digitalmars.D - Suggestion: class object init value change & new compiler warning message
Let we have:
struct Stru {...}
class Obj {
this() {...}
this(int val) {...}
bool f() {...}
}
void func() {
Stru stru; //valid structure, not NULL as 'obj3'
Obj obj1 =3D new Obj;
Obj obj2 =3D new Obj(10);
Obj obj3; //"obj3 =3D NULL;"
}
There will be lot of "object =3D new SomeClass" in code around the globe=
, as =
in the example. I think it's a much more common situation than =
initializing an object with NULL. Hence objects should be initialized wi=
th =
new objects by default. That is, 'func()' would become:
void func() {
Stru stru;
Obj obj1; //"obj1 =3D new Obj;"
Obj obj2(10); //"obj2 =3D new Obj(10);"
Obj obj3 =3D NULL;
}
This reduces redundant code: a class name is not anymore used twice per =
=
object.
The amount of typing required is also smaller; fewer possible typos. (It=
's =
(This suggestion won't break old code.)
Compiler could also optimize the initialization values: if a value is =
assigned to an object before it's used, then it's initialized with NULL =
=
instead of a new object.
void func() {
Stru stru;
Obj obj1; //"obj1 =3D new Obj;"
Obj obj2(10); //"obj2 =3D new Obj(10);"
Obj obj3; //"obj3 =3D NULL;"
if(obj1.f()) {...} //used without initialization
obj3 =3D new Obj; //assigned before used
}
That would be optimal situation of course. Clean and short syntax.
But can the compiler know which init value to use?
The very basic checking is done, of course, as follows:
1) Get the first occurance of an object.
2) If it's on the left side of an assignment statement, it's inited with=
=
NULL.
3) Otherwise it's inited with a new object.
Even this simple solution gets very large proportion of the cases right =
=
(IMHO). (If not, your coding style is weird. ;) )
You can make the checking smarter by taking pointers/references into =
account.
For example, MS Visual Studio works as follows:
void func() {
int a;
int *ptr;
ptr =3D &a; //'ptr' is the same thing as 'a' now
*ptr =3D 1; //this equals to "a =3D 1;"
a++; //so, 'a' is not used here without initialization; no =
warning message
}
I think this kind of checking will get all the cases right (see below =
though).
If I am wrong here, it should not be a problem. If the compiler =
initializes a variable with a new object instead of NULL (which the =
compiler must do if it's not sure), normally it's a very minor =
inefficiency (because it happens very seldom). If necessary (writing som=
e =
complex code, eh?), you could force the right init value to be used:
Obj obj =3D NULL;
If a variable is used without initialization, the compiler should warn =
about it.
Of course, a variable can be set inside another function. For axample:
class Obj {...}
void setObj(out Obj o) {
o =3D new Obj;
}
void func() {
Obj obj; //"obj =3D new Obj;"
setObj(obj);
}
Note that compiler should not initialize 'obj' with NULL here, even if i=
t =
would be possible in this very situation. If an init value would depend =
on =
how functions will handle their parameters, it would be a mess.
It's 'interesting' to notice that Visual Studio does not warn in the =
following cases:
void func2(int val) {
...
}
void func3(int &val) { //in D: "void func3(inout int val)"
...
}
void func() {
int a;
func2(a); //no warning even if 'a' is used wihout initialization!
func3(a); //no warning!
}
In addition VS assumes that 'a' is set in 'func3()' because it _could_ =
modify it. For example:
void func() {
int a;
func3(a);
a++; //no warning!
}
All of these cases should cause a warning message.
That is:
void func() {
int a;
func2(a); //warning
func3(a); //warning
a++; //warning
}
Of course, only one warning message per variable is enough.
Aug 23 2006
xs0 <xs0 xs0.com> Kristian wrote:Let we have: struct Stru {...} class Obj { this() {...} this(int val) {...} bool f() {...} } void func() { Stru stru; //valid structure, not NULL as 'obj3' Obj obj1 = new Obj; Obj obj2 = new Obj(10); Obj obj3; //"obj3 = NULL;" } There will be lot of "object = new SomeClass" in code around the globe, as in the example.That's actually good, as one can easily see that a heap allocation is being done.I think it's a much more common situation than initializing an object with NULL. Hence objects should be initialized with new objects by default. That is, 'func()' would become: void func() { Stru stru; Obj obj1; //"obj1 = new Obj;" Obj obj2(10); //"obj2 = new Obj(10);" Obj obj3 = NULL; } This reduces redundant code: a class name is not anymore used twice per object.The amount of typing required is also smaller; fewer possible typos.auto obj = new LooognClassName(..);(This suggestion won't break old code.)It could - if the constructor does something, it will be done more often than before (and just wildly allocating objects is not a very good idea either, as it's slow even if mem allocation is all that happens). And, there's the obvious: Processor p; while (auto d = readData()) { if (p is null) // construct p only if needed p = new Processor(); p.process(d); } if (p) { // will always execute under your proposal ... }Compiler could also optimize the initialization values: if a value is assigned to an object before it's used, then it's initialized with NULL instead of a new object. [snip] But can the compiler know which init value to use? The very basic checking is done, of course, as follows: 1) Get the first occurance of an object. 2) If it's on the left side of an assignment statement, it's inited with NULL. 3) Otherwise it's inited with a new object.If only it were that simple; in reality, it's quite impossible to determine where an object will be first used (if at all). Which occurrence is first in the following code? Foo foo; if (..) { foo = ..; } else { foo = ..; }If a variable is used without initialization, the compiler should warn about it.This is practically impossible to get right (and it's really annoying when the compiler gets it wrong; trust me, I use Java which tries to do it and it is wrong far too often). Instead, variables in D get initialized to useless values (or 0 for integers), so it's hard to miss usage of an uninitialized value (albeit only at runtime). xs0
Aug 23 2006
I disagree with this.
