user35240: I know, it isn't my original code, but I'm using this for a larger project that I am currently working on. I keep getting this error message: "cannot declare 'client' to be of abstract type 'Client'".
here's the code:

// RoboSapienServer.cpp
// RobosapienServer - Web enable a RoboSapien
// Kevin N. Haw
// http://www.KevinHaw.com/RoboSapienServer.php

// This project combines the default webserver example in the IDE distribution and Karl Castleton's(http://home.mesastate.edu/~kcastlet) RoboSapienIR (http://playground.arduino.cc/Main/RoboSapienIR).
// Source code merged from those two sources.

// Include files
#include <Ethernet.h>
#include <string.h>
#include <WebServer.h>

//////////////////////////////////////////////////////////////////
// Begin Web Server specific variable deinitions
//////////////////////////////////////////////////////////////////

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

// KNH, 02/09/2010 - Change IP address to use local subnet at home
//byte ip[] = { 10, 0, 0, 177 };
byte ip[] = { 192, 168, 1, 177 };

// Server for web requests
EthernetServer server(80);

// Define field name in the submitted form
#define SUBMIT_BUTTON_FIELDNAME "RSCmd"

// String for HTTP request variables
char pcHttpReqRsCmd[20] = {'\0'};


volatile int viRobsapienUrlCmd = -1;  // A robosapien command sent over the URL of a webpage HTTP request


//////////////////////////////////////////////////////////////////
// Begin Robosapien specific variable deinitions
//////////////////////////////////////////////////////////////////


// Some but not all RS commands are defined
#define RSTurnRight       0x80
#define RSRightArmUp      0x81
#define RSRightArmOut     0x82
#define RSTiltBodyRight   0x83
#define RSRightArmDown    0x84
#define RSRightArmIn      0x85
#define RSWalkForward     0x86
#define RSWalkBackward    0x87
#define RSTurnLeft        0x88
#define RSLeftArmUp       0x89
#define RSLeftArmOut      0x8A
#define RSTiltBodyLeft    0x8B
#define RSLeftArmDown     0x8C
#define RSLeftArmIn       0x8D
#define RSStop            0x8E
#define RSWakeUp          0xB1
#define RSBurp            0xC2
#define RSRightHandStrike 0xC0
#define RSNoOp            0xEF

// Subset of additional codes pulled from http://www.contrib.andrew.cmu.edu/~ebuehl/robosapien-lirc/ir_codes.htm
#define RSRightHandSweep  0xC1
#define RSRightHandStrike2 0xC3
#define RSHigh5           0xC4
#define RSFart            0xC7
#define RSLeftHandStrike  0xC8
#define RSLeftHandSweep  0xC9

#define RSWhistle         0xCA
#define RSRoar            0xCE


int IRIn = 2;            // We will use an interrupt
int IROut = 3;           // Where the echoed command will be sent from


boolean RSEcho = true;    // Should Arduino Echo RS commands
boolean RSUsed = true;    // Has the last command been used
volatile int RSBit = 9;   // Total bits of data
volatile int RSCommand;   // Single byte command from IR
int bitTime = 516;        // Bit time (Theoretically 833 but 516)
// works for transmission and is faster
int last;                 // Previous command from IR





//////////////////////////////////////////////////////////////////
// Begin Robosapien specific code
//////////////////////////////////////////////////////////////////



// Receive a bit at a time.
//  NOTE: Unused in the RoboServer aplication
void RSReadCommand() {
  delayMicroseconds(833 + 208); // about 1 1/4 bit times
  int bit = digitalRead(IRIn);
  if (RSBit == 9) { // Must be start of new command
    RSCommand = 0;
    RSBit = 0;
    RSUsed = true;
  }
  if (RSBit < 8) {
    RSCommand <<= 1;
    RSCommand |= bit;
  }
  RSBit++;
  if (RSBit == 9) RSUsed = false;
}

// send the whole 8 bits
void RSSendCommand(int command) {
  digitalWrite(IROut, LOW);
  delayMicroseconds(8 * bitTime);
  for (int i = 0; i < 8; i++) {
    digitalWrite(IROut, HIGH);
    delayMicroseconds(bitTime);
    if ((command & 128) != 0) delayMicroseconds(3 * bitTime);
    digitalWrite(IROut, LOW);
    delayMicroseconds(bitTime);
    command <<= 1;
  }
  digitalWrite(IROut, HIGH);
  delay(250); // Give a 1/4 sec before next
}


