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# Unity A Proposal to Add Strong Type Aliases to the Standard Language

## 29 posts in this topic

Hey, I want strong type aliases in C++. I've never written a standard proposal before, so here's my attempt.

View PDF online

Any suggestions, additions, rewordings, and etc... you are able to offer would be much appreciated. After the community reviews and improves it, I'll email it to the standards committee (which are currently accepting proposals for C++ TR2), so it can hopefully be read and discussed at a committee meeting for addition to the C++ standard.

I would really appreciate it!

Aside from non-static data member initializers, which was added into C++11 (thank you, whoever submitted that proposal!), strong typedefs are one of my most desired C++ features.

Edited by Servant of the Lord
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Hm... Can I ask why this feature is vital? Conversion between

typedef unsigned int Centimeters;
typedef unsigned int Inches;

are legal as for me, as the real types are the same.

For incompatible types gcc (at least) drops an error:

??[santa@yukio ~ $] ? ??> g++ test.cpp -o test -pedantic -Wall test.cpp: In function ‘int main()’: test.cpp:9:8: error: cannot convert ‘MyFloat {aka float}’ to ‘MyInt {aka void*}’ for argument ‘1’ to ‘void test(MyInt)’ ??[santa@yukio ~$]
?
??> cat test.cpp
typedef void* MyInt;
typedef float MyFloat;

void test(MyInt) {
}

int main() {
MyFloat f;
test(f);
return 0;
}

For compatible types you won't get an error even without typedefs:

??[santa@yukio ~ $] ? ??> g++ test.cpp -o test -pedantic -Wall test.cpp: In function ‘int main()’: test.cpp:9:8: warning: ‘f’ is used uninitialized in this function [-Wuninitialized] ??[santa@yukio ~$]
?
??> cat test.cpp
void test(int) {
}

int main() {
float f;
test(f);
return 0;
}
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[quote name='Servant of the Lord' timestamp='1356723633' post='5015148']
I don't want Centimeters and Inches to be convertible
[/quote]

Well and I don't understand why. As I wrote earlier built-in types with similar (in fact equal) semantics _are_ convertible. In your example you are trying to introduce a new type (not an alias in fact, alias is usually just another typename). Simple constructions such as fancy_typedef old_type new_type' aren't able to describe the type semantics at all (both Centimeters and Inches are just lexems for compiler, you don't define what can be done to them). If you suggest semantics should be copied from the old_type than new_type would be just a typename, not a new type.

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[quote name='Álvaro' timestamp='1356725134' post='5015153']
You seriously don't understand why it's desirable to get a compiler error if someone has a quantity in centimeters and tries to use it where a quantity in inches is expected?
[/quote]

This is not my point. My point is thats not a compiler (c++ grammar) problem. Its worth implementing as a part of an STL.

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You seriously don't understand why it's desirable to get a compiler error if someone has a quantity in centimeters and tries to use it where a quantity in inches is expected?

This is not my point. My point is thats not a compiler (c++ grammar) problem. Its worth implementing as a part of an STL.

Well, it's a problem where the compiler could help, if it implemented what we are discussing.

I have felt the need for something like this when I have classes Vector3D and Point3D, which are essentially the same thing, but I need to define them separately if I want the type system to help me make sure my operations make sense (e.g., you are not allowed to add points, but adding vectors is fine, and so is adding a point and a vector).

Since we are dealing with units, I just wrote this little test that seems to work fine:
#include <iostream>

namespace units {
template <int m_pow, int kg_pow, int s_pow>
struct unit {
double value;
explicit unit(double value) : value(value) {
}
};

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator+(unit<m_pow,kg_pow,s_pow> u1, unit<m_pow,kg_pow,s_pow> u2) {
return unit<m_pow,kg_pow,s_pow>(u1.value+u2.value);
}

unit<0,0,0> operator+(unit<0,0,0> u, double d) {
return unit<0,0,0>(u.value+d);
}

unit<0,0,0> operator+(double d, unit<0,0,0> u) {
return unit<0,0,0>(d+u.value);
}

