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What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?
What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
There are two overarching types of variables in Java:
Primitives: variables that contain data. If you want to manipulate the data in a primitive variable you can manipulate that variable directly. By convention primitive types start with a lowercase letter. For example variables of type int or char are primitives.
References: variables that contain the memory address of an Object i.e. variables that refer to an Object. If you want to manipulate the Object that a reference variable refers to you must dereference it. Dereferencing usually entails using . to access a method or field, or using [ to index an array. By convention reference types are usually denoted with a type that starts in uppercase. For example variables of type Object are references.
Consider the following code where you declare a variable of primitive type int and don't initialize it:
int x;
int y = x + x;
These two lines will crash the program because no value is specified for x and we are trying to use x's value to specify y. All primitives have to be initialized to a usable value before they are manipulated.
Now here is where things get interesting. Reference variables can be set to null which means "I am referencing nothing". You can get a null value in a reference variable if you explicitly set it that way, or a reference variable is uninitialized and the compiler does not catch it (Java will automatically set the variable to null).
If a reference variable is set to null either explicitly by you or through Java automatically, and you attempt to dereference it you get a NullPointerException.
The NullPointerException (NPE) typically occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable. So you have a reference to something that does not actually exist.
Take the following code:
Integer num;
num = new Integer(10);
The first line declares a variable named num, but it does not actually contain a reference value yet. Since you have not yet said what to point to, Java sets it to null.
In the second line, the new keyword is used to instantiate (or create) an object of type Integer, and the reference variable num is assigned to that Integer object.
If you attempt to dereference num before creating the object you get a NullPointerException. In the most trivial cases, the compiler will catch the problem and let you know that "num may not have been initialized," but sometimes you may write code that does not directly create the object.
For instance, you may have a method as follows:
public void doSomething(SomeObject obj) {
// Do something to obj, assumes obj is not null
obj.myMethod();
}
In which case, you are not creating the object obj, but rather assuming that it was created before the doSomething() method was called. Note, it is possible to call the method like this:
doSomething(null);
In which case, obj is null, and the statement obj.myMethod() will throw a NullPointerException.
If the method is intended to do something to the passed-in object as the above method does, it is appropriate to throw the NullPointerException because it's a programmer error and the programmer will need that information for debugging purposes.
In addition to NullPointerExceptions thrown as a result of the method's logic, you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method:
// Throws an NPE with a custom error message if obj is null
Objects.requireNonNull(obj, "obj must not be null");
Note that it's helpful to say in your error message clearly which object cannot be null. The advantage of validating this is that 1) you can return your own clearer error messages and 2) for the rest of the method you know that unless obj is reassigned, it is not null and can be dereferenced safely.
Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter and behave differently. You should also explain this in the documentation. For example, doSomething() could be written as:
/**
* #param obj An optional foo for ____. May be null, in which case
* the result will be ____.
*/
public void doSomething(SomeObject obj) {
if(obj == null) {
// Do something
} else {
// Do something else
}
}
Finally, How to pinpoint the exception & cause using Stack Trace
What methods/tools can be used to determine the cause so that you stop
the exception from causing the program to terminate prematurely?
Sonar with find bugs can detect NPE.
Can sonar catch null pointer exceptions caused by JVM Dynamically
Now Java 14 has added a new language feature to show the root cause of NullPointerException. This language feature has been part of SAP commercial JVM since 2006.
In Java 14, the following is a sample NullPointerException Exception message:
in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.List.size()" because "list" is null
List of situations that cause a NullPointerException to occur
Here are all the situations in which a NullPointerException occurs, that are directly* mentioned by the Java Language Specification:
Accessing (i.e. getting or setting) an instance field of a null reference. (static fields don't count!)
Calling an instance method of a null reference. (static methods don't count!)
throw null;
Accessing elements of a null array.
Synchronising on null - synchronized (someNullReference) { ... }
Any integer/floating point operator can throw a NullPointerException if one of its operands is a boxed null reference
An unboxing conversion throws a NullPointerException if the boxed value is null.