// Set up RoboSpapien functionality
void RSSetup()
{
  pinMode(IRIn, INPUT);
  pinMode(IROut, OUTPUT);
  pinMode(10, OUTPUT);
  digitalWrite(IROut, HIGH);

  attachInterrupt(0, RSReadCommand, RISING);

  last = RSNoOp;

  // Make robot burp to indicate setup is complete
  RSSendCommand(RSBurp);

}


// Loop for RoboSapien functionality
// Write only functionality - send the command from the web page to the robot, ignoring any input from the remote
void RSLoop()
{
  // Has a new command come in from the server?
  if (viRobsapienUrlCmd != -1)
  {
    // New command - send it to robot
    Serial.print("Sending command to RoboSapien: ");
    Serial.println(viRobsapienUrlCmd, HEX);
    RSSendCommand(viRobsapienUrlCmd);

    // Now clear command
    viRobsapienUrlCmd = -1;
  }
}

//////////////////////////////////////////////////////////////////
// Begin Webserver Specific Code
//////////////////////////////////////////////////////////////////

// Print ourt MIME and HTML header at top of webpage
void HtmlHeader(Client client)
{
  client.println("HTTP/1.1 200 OK");
  client.println("Content-Type: text/html");
  client.println();
  client.println("<HTML>\n<HEAD>");
  client.println("  <TITLE>Kevin's Arduino Webserver</TITLE>");//
  //  client.println("  <META HTTP-EQUIV=\"refresh\" CONTENT=\"5\">");
  client.println("</HEAD><BODY bgcolor=\"#9bbad6\">");
}

// Print the footer at the bottom of the webpage
void HtmlFooter(Client client)
{
  client.println("</BODY></HTML>");
}

// Print a submit button with the indicated label wrapped in a form for the indicated hex command
void SubmitButton(Client &client, char *pcLabel, int iCmd)
{
  client.print("<form method=post action=\"/?");
  client.print(iCmd, HEX);
  client.print("\"><input type=submit value=\"");
  client.print(pcLabel);
  client.print("\" name=\"" SUBMIT_BUTTON_FIELDNAME "\">");
  client.println("</form>");
}

// Parse an HTTP request header one character at a time, seeking string variables
void ParseHttpHeader(Client &client)
{
  char c;

  // Skip through until we hit a question mark (first one)
  while ((c = client.read()) != '?' && client.available())
  {
    // Debug - print data
    Serial.print(c);
  }

  // Are we here for a question mark or did we run out of data?
  if (client.available() > 2)
  {
    char pcUrlNum[3], *pc;

    // We have enough data for a hex number - read it
    for (int i = 0; i < 2; i++)
    {
      // Read and dump data to debug port
      Serial.print(c = pcUrlNum[i] = client.read());
    }
    // Null terminate string
    pcUrlNum[2] = '\0';

    // Get hex number
    viRobsapienUrlCmd = strtol(pcUrlNum, &pc, 0x10);
  }

  // Skip through and discard all remaining data
  while (client.available())
  {
    // Debug - print data
    Serial.print(c = client.read());
  }
}

// Set up webserver functionality
void WebServerSetup()
{
  Ethernet.begin(mac, ip);
  server.begin();
}

// Web server loop
void WebServerLoop()
{
  Client client = server.available();
  boolean bPendingHttpResponse = false; // True when we've received a whole HTTP request and need to output the webpage
  char c;  // For reading in HTTP request one character at a time

  if (client) {
    // Loop as long as there's a connection
    while (client.connected()) {
      // Do we have pending data (an HTTP request)?
      if (client.available()) {

        // Indicate we need to respond to the HTTP request as soon as we're done processing it
        bPendingHttpResponse = true;

        ParseHttpHeader(client);
      }
      else
      {
        // There's no data waiting to be read in on the client socket.  Do we have a pending HTTP request?
        if (bPendingHttpResponse)
        {
          // Yes, we have a pending request.  Clear the flag and then send the webpage to the client
          bPendingHttpResponse = false;

          // send a standard http response header and HTML header
          HtmlHeader(client);

          // Put out a text header
          client.println("<H1>Kevin's RoboSapien Webserver</H1>");

          client.println("<table border cellspacing=0 cellpadding=5><tr>");
          client.println("<td>");

          // Create buttons
          SubmitButton(client, "WakeUp", RSWakeUp);
          SubmitButton(client, "Roar", RSRoar);
          SubmitButton(client, "Whistle", RSWhistle);
          SubmitButton(client, "High5", RSHigh5);
          client.println("<br>");

          client.println("</td><td>");