It allows an object with an invalid value to pass of as valid while your
program continues executing. This makes it difficult to check for
uninitialized values at runtime. Checking for uninitialized values at
compile time is quite difficult because people can write arbitrarily
complex code, as xs0 pointed out.
Also, I usually want my references to start off knownly invalid and not
allocate unnecessary objects. If this were used, I would have to write
'= null;' very, very, frequently. I suppose it's arguable that
explicitly initializing things to null is a better practice. I wonder
how much current D code is actually written with explicit null
initializiation though.
In a way I wish structs were initialized to null, though I wonder how
that would work with the whole value type idea. Currently I feel that
the wisest thing to do is exactly what D does now - distinguish between
value and reference types, and give each their unique properties.
Kristian wrote:
Let we have:
struct Stru {...}
class Obj {
this() {...}
this(int val) {...}
bool f() {...}
}
void func() {
Stru stru; //valid structure, not NULL as 'obj3'
Obj obj1 = new Obj;
Obj obj2 = new Obj(10);
Obj obj3; //"obj3 = NULL;"
}
There will be lot of "object = new SomeClass" in code around the globe,
as in the example. I think it's a much more common situation than
initializing an object with NULL. Hence objects should be initialized
with new objects by default. That is, 'func()' would become:
void func() {
Stru stru;
Obj obj1; //"obj1 = new Obj;"
Obj obj2(10); //"obj2 = new Obj(10);"
Obj obj3 = NULL;
}
This reduces redundant code: a class name is not anymore used twice per
object.
The amount of typing required is also smaller; fewer possible typos.
(This suggestion won't break old code.)
Compiler could also optimize the initialization values: if a value is
assigned to an object before it's used, then it's initialized with NULL
instead of a new object.
void func() {
Stru stru;
Obj obj1; //"obj1 = new Obj;"
Obj obj2(10); //"obj2 = new Obj(10);"
Obj obj3; //"obj3 = NULL;"
if(obj1.f()) {...} //used without initialization
obj3 = new Obj; //assigned before used
}
That would be optimal situation of course. Clean and short syntax.
But can the compiler know which init value to use?
The very basic checking is done, of course, as follows:
1) Get the first occurance of an object.
2) If it's on the left side of an assignment statement, it's inited
with NULL.
3) Otherwise it's inited with a new object.
Even this simple solution gets very large proportion of the cases right
(IMHO). (If not, your coding style is weird. ;) )
You can make the checking smarter by taking pointers/references into
account.
For example, MS Visual Studio works as follows:
void func() {
int a;
int *ptr;
ptr = &a; //'ptr' is the same thing as 'a' now
*ptr = 1; //this equals to "a = 1;"
a++; //so, 'a' is not used here without initialization; no
warning message
}
I think this kind of checking will get all the cases right (see below
though).
If I am wrong here, it should not be a problem. If the compiler
initializes a variable with a new object instead of NULL (which the
compiler must do if it's not sure), normally it's a very minor
inefficiency (because it happens very seldom). If necessary (writing
some complex code, eh?), you could force the right init value to be used:
Obj obj = NULL;
If a variable is used without initialization, the compiler should warn
about it.
Of course, a variable can be set inside another function. For axample:
class Obj {...}
void setObj(out Obj o) {
o = new Obj;
}
void func() {
Obj obj; //"obj = new Obj;"
setObj(obj);
}
Note that compiler should not initialize 'obj' with NULL here, even if
it would be possible in this very situation. If an init value would
depend on how functions will handle their parameters, it would be a mess.
It's 'interesting' to notice that Visual Studio does not warn in the
following cases:
void func2(int val) {
...
}
void func3(int &val) { //in D: "void func3(inout int val)"
...
}
void func() {
int a;
func2(a); //no warning even if 'a' is used wihout initialization!
func3(a); //no warning!
}
In addition VS assumes that 'a' is set in 'func3()' because it _could_
modify it. For example:
void func() {
int a;
func3(a);
a++; //no warning!
}
All of these cases should cause a warning message.
That is:
void func() {
int a;
func2(a); //warning
func3(a); //warning
a++; //warning
}
Of course, only one warning message per variable is enough.
Aug 23 2006
xs0 and Chad had good points earlier in this thread. In short, they convinced me that the current D style to init class objects to null is a better way than the one I proposed. Though I am surprised that a Java compiler gets the "variable being used without initialization" warnings wrong so often... With a C++ compiler there has been no incorrect warnings (missing warnings, yes). But objects are not reference variables in C++, so it must be it. I came up with another suggestion concerning this issue. It can be read under "Suggestion: shortcut for 'new X'".
Aug 24 2006