Calling super on a null reference throws a NullPointerException. If you are confused, this is talking about qualified superclass constructor invocations:
class Outer {
class Inner {}
}
class ChildOfInner extends Outer.Inner {
ChildOfInner(Outer o) {
o.super(); // if o is null, NPE gets thrown
}
}
Using a for (element : iterable) loop to loop through a null collection/array.
switch (foo) { ... } (whether its an expression or statement) can throw a NullPointerException when foo is null.
foo.new SomeInnerClass() throws a NullPointerException when foo is null.
Method references of the form name1::name2 or primaryExpression::name throws a NullPointerException when evaluated when name1 or primaryExpression evaluates to null.
a note from the JLS here says that, someInstance.someStaticMethod() doesn't throw an NPE, because someStaticMethod is static, but someInstance::someStaticMethod still throw an NPE!
* Note that the JLS probably also says a lot about NPEs indirectly.
NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory (null) as though it were referencing an object. Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException. These are the most common, but other ways are listed on the NullPointerException javadoc page.
Probably the quickest example code I could come up with to illustrate a NullPointerException would be:
public class Example {
public static void main(String[] args) {
Object obj = null;
obj.hashCode();
}
}
On the first line inside main, I'm explicitly setting the Object reference obj equal to null. This means I have a reference, but it isn't pointing to any object. After that, I try to treat the reference as though it points to an object by calling a method on it. This results in a NullPointerException because there is no code to execute in the location that the reference is pointing.
(This is a technicality, but I think it bears mentioning: A reference that points to null isn't the same as a C pointer that points to an invalid memory location. A null pointer is literally not pointing anywhere, which is subtly different than pointing to a location that happens to be invalid.)
What is a NullPointerException?
A good place to start is the JavaDocs. They have this covered:
Thrown when an application attempts to use null in a case where an
object is required. These include:
Calling the instance method of a null object.
Accessing or modifying the field of a null object.
Taking the length of null as if it were an array.
Accessing or modifying the slots of null as if it were an array.
Throwing null as if it were a Throwable value.
Applications should throw instances of this class to indicate other
illegal uses of the null object.
It is also the case that if you attempt to use a null reference with synchronized, that will also throw this exception, per the JLS:
SynchronizedStatement:
synchronized ( Expression ) Block
Otherwise, if the value of the Expression is null, a NullPointerException is thrown.
How do I fix it?
So you have a NullPointerException. How do you fix it? Let's take a simple example which throws a NullPointerException:
public class Printer {
private String name;
public void setName(String name) {
this.name = name;
}
public void print() {
printString(name);
}
private void printString(String s) {
System.out.println(s + " (" + s.length() + ")");
}
public static void main(String[] args) {
Printer printer = new Printer();
printer.print();
}
}
Identify the null values
The first step is identifying exactly which values are causing the exception. For this, we need to do some debugging. It's important to learn to read a stacktrace. This will show you where the exception was thrown:
Exception in thread "main" java.lang.NullPointerException
at Printer.printString(Printer.java:13)
at Printer.print(Printer.java:9)
at Printer.main(Printer.java:19)
Here, we see that the exception is thrown on line 13 (in the printString method). Look at the line and check which values are null by
adding logging statements or using a debugger. We find out that s is null, and calling the length method on it throws the exception. We can see that the program stops throwing the exception when s.length() is removed from the method.
Trace where these values come from
Next check where this value comes from. By following the callers of the method, we see that s is passed in with printString(name) in the print() method, and this.name is null.
Trace where these values should be set
Where is this.name set? In the setName(String) method. With some more debugging, we can see that this method isn't called at all. If the method was called, make sure to check the order that these methods are called, and the set method isn't called after the print method.
This is enough to give us a solution: add a call to printer.setName() before calling printer.print().
Other fixes
The variable can have a default value (and setName can prevent it being set to null):
private String name = "";
Either the print or printString method can check for null, for example:
printString((name == null) ? "" : name);
Or you can design the class so that name always has a non-null value:
public class Printer {
private final String name;
public Printer(String name) {
this.name = Objects.requireNonNull(name);
}
public void print() {
printString(name);
}
private void printString(String s) {
System.out.println(s + " (" + s.length() + ")");
}
public static void main(String[] args) {
Printer printer = new Printer("123");
printer.print();
}
}
See also:
Avoiding “!= null” statements in Java?