          SubmitButton(client, "LeftArmUp", RSLeftArmUp);
          SubmitButton(client, "LeftArmIn", RSLeftArmIn);
          SubmitButton(client, "LeftArmOut", RSLeftArmOut);
          SubmitButton(client, "LeftArmDown", RSLeftArmDown);
          SubmitButton(client, "LeftArmSweep", RSLeftHandSweep);
          client.println("<br>");

          client.println("</td><td>");

          SubmitButton(client, "RightArmUp", RSRightArmUp);
          SubmitButton(client, "RightArmIn", RSRightArmIn);
          SubmitButton(client, "RightArmOut", RSRightArmOut);
          SubmitButton(client, "RightArmDown", RSRightArmDown);
          SubmitButton(client, "RightArmSweep", RSRightHandSweep);
          client.println("<br>");

          client.println("</td></tr></table>");

          client.print("<br><br><br>URL Hex no: ");
          client.print(viRobsapienUrlCmd, HEX);
          client.println("<br />");

          // send HTML footer
          HtmlFooter(client);

          // give the web browser time to receive the data
          delay(1);
          client.stop();
        }
      }
    }  // End while(connected)
  }
}

//////////////////////////////////////////////////////////////////
// Begin arduino entry points
//////////////////////////////////////////////////////////////////

void setup()
{
  // open the serial port at 9600 bps:
  Serial.begin(9600);

  // Print greeting
  Serial.println("Kevin's RobSapien Server");

  RSSetup();
  WebServerSetup();
}

void loop()
{
  RSLoop();
  WebServerLoop();
}


user33871: You can't use Client directly like that.  It's just an abstract class.  You have to use one of the classes like EthernetClient or WiFiClient that implements it.  
Client is like a shell of a class.  It says that there will be classes that act as clients and will have this basic form.  But it doesn't have any code, just the outline.  The base classes that implement that abstract class like EthernetClient or WiFiClient actually have the code to actually BE a client.  

35240: <p>I know, it isn't my original code, but I'm using this for a larger project that I am currently working on. I keep getting this error message: "cannot declare 'client' to be of abstract type 'Client'".
here's the code:
</p>

<pre class="lang-cpp prettyprint-override"><code>// RoboSapienServer.cpp
// RobosapienServer - Web enable a RoboSapien
// Kevin N. Haw
// http://www.KevinHaw.com/RoboSapienServer.php

// This project combines the default webserver example in the IDE distribution and Karl Castleton's(http://home.mesastate.edu/~kcastlet) RoboSapienIR (http://playground.arduino.cc/Main/RoboSapienIR).
// Source code merged from those two sources.

// Include files
#include &lt;Ethernet.h&gt;
#include &lt;string.h&gt;
#include &lt;WebServer.h&gt;

//////////////////////////////////////////////////////////////////
// Begin Web Server specific variable deinitions
//////////////////////////////////////////////////////////////////

byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

// KNH, 02/09/2010 - Change IP address to use local subnet at home
//byte ip[] = { 10, 0, 0, 177 };
byte ip[] = { 192, 168, 1, 177 };

// Server for web requests
EthernetServer server(80);

// Define field name in the submitted form
#define SUBMIT_BUTTON_FIELDNAME "RSCmd"

// String for HTTP request variables
char pcHttpReqRsCmd[20] = {'\0'};


volatile int viRobsapienUrlCmd = -1;  // A robosapien command sent over the URL of a webpage HTTP request


//////////////////////////////////////////////////////////////////
// Begin Robosapien specific variable deinitions
//////////////////////////////////////////////////////////////////


// Some but not all RS commands are defined
#define RSTurnRight       0x80
#define RSRightArmUp      0x81
#define RSRightArmOut     0x82
#define RSTiltBodyRight   0x83
#define RSRightArmDown    0x84
#define RSRightArmIn      0x85
#define RSWalkForward     0x86
#define RSWalkBackward    0x87
#define RSTurnLeft        0x88
#define RSLeftArmUp       0x89
#define RSLeftArmOut      0x8A
#define RSTiltBodyLeft    0x8B
#define RSLeftArmDown     0x8C
#define RSLeftArmIn       0x8D
#define RSStop            0x8E
#define RSWakeUp          0xB1
#define RSBurp            0xC2
#define RSRightHandStrike 0xC0
#define RSNoOp            0xEF

// Subset of additional codes pulled from http://www.contrib.andrew.cmu.edu/~ebuehl/robosapien-lirc/ir_codes.htm
#define RSRightHandSweep  0xC1
#define RSRightHandStrike2 0xC3
#define RSHigh5           0xC4
#define RSFart            0xC7
#define RSLeftHandStrike  0xC8
#define RSLeftHandSweep  0xC9

