std::string smartReverse::rev_recursive() const
{
if (str.length() <= 1) {
return str;
}
char first_char = str[0];
smartReverse remainder(str.substr(1));
return remainder.rev_recursive() + first_char;
}

Answer 2

There is no any need to allocate an object of the type std::string dynamically. It is just a bad idea.
I do not know how the class smartReverse looks but here is its simplified version that has only one member function rev_recursive that reverses the stored string.
#include <iostream>
#include <string>
class smartReverse
{
public:
smartReverse( const std::string &s ) : s( s ) {}
std::string rev_recursive()
{
if ( s.size() < 2 ) return s;
char first = s.front(), last = s.back();
s = s.substr( 1, s.size() - 2 );
return s = last + rev_recursive() + first;
}
private:
std::string s;
};
int main()
{
smartReverse obj( "Hello Brennen Green" );
std::cout << obj.rev_recursive() << '\n';
return 0;
}
The program output is
neerG nennerB olleH
If the function shall be a constant member function then its implementation can look the following way
#include <iostream>
#include <string>
class smartReverse
{
public:
smartReverse( const std::string &s ) : s( s ) {}
std::string getString() const
{
return s;
}
std::string rev_recursive() const
{
if ( s.size() < 2 ) return s;
char first = s.front(), last = s.back();
return last + smartReverse( s.substr( 1, s.size() - 2 ) ).rev_recursive() + first;
}
private:
std::string s;
};
int main()
{
smartReverse obj( "Hello Brennen Green" );
std::cout << obj.getString() << '\n';
std::cout << obj.rev_recursive() << '\n';
return 0;
}
The program output is
Hello Brennen Green
neerG nennerB olleH
Pay attention to that the used approach is more efficient than when only one character is removed from the beginning of the string and then appended to the end because in the used approach the number of recursions is less than or equal to s.size() / 2 of the original string.