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator-(unit<m_pow,kg_pow,s_pow> u1, unit<m_pow,kg_pow,s_pow> u2) {
return unit<m_pow,kg_pow,s_pow>(u1.value-u2.value);
}

unit<0,0,0> operator-(unit<0,0,0> u, double d) {
return unit<0,0,0>(u.value-d);
}

unit<0,0,0> operator-(double d, unit<0,0,0> u) {
return unit<0,0,0>(d-u.value);
}

template <int m_pow1, int kg_pow1, int s_pow1, int m_pow2, int kg_pow2, int s_pow2>
unit<m_pow1+m_pow2,kg_pow1+kg_pow2,s_pow1+s_pow2> operator*(unit<m_pow1,kg_pow1,s_pow1> u1, unit<m_pow2,kg_pow2,s_pow2> u2) {
return unit<m_pow1+m_pow2,kg_pow1+kg_pow2,s_pow1+s_pow2>(u1.value*u2.value);
}

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator*(unit<m_pow,kg_pow,s_pow> u, double d) {
return unit<m_pow,kg_pow,s_pow>(u.value*d);
}

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator*(double d, unit<m_pow,kg_pow,s_pow> u) {
return unit<m_pow,kg_pow,s_pow>(d*u.value);
}

template <int m_pow1, int kg_pow1, int s_pow1, int m_pow2, int kg_pow2, int s_pow2>
unit<m_pow1-m_pow2,kg_pow1-kg_pow2,s_pow1-s_pow2> operator/(unit<m_pow1,kg_pow1,s_pow1> u1, unit<m_pow2,kg_pow2,s_pow2> u2) {
return unit<m_pow1-m_pow2,kg_pow1-kg_pow2,s_pow1-s_pow2>(u1.value/u2.value);
}

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator/(unit<m_pow,kg_pow,s_pow> u, double d) {
return unit<m_pow,kg_pow,s_pow>(u.value/d);
}

template <int m_pow, int kg_pow, int s_pow>
unit<m_pow,kg_pow,s_pow> operator/(double d, unit<m_pow,kg_pow,s_pow> u) {
return unit<-m_pow,-kg_pow,-s_pow>(d/u.value);
}

template <int m_pow, int kg_pow, int s_pow>
std::ostream &operator<<(std::ostream &os, unit<m_pow,kg_pow,s_pow> u) {
os << u.value;
if (m_pow != 0) {
os << "m";
if (m_pow != 1)
os << "^" << m_pow;
}
if (kg_pow != 0) {
os << "Kg";
if (kg_pow != 1)
os << "^" << kg_pow;
}
if (s_pow != 0) {
os << "s";
if (s_pow != 1)
os << "^" << s_pow;
}
return os;
}

unit<1,0,0> meter(1);
unit<1,0,0> centimeter(0.01);
unit<1,0,0> inch(0.0254);
unit<0,0,1> second(1.0);
unit<0,0,1> minute(60.0);
unit<0,0,1> hour(3600.0);
unit<0,1,0> gram(0.001);
// etc.
}

using namespace units;

int main() {
std::cout << "100 inches/hour = " << (100.0*inch/hour)/(centimeter/minute) << " centimeters/minute\n";
}


Does anyone know if there is a C++ library that does this type of thing? (Oh, and sorry about the hijack...)
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boost::units seems to do what you want.
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It would be cool if you could define casting operators for opaque typedefs too*:

// Obviously this would require a few changes to the Standard
Inches operator Centimeters() (Centimeters cm)
{
// Estimate
return (cm * 5) / 2;
// Note that the above is merely example code; I realize that a) the above isn't necessarily the
// the right data type, and b) casting it to the right data type could cause recursion without some
// changes or facilities added to the Standard.
}

inline constexpr Centimeters operator"" _cm (int cm)
{
return (Centimeters)cm;
}

Inches i = (Inches)(12_cm); // The point of this code is to show this line

Also, what would be the implications with promotions? What if you multiplied Inches (which is typedefd as an int) by a float? Or multiplied it by an int?