I still can't find the problem
If you tried to debug the problem and still don't have a solution, you can post a question for more help, but make sure to include what you've tried so far. At a minimum, include the stacktrace in the question, and mark the important line numbers in the code. Also, try simplifying the code first (see SSCCE).
Question: What causes a NullPointerException (NPE)?
As you should know, Java types are divided into primitive types (boolean, int, etc.) and reference types. Reference types in Java allow you to use the special value null which is the Java way of saying "no object".
A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference. For example, if you write this:
public class Test {
public static void main(String[] args) {
String foo = null;
int length = foo.length(); // HERE
}
}
the statement labeled "HERE" is going to attempt to run the length() method on a null reference, and this will throw a NullPointerException.
There are many ways that you could use a null value that will result in a NullPointerException. In fact, the only things that you can do with a null without causing an NPE are:
assign it to a reference variable or read it from a reference variable,
assign it to an array element or read it from an array element (provided that array reference itself is non-null!),
pass it as a parameter or return it as a result, or
test it using the == or != operators, or instanceof.
Question: How do I read the NPE stacktrace?
Suppose that I compile and run the program above:
$ javac Test.java
$ java Test
Exception in thread "main" java.lang.NullPointerException
at Test.main(Test.java:4)
$
First observation: the compilation succeeds! The problem in the program is NOT a compilation error. It is a runtime error. (Some IDEs may warn your program will always throw an exception ... but the standard javac compiler doesn't.)
Second observation: when I run the program, it outputs two lines of "gobbledy-gook". WRONG!! That's not gobbledy-gook. It is a stacktrace ... and it provides vital information that will help you track down the error in your code if you take the time to read it carefully.
So let's look at what it says:
Exception in thread "main" java.lang.NullPointerException
The first line of the stack trace tells you a number of things:
It tells you the name of the Java thread in which the exception was thrown. For a simple program with one thread (like this one), it will be "main". Let's move on ...
It tells you the full name of the exception that was thrown; i.e. java.lang.NullPointerException.
If the exception has an associated error message, that will be output after the exception name. NullPointerException is unusual in this respect, because it rarely has an error message.
The second line is the most important one in diagnosing an NPE.
at Test.main(Test.java:4)
This tells us a number of things:
"at Test.main" says that we were in the main method of the Test class.
"Test.java:4" gives the source filename of the class, AND it tells us that the statement where this occurred is in line 4 of the file.
If you count the lines in the file above, line 4 is the one that I labeled with the "HERE" comment.
Note that in a more complicated example, there will be lots of lines in the NPE stack trace. But you can be sure that the second line (the first "at" line) will tell you where the NPE was thrown1.
In short, the stack trace will tell us unambiguously which statement of the program has thrown the NPE.
See also: What is a stack trace, and how can I use it to debug my application errors?
1 - Not quite true. There are things called nested exceptions...
Question: How do I track down the cause of the NPE exception in my code?
This is the hard part. The short answer is to apply logical inference to the evidence provided by the stack trace, the source code, and the relevant API documentation.
Let's illustrate with the simple example (above) first. We start by looking at the line that the stack trace has told us is where the NPE happened:
int length = foo.length(); // HERE
How can that throw an NPE?
In fact, there is only one way: it can only happen if foo has the value null. We then try to run the length() method on null and... BANG!
But (I hear you say) what if the NPE was thrown inside the length() method call?
Well, if that happened, the stack trace would look different. The first "at" line would say that the exception was thrown in some line in the java.lang.String class and line 4 of Test.java would be the second "at" line.
So where did that null come from? In this case, it is obvious, and it is obvious what we need to do to fix it. (Assign a non-null value to foo.)
OK, so let's try a slightly more tricky example. This will require some logical deduction.
public class Test {
private static String[] foo = new String[2];
private static int test(String[] bar, int pos) {
return bar[pos].length();
}
public static void main(String[] args) {
int length = test(foo, 1);
}
}
$ javac Test.java
$ java Test
Exception in thread "main" java.lang.NullPointerException
at Test.test(Test.java:6)
at Test.main(Test.java:10)
$
So now we have two "at" lines. The first one is for this line:
return args[pos].length();
and the second one is for this line:
int length = test(foo, 1);
Looking at the first line, how could that throw an NPE? There are two ways:
If the value of bar is null then bar[pos] will throw an NPE.