#define RSWhistle         0xCA
#define RSRoar            0xCE


int IRIn = 2;            // We will use an interrupt
int IROut = 3;           // Where the echoed command will be sent from


boolean RSEcho = true;    // Should Arduino Echo RS commands
boolean RSUsed = true;    // Has the last command been used
volatile int RSBit = 9;   // Total bits of data
volatile int RSCommand;   // Single byte command from IR
int bitTime = 516;        // Bit time (Theoretically 833 but 516)
// works for transmission and is faster
int last;                 // Previous command from IR





//////////////////////////////////////////////////////////////////
// Begin Robosapien specific code
//////////////////////////////////////////////////////////////////



// Receive a bit at a time.
//  NOTE: Unused in the RoboServer aplication
void RSReadCommand() {
  delayMicroseconds(833 + 208); // about 1 1/4 bit times
  int bit = digitalRead(IRIn);
  if (RSBit == 9) { // Must be start of new command
    RSCommand = 0;
    RSBit = 0;
    RSUsed = true;
  }
  if (RSBit &lt; 8) {
    RSCommand &lt;&lt;= 1;
    RSCommand |= bit;
  }
  RSBit++;
  if (RSBit == 9) RSUsed = false;
}

// send the whole 8 bits
void RSSendCommand(int command) {
  digitalWrite(IROut, LOW);
  delayMicroseconds(8 * bitTime);
  for (int i = 0; i &lt; 8; i++) {
    digitalWrite(IROut, HIGH);
    delayMicroseconds(bitTime);
    if ((command &amp; 128) != 0) delayMicroseconds(3 * bitTime);
    digitalWrite(IROut, LOW);
    delayMicroseconds(bitTime);
    command &lt;&lt;= 1;
  }
  digitalWrite(IROut, HIGH);
  delay(250); // Give a 1/4 sec before next
}


// Set up RoboSpapien functionality
void RSSetup()
{
  pinMode(IRIn, INPUT);
  pinMode(IROut, OUTPUT);
  pinMode(10, OUTPUT);
  digitalWrite(IROut, HIGH);

  attachInterrupt(0, RSReadCommand, RISING);

  last = RSNoOp;

  // Make robot burp to indicate setup is complete
  RSSendCommand(RSBurp);

}


// Loop for RoboSapien functionality
// Write only functionality - send the command from the web page to the robot, ignoring any input from the remote
void RSLoop()
{
  // Has a new command come in from the server?
  if (viRobsapienUrlCmd != -1)
  {
    // New command - send it to robot
    Serial.print("Sending command to RoboSapien: ");
    Serial.println(viRobsapienUrlCmd, HEX);
    RSSendCommand(viRobsapienUrlCmd);

    // Now clear command
    viRobsapienUrlCmd = -1;
  }
}

//////////////////////////////////////////////////////////////////
// Begin Webserver Specific Code
//////////////////////////////////////////////////////////////////

// Print ourt MIME and HTML header at top of webpage
void HtmlHeader(Client client)
{
  client.println("HTTP/1.1 200 OK");
  client.println("Content-Type: text/html");
  client.println();
  client.println("&lt;HTML&gt;\n&lt;HEAD&gt;");
  client.println("  &lt;TITLE&gt;Kevin's Arduino Webserver&lt;/TITLE&gt;");//
  //  client.println("  &lt;META HTTP-EQUIV=\"refresh\" CONTENT=\"5\"&gt;");
  client.println("&lt;/HEAD&gt;&lt;BODY bgcolor=\"#9bbad6\"&gt;");
}

// Print the footer at the bottom of the webpage
void HtmlFooter(Client client)
{
  client.println("&lt;/BODY&gt;&lt;/HTML&gt;");
}

// Print a submit button with the indicated label wrapped in a form for the indicated hex command
void SubmitButton(Client &amp;client, char *pcLabel, int iCmd)
{
  client.print("&lt;form method=post action=\"/?");
  client.print(iCmd, HEX);
  client.print("\"&gt;&lt;input type=submit value=\"");
  client.print(pcLabel);
  client.print("\" name=\"" SUBMIT_BUTTON_FIELDNAME "\"&gt;");
  client.println("&lt;/form&gt;");
}

// Parse an HTTP request header one character at a time, seeking string variables
void ParseHttpHeader(Client &amp;client)
{
  char c;