*This is, I would say, related to opaque typedefs, but is also a bit of a separate issue (because declaring a casting operator makes implicit conversions possible, and AFAIK you can't make a casting operator that requires explicit casting). I suppose you could also say it could work for non-opaque typedefs/implicit conversions, but I don't like the ambiguity of Inches i = 12_cm;

Edited by Cornstalks
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[quote name='Álvaro' timestamp='1356728174' post='5015178']
I have felt the need for something like this when I have classes Vector3D and Point3D, which are essentially the same thing, but I need to define them separately if I want the type system to help me make sure my operations make sense (e.g., you are not allowed to add points, but adding vectors is fine, and so is adding a point and a vector).
[/quote]

Ok, I got the idea. Still thats more like a copy of the type, rather than a typedef, so i suggest something like:

typecopy unsigned int Centimeters;
typecopy unsigned int Inches;

which should make an exact type copy under the specified name.

Also its not clear what to do with types hierarchy. Lets say:

class A {};
class B: public class A {};
typecopy B MyB; // should MyB also be a subclass of A or just copy the public part of an A interface?
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I can see how this would be useful, but there was one (non technical) thing about the paper that didn't feel right.

"(The author of this proposal does not seriously endorse the use of macros for type aliases, and the

above example was merely used for dramatic effect to trigger the gag reflex of any committee

I'm not sure if papers like these are an appropriate place for smug jokes, but I would venture they're not.
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I agree - know your audience.
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I don't want Centimeters and Inches to be convertible

Well and I don't understand why. As I wrote earlier built-in types with similar (in fact equal) semantics _are_ convertible.

You seriously don't understand why it's desirable to get a compiler error if someone has a quantity in centimeters and tries to use it where a quantity in inches is expected? I am not sure how else to explain it, since SOTL has been very clear.

class Inch
{
float mValue;
};

class Centimeter
{
float mValue;
};

int main(void)
{
Inch i;
Centimeter c = i;

return 0;
}
test.cpp : error C2440: 'initializing' : cannot convert from 'Inch' to 'Centimeter'
No constructor could take the source type, or constructor overload resolution was ambiguous

Am I missing something?

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I can see how this would be useful, but there was one (non technical) thing about the paper that didn't feel right.

"(The author of this proposal does not seriously endorse the use of macros for type aliases, and the
above example was merely used for dramatic effect to trigger the gag reflex of any committee

I'm not sure if papers like these are an appropriate place for smug jokes, but I would venture they're not.

Good point - I was trying to inject a spot of humor, but yeah, probably not the right place to do so.

class Inch
{
float mValue;
};

class Centimeter
{
float mValue;
};

int main(void)
{
Inch i;
Centimeter c = i;

return 0;
}

Am I missing something?

Yes, that makes the entire language very clunky. I would hate to have to do this:
myInch.mValue = 17;
myCentimeter.mValue = 35;
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class Inch
{
float mValue;
};

class Centimeter
{
float mValue;
};

int main(void)
{
Inch i;
Centimeter c = i;

return 0;
}

Am I missing something?

Yes, that makes the entire language very clunky. I would hate to have to do this:
myInch.mValue = 17;
myCentimeter.mValue = 35;

First, that code will not compile because mValue is a private member.  Second, you can define the constructors and assignment operators so that you dont need to directly access mValue at all.  Yes, there's some code you need to write for the class, but using it would be no different from using a built-in type, and you actually now have more flexibility in what operations you can do on your types and how to convert them, which get automatically converted and which result in compile errors, etc.

So I still dont understand why simply using a class is not an acceptable solution.  You want a new type with your own defined data and behaviors... which is exactly what classes are meant for.

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So I still dont understand why simply using a class is not an acceptable solution.  You want a new type with your own defined data and behaviors... which is exactly what classes are meant for.

The point is that we don't want to have to write identical classes if we can help it.

Here's another example:
struct Vector3D {
double x, y, z;

Vector3D(double x, double y, double z) : x(x), y(y), z(z) {
}

void print(std::ostream &os) const {
os << '(' << x << ',' << y << ',' << z << ')';
}
};

struct Point3D {
double x, y, z;

Point3D(double x, double y, double z) : x(x), y(y), z(z) {
}

void print(std::ostream &os) const {
os << '(' << x << ',' << y << ',' << z << ')';
}
};


It would be nice to say "a Point3D works exactly as a Vector3D". A typedef would allow that, but then you won't actually get two separate types.
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First, that code will not compile because mValue is a private member.  Second, you can define the constructors and assignment operators so that you dont need to directly access mValue at all.  Yes, there's some code you need to write for the class, but using it would be no different from using a built-in type, and you actually now have more flexibility in what operations you can do on your types and how to convert them, which get automatically converted and which result in compile errors, etc.

