#include <string>
#include <cctype>
 
using namespace std;
 
// Check if a series of characters is on a word boundary
bool isWord( string source, int start, int length ) {
  bool isLeftBoundary = ( start == 0
    || isspace( source[start - 1] )
    || ispunct( source[start - 1] ) );
 
  bool isRightBoundary = ( start + length == source.length()
    || isspace( source[start + length] )
    || ispunct( source[start + length] ) );
 
  return isLeftBoundary && isRightBoundary;
}
 
// Find the starting index of a word in the string
int findWord( string source, string word ) {
  int index = 0;
 
  while ( ( index = source.find( word, index ) ) != string::npos ) {
    if ( isWord( source, index, word.length() ) ) {
      return index;
    }
 
    index += word.length();
  }
 
  return string::npos;
}

#include <iostream>
#include <string>
#include <algorithm>
 
int main()
{
    const std::string line_in("the cat sat on the mat");
    const std::string match("cat");
    std::string::const_iterator iter =
        std::search(
            line_in.begin(), 
            line_in.end(), 
            match.begin(), 
            match.end() 
    );
 
    if (iter != line_in.end() )
        std::cout << std::string(iter, line_in.end() ) << std::endl;
}

bool is_space_punct( std::string::const_iterator iter )
{
    return isspace( *iter ) || ispunct( *iter ) ;
}

    bool is_word = true;
    if (iter != line_in.begin() )  //Don't check the beginning
        is_word = is_space_punct( iter-1 );
 
    if ( iter+match.size() != line_in.end() ) //Don't check the end
        is_word = is_word && is_space_punct( iter+match.size() );