  // Skip through until we hit a question mark (first one)
  while ((c = client.read()) != '?' &amp;&amp; client.available())
  {
    // Debug - print data
    Serial.print(c);
  }

  // Are we here for a question mark or did we run out of data?
  if (client.available() &gt; 2)
  {
    char pcUrlNum[3], *pc;

    // We have enough data for a hex number - read it
    for (int i = 0; i &lt; 2; i++)
    {
      // Read and dump data to debug port
      Serial.print(c = pcUrlNum[i] = client.read());
    }
    // Null terminate string
    pcUrlNum[2] = '\0';

    // Get hex number
    viRobsapienUrlCmd = strtol(pcUrlNum, &amp;pc, 0x10);
  }

  // Skip through and discard all remaining data
  while (client.available())
  {
    // Debug - print data
    Serial.print(c = client.read());
  }
}

// Set up webserver functionality
void WebServerSetup()
{
  Ethernet.begin(mac, ip);
  server.begin();
}

// Web server loop
void WebServerLoop()
{
  Client client = server.available();
  boolean bPendingHttpResponse = false; // True when we've received a whole HTTP request and need to output the webpage
  char c;  // For reading in HTTP request one character at a time

  if (client) {
    // Loop as long as there's a connection
    while (client.connected()) {
      // Do we have pending data (an HTTP request)?
      if (client.available()) {

        // Indicate we need to respond to the HTTP request as soon as we're done processing it
        bPendingHttpResponse = true;

        ParseHttpHeader(client);
      }
      else
      {
        // There's no data waiting to be read in on the client socket.  Do we have a pending HTTP request?
        if (bPendingHttpResponse)
        {
          // Yes, we have a pending request.  Clear the flag and then send the webpage to the client
          bPendingHttpResponse = false;

          // send a standard http response header and HTML header
          HtmlHeader(client);

          // Put out a text header
          client.println("&lt;H1&gt;Kevin's RoboSapien Webserver&lt;/H1&gt;");

          client.println("&lt;table border cellspacing=0 cellpadding=5&gt;&lt;tr&gt;");
          client.println("&lt;td&gt;");

          // Create buttons
          SubmitButton(client, "WakeUp", RSWakeUp);
          SubmitButton(client, "Roar", RSRoar);
          SubmitButton(client, "Whistle", RSWhistle);
          SubmitButton(client, "High5", RSHigh5);
          client.println("&lt;br&gt;");

          client.println("&lt;/td&gt;&lt;td&gt;");

          SubmitButton(client, "LeftArmUp", RSLeftArmUp);
          SubmitButton(client, "LeftArmIn", RSLeftArmIn);
          SubmitButton(client, "LeftArmOut", RSLeftArmOut);
          SubmitButton(client, "LeftArmDown", RSLeftArmDown);
          SubmitButton(client, "LeftArmSweep", RSLeftHandSweep);
          client.println("&lt;br&gt;");

          client.println("&lt;/td&gt;&lt;td&gt;");

          SubmitButton(client, "RightArmUp", RSRightArmUp);
          SubmitButton(client, "RightArmIn", RSRightArmIn);
          SubmitButton(client, "RightArmOut", RSRightArmOut);
          SubmitButton(client, "RightArmDown", RSRightArmDown);
          SubmitButton(client, "RightArmSweep", RSRightHandSweep);
          client.println("&lt;br&gt;");

          client.println("&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;");

          client.print("&lt;br&gt;&lt;br&gt;&lt;br&gt;URL Hex no: ");
          client.print(viRobsapienUrlCmd, HEX);
          client.println("&lt;br /&gt;");

          // send HTML footer
          HtmlFooter(client);

          // give the web browser time to receive the data
          delay(1);
          client.stop();
        }
      }
    }  // End while(connected)
  }
}

//////////////////////////////////////////////////////////////////
// Begin arduino entry points
//////////////////////////////////////////////////////////////////

void setup()
{
  // open the serial port at 9600 bps:
  Serial.begin(9600);

  // Print greeting
  Serial.println("Kevin's RobSapien Server");

  RSSetup();
  WebServerSetup();
}

void loop()
{
  RSLoop();
  WebServerLoop();
}
</code></pre>
 33871: <p>You can't use Client directly like that.  It's just an abstract class.  You have to use one of the classes like EthernetClient or WiFiClient that implements it.  </p>

<p>Client is like a shell of a class.  It says that there will be classes that act as clients and will have this basic form.  But it doesn't have any code, just the outline.  The base classes that implement that abstract class like EthernetClient or WiFiClient actually have the code to actually BE a client.  </p>
