Simple solution if you are using C++11. Probably O(1) time as well:

st.pop_back();

if (str.size () > 0)  str.resize (str.size () - 1);

An std::erase alternative is good, but I like the "- 1" (whether based on a size or end-iterator) - to me, it helps expresses the intent.

BTW - Is there really no std::string::pop_back ? - seems strange.

buf.erase(buf.size() - 1);

This assumes you know that the string is not empty. If so, you'll get an out_of_range exception.

That's all you need:

#include <string>  //string::pop_back & string::empty

if (!st.empty())
    st.pop_back();

str.erase( str.end()-1 )

Reference: std::string::erase() prototype 2

no c++11 or c++0x needed.

int main () {

  string str1="123";
  string str2 = str1.substr (0,str1.length()-1);

  cout<<str2; // output: 12

  return 0;
}

With C++11, you don't even need the length/size. As long as the string is not empty, you can do the following:

if (!st.empty())
  st.erase(std::prev(st.end())); // Erase element referred to by iterator one
                                 // before the end

str.erase(str.begin() + str.size() - 1)

str.erase(str.rbegin()) does not compile unfortunately, since reverse_iterator cannot be converted to a normal_iterator.

C++11 is your friend in this case.

#include<iostream>
using namespace std;
int main(){
  string s = "Hello";// Here string length is 5 initially
  s[s.length()-1] = '\0'; //  marking the last char to be null character
  s = &s[0]; // using ampersand infront of the string with index will render a string from the index until null character discovered
  cout<<"the new length of the string "<<s + " is " <<s.length();
  return 0;
}