So I still dont understand why simply using a class is not an acceptable solution.  You want a new type with your own defined data and behaviors... which is exactly what classes are meant for.

No, I want a new type with identical data and identical behavior, with the only change being no implicit conversion between the original type and the derivative.
It'd be a huge amount of unnecessary boilerplate code for virtually identical classes.

Here's a copy+paste of real code from my current project, using my macro version (which hides all the boilerplate, but has some flaws):
strong_typedef(cPoint, SubTileLoc, Point); //The position (in tiles, not in pixels) of a tile within a tileset image.
strong_typedef(cPoint, PosInImage, Point); //The pixel position in an image.

strong_typedef(cPoint, PosInMonitor, Point); //The position (in pixels) in the monitor itself.
strong_typedef(cPoint, PosInWindow, Point); //The position (in pixels) in the game window's client area.
strong_typedef(cPoint, PosInVirtualWindow, Point); //The position (in pixels) in the game's window, after accounting for the virtual window size.

strong_typedef(cPoint, PosInWorld, Point); //Position within the world, in pixels.
strong_typedef(cPoint, VisiblePosInWorld, Point); //Position within the loaded portion of the world, in pixels.
//strong_typedef(cPoint, PosInArea, Point); //Might be needed in the future (if I ever allow multiple _areas_ side by side).
strong_typedef(cPoint, PosInCell, Point); //Position within a cell, in pixels.
strong_typedef(cPoint, PosInTile, Point); //Position within a tile, in pixels.

strong_typedef(cPoint, TileLoc, Point); //The position of a tile within a cell, in tiles (not pixels).
strong_typedef(cPoint, CellLoc, Point); //The position of a cell within the area, in cells (not pixels).
strong_typedef(cPoint, VisibleCellLoc, Point); //The position of a cell within the loaded chunks, in cell (not pixels). VisibleCellLoc(0,0) is usually the cell the player is in.

strong_typedef(cPoint, TileOffset, Point); //The offset from the grid, in pixels, at which a tile is drawn. (0,0) is aligned with grid.

strong_typedef(cRect, AreaBounds, Rect); //Bounds of the area, in cells.
Ignore the third parameter, and read it as "typedef cRect AreaBounds;". (The 'c' in my classes are part of an abbreviated namespace, and do not stand for 'class'. Only a few basic times have that prefix)

Notice a common theme there? 13 of the 14 strong typedefs actually are identical classes. You'd have me manually create 13 different classes? No, I wouldn't bother. One or two, maybe. But since it's so easy with a good strong typedef, I can do a dozen with zero extra programming work, and gain the following benefits for free:
1) The code is more self-descriptive. cPoint is generic name, CellLoc tells much better what the variable is, so the variable name can tell its use. Example: "CellLoc centerCell;"
2) Compile-time bug catching of simple mistakes where data of one type (like inches, or tiles) is given when another measurement (like centimeters, or pixels) is expected. This is the primary motivation for me doing this, after several times (over the course of my current multi-year project) I got burned by subtle bugs that the compiler could've caught (and now does!) where I made a simple mistake that went uncaught for quite awhile. After implementing the system and recompiling, the compiler balked and gave a false positive... so I thought, until I looked closer and realized it caught a bug that I didn't yet know existed.

Again, it takes zero programming effort to get these benefits, if strong typedefs are available. Regular typedefs provide the first benefit only, and people only use regular typedefs in less than half the situations where they'd be beneficial, because the benefit they provide is so minor. Mostly people use typedefs for minimizing the typing work they have to do on long class names (which is a valid usage), instead of using the benefits of typedefs to increase code clarity.

