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digitalmars.D.learn - Passing rvalues to functions expecting const ref

reply "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
Hi. In C++, I can do this:

struct Vector3
{
   Vector3(_x, _y, _z); // constructor
};

float dot(const Vector3 &v, const Vector3 &u);

dot(Vector3(0, 0, 0), Vector3(1, 1, 1));
------------------------------------

In D, I can't -- and it's really annoying, because I am forced to 
make a copy, i.e:
dot(Vector3 v, Vector3 u);

I don't want this.

The other solution is this:
Vector3 v = {0, 0, 0};
Vector3 u = {1, 1, 1};

dot(v,u); // dot(const ref Vector3 v, const ref Vector3 u);

But it's not as clean.
Why can't I do what I can in C++? Is it a technical or a design 
decision? Are there plans to support it?
Dec 23 2012
parent reply "Namespace" <rswhite4 googlemail.com> writes:
As long as you use structs this should work, as you can see here: 
http://dpaste.dzfl.pl/03adf3d1
But if you use classes, it does not work anymore. So I like it a 
lot, that it works with structs. :)
Dec 23 2012
next sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Sunday, 23 December 2012 at 12:08:47 UTC, Namespace wrote:

 As long as you use structs this should work, as you can see 
 here: http://dpaste.dzfl.pl/03adf3d1
 But if you use classes, it does not work anymore. So I like it 
 a lot, that it works with structs. :)


Thank you. I had forgotten to supply the constructor to the 
struct :/
Dec 23 2012
prev sibling next sibling parent reply "John Chapman" <johnch_atms hotmail.com> writes:
On Sunday, 23 December 2012 at 12:08:47 UTC, Namespace wrote:

 As long as you use structs this should work, as you can see 
 here: http://dpaste.dzfl.pl/03adf3d1
 But if you use classes, it does not work anymore. So I like it 
 a lot, that it works with structs. :)


I don't think this will work after 2.061 is released. The 
behaviour is changing to disallow struct literals as lvalues.
Dec 23 2012
parent reply "Namespace" <rswhite4 googlemail.com> writes:
On Sunday, 23 December 2012 at 18:32:47 UTC, John Chapman wrote:

 On Sunday, 23 December 2012 at 12:08:47 UTC, Namespace wrote:

 As long as you use structs this should work, as you can see 
 here: http://dpaste.dzfl.pl/03adf3d1
 But if you use classes, it does not work anymore. So I like it 
 a lot, that it works with structs. :)


 I don't think this will work after 2.061 is released. The 
 behaviour is changing to disallow struct literals as lvalues.


I hope not. As long as "auto ref" don't work for normal 
functions, this change would be very inconvenient...

Minas Mina:
Show me the whole code, I think that your opBinary functions 
returns rvalues.
This would be a good and important case for "auto ref". But until 
now it is only for template paramters...
Dec 23 2012
parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
On Sunday, 23 December 2012 at 20:40:09 UTC, Namespace wrote:

 Minas Mina:
 Show me the whole code, I think that your opBinary functions 
 returns rvalues.
 This would be a good and important case for "auto ref". But 
 until now it is only for template paramters...


struct Vector3
{
	float x, y, z;
	
	this(float _x, float _y, float _z)
	{
		x = _x;
		y = _y;
		z = _z;
	}
	
	// negate operator
	Vector3 opUnary(string s)() const if( s == "-" )
	{
		Vector3 temp = this;
		
		temp.x = -temp.x;
		temp.y = -temp.y;
		temp.z = -temp.z;
		
		return temp;
	}
	
	// + operator for completeness
	Vector3 opUnary(string s)() const if( s == "+" )
	{
		return this;
	}
	
	// binary operators
	Vector3 opBinary(string op) (float val) const
	{
		static if( op == "+" )
		{
			Vector3 temp = this;
			temp.x += val;
			temp.y += val;
			temp.z += val;
		}
		else static if( op == "-" )
		{
			Vector3 temp = this;
			temp.x -= val;
			temp.y -= val;
			temp.z -= val;
		}
		else static if( op == "*" )
		{
			Vector3 temp = this;
			temp.x *= val;
			temp.y *= val;
			temp.z *= val;
		}
		else static if( op == "/" )
		{
			Vector3 temp = this;
			temp.x /= val;
			temp.y /= val;
			temp.z /= val;
		}
		
		return temp;
	}
	
	Vector3 opBinary(string op) (Vector3 v) const
	{
		static if( op == "+" )
		{
			Vector3 temp = this;
			temp.x += v.x;
			temp.y += v.y;
			temp.z += v.z;
		}
		static if( op == "-" )
		{
			Vector3 temp = this;
			temp.x -= v.x;
			temp.y -= v.y;
			temp.z -= v.z;
		}
		static if( op == "*" )
		{
			Vector3 temp = this;
			temp.x *= v.x;
			temp.y *= v.y;
			temp.z *= v.z;
		}
		
		return temp;
	}
}

/// dot product of two Vector3 vectors
 safe pure float dot(Vector3 u, Vector3 v)
{
	return u.x * v.x + u.y * v.y + u.z * v.z;
}

dot is making copies now because what I have shown earlier does 
not work... It's what I'm using now.
Dec 23 2012
prev sibling parent "Minas Mina" <minas_mina1990 hotmail.co.uk> writes:
There's another problem though:

struct Vector3
{
	float x, y, z;
	
	this(float _x, float _y, float _z)
	{
		x = _x;
		y = _y;
		z = _z;
	}
	
	// the binary +, - operators are defined
}

Vector3 cross(const ref Vector3 a, const ref Vector3 b);
...

Vector3 n = cross(b-a, c-a);


raytracing/triangle.d(58): Error: function 
raytracing.vector.cross (ref const(Vector3) a, ref const(Vector3) 
b) is not callable using argument types (Vector3,Vector3)
raytracing/triangle.d(58): Error: this.b.opBinary(this.a) is not 
an lvalue
raytracing/triangle.d(58): Error: this.c.opBinary(this.a) is not 
an lvalue

That's what I actually want.
Dec 23 2012