If the value of bar[pos] is null then calling length() on it will throw an NPE.
Next, we need to figure out which of those scenarios explains what is actually happening. We will start by exploring the first one:
Where does bar come from? It is a parameter to the test method call, and if we look at how test was called, we can see that it comes from the foo static variable. In addition, we can see clearly that we initialized foo to a non-null value. That is sufficient to tentatively dismiss this explanation. (In theory, something else could change foo to null ... but that is not happening here.)
So what about our second scenario? Well, we can see that pos is 1, so that means that foo[1] must be null. Is this possible?
Indeed it is! And that is the problem. When we initialize like this:
private static String[] foo = new String[2];
we allocate a String[] with two elements that are initialized to null. After that, we have not changed the contents of foo ... so foo[1] will still be null.
What about on Android?
On Android, tracking down the immediate cause of an NPE is a bit simpler. The exception message will typically tell you the (compile time) type of the null reference you are using and the method you were attempting to call when the NPE was thrown. This simplifies the process of pinpointing the immediate cause.
But on the flipside, Android has some common platform-specific causes for NPEs. A very common is when getViewById unexpectedly returns a null. My advice would be to search for Q&As about the cause of the unexpected null return value.
It's like you are trying to access an object which is null. Consider below example:
TypeA objA;
At this time you have just declared this object but not initialized or instantiated. And whenever you try to access any property or method in it, it will throw NullPointerException which makes sense.
See this below example as well:
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
A null pointer exception is thrown when an application attempts to use null in a case where an object is required. These include:
Calling the instance method of a null object.
Accessing or modifying the field of a null object.
Taking the length of null as if it were an array.
Accessing or modifying the slots of null as if it were an array.
Throwing null as if it were a Throwable value.
Applications should throw instances of this class to indicate other illegal uses of the null object.
Reference: http://docs.oracle.com/javase/8/docs/api/java/lang/NullPointerException.html
A null pointer is one that points to nowhere. When you dereference a pointer p, you say "give me the data at the location stored in "p". When p is a null pointer, the location stored in p is nowhere, you're saying "give me the data at the location 'nowhere'". Obviously, it can't do this, so it throws a null pointer exception.
In general, it's because something hasn't been initialized properly.
A lot of explanations are already present to explain how it happens and how to fix it, but you should also follow best practices to avoid NullPointerExceptions at all.
See also:
A good list of best practices
I would add, very important, make a good use of the final modifier.
Using the "final" modifier whenever applicable in Java
Summary:
Use the final modifier to enforce good initialization.
Avoid returning null in methods, for example returning empty collections when applicable.
Use annotations #NotNull and #Nullable
Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn't be null.
Use equals with a known object first: if("knownObject".equals(unknownObject)
Prefer valueOf() over toString().
Use null safe StringUtils methods StringUtils.isEmpty(null).
Use Java 8 Optional as return value in methods, Optional class provide a solution for representing optional values instead of null references.
A null pointer exception is an indicator that you are using an object without initializing it.
For example, below is a student class which will use it in our code.
public class Student {
private int id;
public int getId() {
return this.id;
}
public setId(int newId) {
this.id = newId;
}
}
The below code gives you a null pointer exception.
public class School {
Student student;
public School() {
try {
student.getId();
}
catch(Exception e) {
System.out.println("Null pointer exception");
}
}
}
Because you are using student, but you forgot to initialize it like in the
correct code shown below:
public class School {
Student student;
public School() {
try {
student = new Student();
student.setId(12);
student.getId();
}
catch(Exception e) {
System.out.println("Null pointer exception");
}
}
}
In Java, everything (excluding primitive types) is in the form of a class.
If you want to use any object then you have two phases:
Declare
Initialization
Example:
Declaration: Object object;
Initialization: object = new Object();
Same for the array concept:
Declaration: Item item[] = new Item[5];
Initialization: item[0] = new Item();
If you are not giving the initialization section then the NullPointerException arise.
In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.
When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.
