Error: initial value of reference to non-const must be an lvalue

[Rii]

Hello there, I've been trying to draw some lines with caclulations, but I've been unable to due to one error I don't understand. (intellisence error(mouseover) is in title)

#include <windows.h>
#include <string>
using namespace std;

void ReadFiles(int &id1, int &x1, int &y1, 
	       int &id2, int &x2, int &y2, 
	       char* &t, int &p5)
{
	p5 = 5;
	// other stuff
}

void DrawLine(HDC hdc, int &x, int &y, int &xx, int &yy)
{
	POINT pt[2];
	pt[0].x = x;
	pt[0].y = y;
	pt[1].x = xx;
	pt[1].y = yy;

	Polyline (hdc, pt, 2);
}

void DrawEdge(HDC hdc)
{
	int id1, x1, y1, id2, x2, y2, tx, ty, p5;
	char* t = "";
	ReadFiles(id1, x1, y1, id2, x2, y2, t, p5);
	string treat = t;
	
	if (x1 < x2 && y1 == y2) /*IF toLEFT*/
	{
		DrawLine(hdc, x1, y1, x2, y2);
		// method of adding reference to reference
		DrawLine(hdc, x1, y1, x1+p5, y1+p5);
		DrawLine(hdc, x1, y1, x1+p5, y1-p5);
		TextOut(hdc,tx,ty,t,treat.size());
	}
	else if (x1 == x2 && y1 < y2) /*IF toUP*/
	{
		DrawLine(hdc, x1, y1, x2, y2);
		// method of adding regular int to reference
		DrawLine(hdc, x1, y1, x1+5, y1+5);
		DrawLine(hdc, x1, y1, x1-5, y1+5);
		TextOut(hdc,tx,ty,t,treat.size());
	}
	else MessageBox(NULL, "Diagonal paths not allowed!", "ERROR",
			MB_ABORTRETRYIGNORE | MB_ICONWARNING);
}

both methods give the same errors

main.cpp(35): error C2664: 'DrawLine' : cannot convert parameter 4 from 'int' to 'int &'
main.cpp(36): error C2664: 'DrawLine' : cannot convert parameter 4 from 'int' to 'int &'
main.cpp(43): error C2664: 'DrawLine' : cannot convert parameter 4 from 'int' to 'int &'
main.cpp(44): error C2664: 'DrawLine' : cannot convert parameter 4 from 'int' to 'int &'

is there a way to easily do the x1+5 in the DrawLine without having to do stuff like int xl1a = x1+5; int yl1a = y1+5 every single time?

Thanks in advance!
Rii

EDIT: in short it can't calculate with pass by reference.

[coder777]

You don't need the reference in DrawLine(). Just remove them

[kbw]

It would help if you identified the line of code that the error is on. Without that I'll just speculate.

void DrawLine(HDC hdc, int &x, int &y, int &xx, int &yy)
means the parameters x, y, xx, yy will be modified by DrawLine, so you have to pass variables.

That means you can't write code like:

DrawLine(hMyDc, 0, 0, 70, 12);

As DrawLine doesn't actually change these parameters, you should pass them by value, so you get:

void DrawLine(HDC hdc, int x, int y, int xx, int yy)
{
	POINT pt[2];
	pt[0].x = x;
	pt[0].y = y;
	pt[1].x = xx;
	pt[1].y = yy;

	Polyline (hdc, pt, 2);
}

You can pass constants to this version of DrawLine.

[vlad from moscow]

The error means that you are trying to initialize non-const reference with non lvalue. The problem is that the expressions x1 + p5 and y1 + p5 are not lvalues in the call below.

DrawLine(hdc, x1, y1, x1+p5, y1+p5);

Either you shall declare parameters of the function as const references as, for example,

void DrawLine(HDC hdc, const int &x, const int &y, const int &xx, const int &yy);

or remove references from the parameter declarations because they are not needed.

[Rii]

@all: thank you all for the replies! I removed the references, it's working properly like that now.
bah so foolish of me, sorry for the dumb mistake >_<"
Rii