Error: within this context

Technical C/C++
1

I’m trying to add the code for the Axis camera and I keep seeing this error when I build the project “error: within this context”.

#include “WPILib.h”
#include “Gamepad.h”
#include “Vision/AxisCamera.h”

class RobotDemo : public SimpleRobot
{
RobotDrive myRobot; // robot drive system
Gamepad controller;
AxisCamera camera;

public:
RobotDemo(void):
myRobot(10, 9, 2, 1), // these must be initialized in the same order
controller(1)
{ <------ERROR HERE

All I did was add the AxisCamera header file and the identifier (AxisCamera camera).

What am I missing? Thanks.

2

You’ve added a data member called camera, of type AxisCamera, no problem there. But the code you’ve included has a constructor with an initialization list, and you haven’t added a value for camera field.

Not being that familiar with the C++ WPI code, the issue is likely that the AxisCamera doesn’t have a default constructor or for some other reason, the compiler wants camera to be assigned using the initialization list.

Try putting

camera(???)

after the controller(1) line and before the {. Replace ??? with the parameters required for an AxisCamera object to be initialized.

Gotta love compiler error messages sometimes.

Greg McKaskle

3

AxisCamera is a singleton. You shouldn’t be calling the constructor directly. Try this.

AxisCamera& camera;

//In your constructor do this
camera = AxisCamera::GetInstance();
4

Thanks for the reply. It looks like I have a different problem now. When I try to implement your suggestion and use:

camera = AxisCamera::GetInstance();

I get an error that says “cannot convert ‘AxisCamera’ to ‘AxisCamera*’ in assignment”. Can you explain what this means?

Thanks again.

5

You should declare the camera as:


AxisCamera &camera;

camera = AxisCamera::GetInstance();
6

Thanks for the correction mikets. I misread the header when I wrote my initial post. I updated it to have the reference declaration instead of a pointer.

7

I really appreciate both of your replies, but making the corrections you suggested has led me to another problem.

class RobotDemo : public SimpleRobot
{
	RobotDrive myRobot; // robot drive system
	Gamepad controller;
	AxisCamera& camera;
	
public:
	RobotDemo(void):
		myRobot(10, 9, 2, 1),	
		controller(1)
	{                                     <-------------------------ERROR HERE
		GetWatchdog().SetExpiration(0.1);
	
		camera = AxisCamera::GetInstance();  <--------ERROR HERE

The first error says “uninitialized reference”. How do I initialize it?
The second error gets me back to where I started. It says “error within this context”.

This feels like one step forward then two steps back. Thanks for your help.

8

You need to initialize all non pointer members before the body (and especially references, they only have one life)

try this:

class RobotDemo : public SimpleRobot
{
	RobotDrive myRobot; // robot drive system
	Gamepad controller;
	AxisCamera& camera;
	
public:
	RobotDemo(void):
		myRobot(10, 9, 2, 1),	
		controller(1),
camera(AxisCamera::GetInstance())
	{                                    
		GetWatchdog().SetExpiration(0.1);

Also, “error within this context” is part of a larger error, open the build console and find the red lines to find the rest of the message. many messages are 2 or more parts.
Ex (bold is error in code)
file.cpp: In constructor ZomBDashboardPacketSender::ZomBDashboardPacketSender()':** file.cpp:301: error: uninitialized reference member ZomBDashboardPacketSender::hp’
**WPILib/Dashboard.h:58: error: `void Dashboard::operator=(const Dashboard&)’ is private
file.cpp:302: error: within this context