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Is this code ok, or should I create a separate lock object? I am asking if there is a potential for some deadlock or something like that. I assume it should work since the language allows it, but just to be sure I would rather understand how it works and why it is ok or why it is not ok.
var foo = false
fun bar()
{
synchronized(foo) {
foo = !foo
}
}
Your code is broken in a way you might not expect. Here's the simplified version of the bytecode it generates, decompiled to Java:
private static boolean foo;
public static final void bar() {
Boolean var0 = Boolean.valueOf(foo);
synchronized(var0) {
foo = !foo;
}
}
So you're essentially locking on whatever object the valueOf function returns, which in my JRE is either the TRUE or FALSE singletons inside the Boolean class (this is also simplified):
public class Boolean {
public static final Boolean TRUE = new Boolean(true);
public static final Boolean FALSE = new Boolean(false);
public static Boolean valueOf(boolean var0) {
return var0 ? TRUE : FALSE;
}
}
Your best bet for certain, runtime independent code is probably to create a separate instance of Any to synchronize on.
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I want to create a unique pointer in one class, class A, then pass on the ownership to another class, class B. Am I ok doing such a thing?
The code below gives me error in getC:
error: call to deleted constructor of 'std::unique_ptr<C>
What am I doing wrong?
class A {
...
void func(shared_ptr<B> Bptr) {
A_pass = make_unique<C>();
Bptr->setPass(move(A_pass));
}
unique_ptr<C> getC()
{
return A_pass;
}
unique_ptr<C> A_pass;
};
class B {
...
void setPass(unique_ptr<C> pass_ptr){
B_pass = move(pass_ptr);
}
unique_ptr<C> B_pass;
}
edit: update the question
Your question does not state where the compiler error occurs, but I would guess it’s the getC() member function:
class A {
...
unique_ptr<C> getC()
{
return A_pass;
}
unique_ptr<C> A_pass;
};
The function as written is attempting to copy A_pass, which of course is not possible for the std::unique_ptr<T> type.
You can rewrite it to explicitly move from the source (I’m not able to test this):
unique_ptr<C> A::getC() {
return A_pass.release();
// alternative: return unique_ptr<C>(std::move(A_pass));
}
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I had a situation where I wanted to use a default paramenter for a reference in a VERY large legacy codebase for a fix.
static bool _defaultValue = false;
bool SomeFunction(const SomeComplexObject& iObj, bool& isSomeVal = _defaultValue )
{
// ... code
}
My issue is with using a static variable inside a namespace just dangling there by itself.
This code is going to be reviewed before being shipped but I'm unsure if it would be considered bad practice to have a dangling static variable like that.
Without the variable you can't have a default value for a reference. My options are very limited to make other changes to get the desired effect.
Would this be considered "hacky unprofessional coding"?
My suggestion would be:
Remove the global variable.
Don't use a default value for the reference argument.
Create a function overload that has only one argument.
Call the first function from the second function.
bool SomeFunction(const SomeComplexObject& iObj, bool& isSomeVal)
{
// ... code
}
bool SomeFunction(const SomeComplexObject& iObj)
{
bool dummy;
return SomeFunction(iObj, dummy);
}
Client code can call whichever function is appropriate in their context.
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I have got some related answers like Why the size of empty class that is derived from two empty classes is 2? but not get the answer to my question clearly.
interface PI1
{
default void show()
{
System.out.println("Default PI1");
}
}
interface PI2
{
default void show()
{
System.out.println("Default PI2");
}
}
class TestClass implements PI1, PI2
{
public void show()
{
PI1.super.show();
PI2.super.show();
}
public static void main(String args[])
{
TestClass d = new TestClass();
d.show();
}
}
Does this JAVA program show multiple inheritance?
In C++ the minimum size is 1.
However, the other question is about multiple inheritance from base classes of the same type. Two objects of the same type cannot have the same address, because then they would not be different objects.
The address is an important part of the identity of an object.
So, if you have two objects of the same type, the minimum size would be 2.
None of this happens in Java, because there is no multiple inheritance.
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I have a question that is sort of a follow up to this:
Initializing default values in a struct
I've got a struct that's already got 17 bools, and a clear() method that sets them all to false. It's a long term project; the code could still be in use years from now and get added to. Is there a way to initialize all members that will extend automatically, so that someone adding new members to the struct doesn't need to remember to add them to the clear() method (other than a comment saying "please don't forget")?
This code base is not C++11 at this time, so I don't think I can initialize in the declaration.
The code is like this:
typedef struct {
bool doThingA;
bool doThingB;
bool doThingC;
bool doThingD;
// etc for several more bools
void clear() {
doThingA = false;
doThingB = false;
doThingC = false;
doThingD = false;
// etc...
}
} EnableFlags;
struct EnableFlags {
bool doThingA;
bool doThingB;
bool doThingC;
bool doThingD;
// etc for several more bools
void clear() {
*this = EnableFlags();
}
};
This will create a temporary with all members set to zero and then make *this a copy of it. So it sets all the members to zero, no matter how many there are.
This assumes that you haven't defined a default constructor that does something other than set all the flags to false. If you have no user-defined constructors then that assumption holds.
Since C++11 it's even simpler:
void clear() {
*this = {};
}
One option is to use a static assertion about the size of the structure inside the clear function.
First determine the current size of the struct. Let's say it's 68. Then, add this:
void clear()
{
BOOST_STATIC_ASSERT(sizeof(EnableFlags) == 68 && "You seem to have added a data member. Clear it before changing the number 68 to match current struct size!!");
// the rest as before ...
}
I've used Boost's static assertion macro, but you can use any other one you want, or roll out your own.
With such an assertion, the code will fail to compile when the size of the structure changes. It's not a catch-all solution, but does provide a safeguard.
Just use objetcs.
So you can use a 'for' loop to check in a std::vector if their values are false or true.
So you don't have your futurs co-workers put the "false" value each time they create a new boolean variable.
Structures are inapropriates here.
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How to use 'reference variables' in C++ classes?
I have the following code that I want to put into a class: (note KinectCV&)
KinectCV& kinect = freenect.createDevice(0);
kinect.some_init_functions();
while(condition) {
// getting frames from kinect and processing
kinect.some_processing_functions();
}
kinect.some_stopping_functions();
I'm trying to make a class and separate init, process and stop functions:
class MyKinect {
public:
KinectCV kinect;
void init(){
/* I cannot use the '& kinect = freenect.createDevice(0);' syntax, help me in this */
}
void process(){
kinect.some_processing_functions();
}
void stop(){
kinect.some_stopping_functions();
}
}
I cannot use the '& kinect = freenect.createDevice(0)
That is right, you cannot assign references; once initialized, they refer to the same object forever. What looks like an assignment in your code that works
KinectCV& kinect = freenect.createDevice(0);
is not an assignment, it's initialization. It can be rewritten using the initialization syntax instead of the assignment syntax, like this:
KinectCV& kinect(freenect.createDevice(0));
The reason behind it is that in C++ all initialization must happen in the constructor, not in a "designated initialization function". C++ has no idea that init is your initialization function; all it knows is that once the constructor is over, the object must be in a consistent state, included with all the references that it might hold.
Moving the initialization code into MyKinect's constructor will fix the problem:
class MyKinect {
public:
KinectCV kinect;
MyKinect() : kinect(freenect.createDevice(0)) {
}
}