My typedef'd items above may have gone a little overboard, and two or three of them probably don't make much sense (but do no harm either). The rest are very significant improvements to the readability and stability of my code. Edited by Servant of the Lord
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So I still dont understand why simply using a class is not an acceptable solution.  You want a new type with your own defined data and behaviors... which is exactly what classes are meant for.

The point is that we don't want to have to write identical classes if we can help it.

Here's another example:
struct Vector3D {
double x, y, z;

Vector3D(double x, double y, double z) : x(x), y(y), z(z) {
}

void print(std::ostream &os) const {
os << '(' << x << ',' << y << ',' << z << ')';
}
};

struct Point3D {
double x, y, z;

Point3D(double x, double y, double z) : x(x), y(y), z(z) {
}

void print(std::ostream &os) const {
os << '(' << x << ',' << y << ',' << z << ')';
}
};


It would be nice to say "a Point3D works exactly as a Vector3D". A typedef would allow that, but then you won't actually get two separate types.

So, like this?

class UnitOfLength
{
public:

private:
float mValue;
};

class Inch : public UnitOfLength {};
class Centimeter : public UnitOfLength {};

int main(void)
{
Inch i;
Centimeter c = i;	// <--- compile error

return 0;
}`
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The constructors don't get exposed when you do that, so you'll have to rewrite them for Inch and Centimeter. Also, you can't do that with primitive types.

Of course there are alternative ways to do this, but having this feature in the language would make certain things easier, that's all.

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The constructors don't get exposed when you do that, so you'll have to rewrite them for Inch and Centimeter. Also, you can't do that with primitive types.

Of course there are alternative ways to do this, but having this feature in the language would make certain things easier, that's all.

Ahh, that's right, I forgot that the constructors and operators wont be inherited and you'll need to declare them in the child classes.

Ok, I can see where the motivation for this comes from, but on the other hand I've never seen a real case where I'd save enough typing/code duplication to make me want this.  In the Inch/Centimeter example, it's only 2 classes.  But even if more, this is stuff that you write once and then it'll sit in a lib untouched afterwards.  And, with a class you do get the additional abilities like operator overloading, and controlling which ones to allow and which to not allow.

In the code sample you gave... it just seems out of control.  You really just need a single 2D Point object (you have 13!), but somehow it looks like you're trying to prevent your high-level code from doing the wrong thing by moving your error checking to the lowest levels, and in doing so actually making more work for yourself with all these different typedefs that all cant be automatically cast to each other.  If I saw this code anyplace I worked I'd seriously have to wonder what the heck was going on.

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[quote name='0r0d' timestamp='1356845707' post='5015653']
In the code sample you gave... it just seems out of control.  You really just need a single 2D Point object (you have 13!), but somehow it looks like you're trying to prevent your high-level code from doing the wrong thing by moving your error checking to the lowest levels, and in doing so actually making more work for yourself with all these different typedefs that all cant be automatically cast to each other.  If I saw this code anyplace I worked I'd seriously have to wonder what the heck was going on.
[/quote]

I guess that paragraph was responding to SOTL, not me. I'll reply anyway. :)

I have to admit that those 13 classes seem a little overwhelming, but I don't see his code as moving error checking to the lowest levels. It is simply making the type system work for him, by forbidding operations that are most likely bugs. I can see myself doing something similar for vertices in a 3D pipeline: If you add model coordinates and world coordinates together, you probably have a bug, and having separate types for them makes total sense.

At work we deal a lot with orders to buy or sell stocks. Whenever I use a double in that context, I actually know more about it than that: It's an order size in shares, or a price in dollars/share, or a price in euros/share, or a dollar value for the order (or sometimes more exotic things, but let's keep it simple). It would make total sense to use separate types for these things, although in the end they are all just doubles when you get down to assembly level. I just would like my compiler to bomb if I ever try to add a quantity and a price.

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Ahh, that's right, I forgot that the constructors and operators wont be inherited and you'll need to declare them in the child classes.
Though see n2141 and forwarding constructors in C++11.
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• By NA-45
EDIT: We've found a designer/composer and an artist.  I'm looking for one more artist!

I'm currently working on Metroidvania style game that I was inspired to start by Hollow Knight and Beksiński's art.

