C++ substr function

The STL provides us with substr function, but it is lacking. For one it requires 2 parameters, instead of defaulting on the second. The php substr function also provides the ability to use negative 'start' position and negative lengths.

#include <iostream>
#include <string>
 
using namespace std;
 
string substr(const string &str, int start);
string substr(const string &str, int start, int length);
 
int main(int argc, char *argv[])
{
    cout << substr("abcdef", -1) << endl;     // "f"
    cout << substr("abcdef", -2) << endl;     // "ef"
    cout << substr("abcdef", -3, 1) << endl;  // "d"
 
    cout << substr("abcdef", 0, -1) << endl;  // "abcde"
    cout << substr("abcdef", 2, -1) << endl;  // "cde"
    cout << substr("abcdef", 4, -4) << endl;  // ""
    cout << substr("abcdef", -3, -1) << endl; // "de"
 
    cout << substr("abcdef", 1) << endl;      // "bcdef"
    cout << substr("abcdef", 1, 3) << endl;   // "bcd"
    cout << substr("abcdef", 0, 4) << endl;   // "abcd"
    cout << substr("abcdef", 0, 8) << endl;   // "abcdef"
    cout << substr("abcdef", -1, 1) << endl;  // "f"
}
 
string substr(const string &str, int start)
{
    return substr(str,start,str.length());
}
string substr(const string &str, int start, int length)
{
    if (start  < 0 ) start+=str.length();
    if (length < 0 ) length=str.length()+length-start;
    if (length < 0 ) return "";
    return str.substr(start,length);
}

I used test cases from the php website. Having 2 functions the same name with slightly different parameters is an example of function overloading. This function allows for negative starts and negative lengths, to mimic the php string function.

code snippets are licensed under Creative Commons CC-By-SA 3.0 (unless otherwise specified)

zedwood.com

C++ substr function