Do I need to have constructors in a class? Always? Supposing I am doing some vector maths, and I do the following: float distance_to_target = (this.position - target.position).get_length(); or supposing I use planes and points in boolean logic: if(point_vs_plane((this.position+this.direction),floor_plane))) this makes sense to me. [edited it because it actually didn't. Sorry.] If I do or don't have constructors here, what are the consequences? This kind of code is throughout my game engine. I find that by overloading operators and treating the classes as simple data types i get a slight speed boost and the maths looks more like the formulae you see in tutorials, which would otherwise complicate my learning process. My vector, ray, triangle, and line classes don't have constructors. Instead, i define them at statement scope, and let the compiler expand the code. I've heard it said that this might cause problems. Any thoughts?

If you don't have a constructor, with MSVC at least, it will create one for you. It's empty though. Have you tested this before you asked? Just remove the constructor and compile. If it works, tada.

All a constructor is intended to do for you is put your class object "in a good state." That means that nothing your class object contains, if used, could lead to undefined behavior-- like uninitialized pointers.

But although C++ gives you enough bullets to shoot yourself in the foot this way, I like to think those bullets could be "useful". Just look at each class on a case-by-case basis and decide which things need to be set into a good state.


C

my_life:          nop          jmp my_life

Quote:Original post by speciesUnknown
Do I need to have constructors in a class? Always?

You need a constructor to be able to construct the class. If you don't write any, the compiler will generate a default one, which simply calls the default constructors of the base class and all the members. Note that the default constructor of POD types is a no-op (it does nothing).

Quote:Any thoughts?

You're trying to save speed in the wrong place. A typical optimization for vectors in C++ is to use expression templates. This is because, with or without inlining and/or constructors, expressions such as vector a = b + c + d; will involve creation of two temporary vectors (one per add) where none is needed. You could of course use vector a = b; (a += c) += d; but it's less readable.

With expression templates, b + c + d would not create a vector, but rather an object of type, say:
vect::wrap<vect::add<vect::add<vect::const,vect::const>,vect::const> >
This object could then be used to initialize a by effectively performing the steps a = b; (a += c) += d;. So, even though the code would still look like vector a = b + c + d;, it would act as vector a = b; (a += c) += d;.

Thanks for your replies. Some useful comments here.

Quote:
If you don't have a constructor, with MSVC at least, it will create one for you. It's empty though. Have you tested this before you asked? Just remove the constructor and compile. If it works, tada.

Ive been using my vector class without a constructor for 18 months. As yet, no bugs, but I've been using the same c++ compiler the while time, gcc 4.0.1

Quote:
Making your classes behave in logical ways is certainly making good use of C++, but I do not see how you achieved a speed boost.

When i first tested it for speed, i had the following constructor:

g_vector::g_vector()
{
x=0;y=0;z=1;w=1;
};


I found that this was slower than not having a constructor. constructor. I tested it by creating a vector, adding it to another, multiplying it by a scalar, then dividing it by a scalar, all in one statement:

void test_vector()
{
static g_vector test_subject;
test_subject.set_vector(12,3,9,0);
test_vector=(test_vector+test_vector*96.3)/12 - test_vector;
}

The code ran faster without the constructor. I didn't test it with the constructor empty, however. Its possible that the constructor being created, empty, by the compiler rather than containing four floating point assignments is the cause of the speed increase.

Quote:
And yes, using constructors to initialize classes can absolutely help with debugging time/stability. Making the call to not initialize a class is an optimization call that should be considered like all others - with care and measurement to insure the cost justifies the added complexity/risk.

quite, and I am aware of the risk. My vector class is used in other classes, which do have constructors, and which call the set_vector method to ensure the initial values don't send, for example, an NPC flying off into space at massive speeds. This is just a game engine I'm building for fun and for learning purposes, so it wont be used in any critical systems. Probably.

Quote:

With expression templates, b + c + d would not create a vector, but rather an object of type, say:
vect::wrap<vect::add<vect::add<vect::const,vect::const>,vect::const> >
This object could then be used to initialize a by effectively performing the steps a = b; (a += c) += d;. So, even though the code would still look like vector a = b + c + d;, it would act as vector a = b; (a += c) += d;.

Thanks, I'm sure thats good advice, I don't quite understand it but ill read up on expression templates.

Quote:
I must say though that you could run into obscure bugs later down the track because you didn't want to use a constructor to initialize your member variables. They take 5 seconds to write and their use is considered good coding practice.

Good practice in programming is rather like good practice in real life. Its usually slower, but much safer. Like stopping and looking left and right before crossing the road. You have to stop, but your less likely that your legs will run into "design issues".


Is the compilers creation of empty constructors and destructors a part of the language specification? If not, to ensure compatibility I think I'll have two constructors: one which takes initialization parameters, and one which is totally empty and takes no parameters.