It's built in Unity using C# and has quite a bit done already.  I'm handling the programming myself and have a working model (besides combat which is a WIP) that can be expanded greatly depending on where we decide to take the project.  You can see the current test area here: https://streamable.com/mp5o8  Since I'm not artistically gifted, its all rectangles but can easily be skinned once we've desired on designs.
I have professional experience using Unity and C# working on both a released game and a prototype as well as having extensive Java knowledge.  I also dabble in Python with a little bit of C++.
I have worked on and completed many projects before, the most recent being a 2D stick fighting game written ground up in Java Swing (don't ask why): https://www.youtube.com/watch?v=V4Bkoyp_f0o
I'm looking for a 2D artist (potentially more than one) to create concept and game art and a designer/writer who can help flesh out the story as well as map out and create challenging and eye catching areas.  I can handle most if not all of the programming side of things though if there is anyone who is extremely passionate about this sort of thing, I'd consider splitting the load.
The end goal is a completed game that can be sold however profit isn't really a concern to me as it's mostly a labor of love from my part.  Any profits would be split between team members however that's pretty far off so don't make that a reason to join.
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The story I have in mind is something like this:
A man wakes up in a chasm that stretches seemingly endlessly in both directions lined with enormous statues.

He discovers a temple with text above a closed gate that tells of the failed kingdom that lies below.  After finding a way around this, he drops down into the subterranean kingdom.  Adventuring through the labrynth below, he comes across different cities in which the residents succumbed to different sins such as Greed, Wrath, etc.  Each city tells a story of how its fixation on something lead to their demise leading up to a fight with the personification of their mistake.
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An very rough idea for Waterways, a potential area:
- To enter you must be wearing a pair of glasses that you find somewhere earlier in the ruins.  There are similar glasses found in every home.  Everything appears incredibly beautiful however something seems wrong.  After triggering some event, the glasses break and it's revealed that the glasses are made of some sort of stone that makes everything appear differently.  The city is in ruins and absolutely disgusting as everything was neglected.
- The only thing that remains intact is in the center of the city, an incredible statue of a goddess holding up a large sphere of the same material that was used for the glass.  You slowly learn the story behind the statue: the goddess came from the sea that the city lies on and brought prosperity to them.
- After opening up the the temple of the goddess that lies right on the edge of the waters, a giant sheet of the glass covers an opening in the back of the temple that reveals the goddess behind it.  You shatter the glass and it becomes apparent that the goddess is actually a disgusting creature half beached and mostly immobile that appears to secrete the material that makes up the glass. Fight ensues.
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The combat is pretty up in the air and part of the reason I need a designer to bounce ideas off of but I think it will be something like this:
- 4 orbs equipped at a time
- 2 orbs selected at a time
- Pressing the cast button will cast a spell determined by the 2 orbs that are selected
- Spells cost mana however you can use spells with 0 mana and it will cost health instead
- These spells in addition to being useful for combat, are the Metroidvania "gating" metchanic.  For instance, one of the conceptualized spells is a water orb + water orb to create a ice pillar that can be either used to block projectiles/enemy paths or to jump on to reach high areas
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If you're interested or have any questions, contact me through discord.  My id is NA-45#3692.
• By sZokka

Radio Rabbit is a local coop shoot ‘em up where two players control one more or less combined character. The character exists out of a rabbit’s body and a floating, still to the body connected, giant eye.
Each player controls one of them.

The rabbit’s goal is to fly safely through the level and to avoid enemies to reach the goal.  The Eye on the other hand can shoot. He is the one who clears the way. One character can move the other can shoot. So both player need to work together to fight of evil creatures and to complete the level.

•    explore the level to find the key which activates the portal gate
•    escape through the portal before the timer runs out
•    if you are to slow, the nuke will explode
•    use your character abilities, the rabbit can boost while the eye got the vision
•    you’ll get more powerful abilities from items such as a supershot
•    shoot as many enemies as possible to gain score
•    remaining time at the end of each level gets added to the score

Features
•    2 Player couch coop
•    4 level + tutorial
•    an epic boss fight
•    fully gamepad supported (XBox or equivalent)
•    local high score

Grab a friend and check it out!