g++output:

main.cpp: In function 'std::string rev(std::string)':
main.cpp:20: warning: statement has no effect
main.cpp:21: error: invalid conversion from 'char' to 'const char*'
main.cpp:21: error:   initializing argument 2 of 'std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::insert(typename _Alloc::size_type, const _CharT*) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]'

Code:

#include <iostream>
#include <string>
using namespace std;

string rev (string foo);

int main () {
	string input;
	string bar;
	cout << "Enter your text: ";
	getline(cin,input);
	bar = rev(input);
	cout << bar << endl;
	return 0;
}

string rev (string foo) {
	int s = foo.size();
	string bar;
	for(s ; s >= 0; s--) {
		bar.insert(1,foo[s]);
	}
	return bar;
}