Your reference is "pointing" to null, thus "Null -> Pointer".
The object lives in the VM memory space and the only way to access it is using this references. Take this example:
public class Some {
private int id;
public int getId(){
return this.id;
}
public setId( int newId ) {
this.id = newId;
}
}
And on another place in your code:
Some reference = new Some(); // Point to a new object of type Some()
Some otherReference = null; // Initiallly this points to NULL
reference.setId( 1 ); // Execute setId method, now private var id is 1
System.out.println( reference.getId() ); // Prints 1 to the console
otherReference = reference // Now they both point to the only object.
reference = null; // "reference" now point to null.
// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );
// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...
This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.
Another occurrence of a NullPointerException occurs when one declares an object array, then immediately tries to dereference elements inside of it.
String[] phrases = new String[10];
String keyPhrase = "Bird";
for(String phrase : phrases) {
System.out.println(phrase.equals(keyPhrase));
}
This particular NPE can be avoided if the comparison order is reversed; namely, use .equals on a guaranteed non-null object.
All elements inside of an array are initialized to their common initial value; for any type of object array, that means that all elements are null.
You must initialize the elements in the array before accessing or dereferencing them.
String[] phrases = new String[] {"The bird", "A bird", "My bird", "Bird"};
String keyPhrase = "Bird";
for(String phrase : phrases) {
System.out.println(phrase.equals(keyPhrase));
}
I have dynamically created an Sensor object with new. However, when I try to use the object, it looks like the Sensor's members are uninitialized. I have verified that the creation and initialization of the object is successful. I don't believe this problem is due to the global object not being static but, changing it doesn't resolve the issue either.
class SensorModule
{
public:
SPI_BUS* spi;
ADX123* accel;
SENSOR* sensor2;
...
public:
SensorModule();
~SensorModule();
public:
sendCommand();
...
}
SensorModule::SensorModule()
{
spi = new SPI_BUS(1,2,3);
accel = new ADX123(spi,4);
sensor2 = new SENSOR(spi,5);
...// more initialization
}
void SensorModule::sendCommand(void){
accel->accelCommand(); // When I debug and stop here, I see that my "this"
// accel pointer are uninitialized
// however, the SensorModule object pointer is
// valid in the heap
}
main.cpp
SensorModule* pModule = NULL;
int main(void)
{
pModule = new SensorModule();
pModule->sendCommand();
...
for(;;)
{}
}
your code seems fine so far, although you have to be very careful to also call delete. In C++ you should usually use unique_ptr to make sure that the elements are deallocated.
Do you debug in debug or release mode? Release mode sometimes messes up some debug symbols. Do you change accel in the course of the constructor or with another method? There has to be something because accel is definitely initialized here. You might also step through the different lines of your constructor to check this.
so there is a main_window class that handling everything the thing that's important here is the Vector of enemy, enemies, at the time enemy is dead, it will push back a effect object into the vector of Effect. and here come the the handy point
lua_State *G = luaL_newstate(); // I need this thing to be global
// class constructor
Effect::Effect(int ix, int iy)
{
// not important codes deleted
luaL_openlibs(G);
luaL_dofile(G, "script/effect/blue_explosion.lua");
lua_getglobal(G, "draw_x");
draw_x = lua_tointeger(G, -1);
lua_getglobal(G, "draw_y");
draw_y = lua_tointeger(G, -1);
/* the function name */
lua_getfield(G, LUA_GLOBALSINDEX, "setup");
/* the first argument */
lua_pushnumber(G, ix);
/* the second argument */
lua_pushnumber(G, iy);
/* call the function with 2 arguments, return 1 result */
lua_call(G, 2, 0);
}
these things are going fine by itself, but here comes the problem
void Effect::close_lua()
{
lua_close(G);
}
which is called upon the end of life of the effect
with out this closure, it will start eat up my Ram
but if I use this, and lot's Effect object was created, the thing will crash with a
Access violation (segmentation fault)
anyone that have an idea how to fix this?
or should I just change an scripting language?
it seems like i had close one state twice, but the lua_close(G) is in different objects that's in the same vector, i just think up a way, maybe i can put lua_states in a vector too?
problem solved! what i did was make the lua_state object to private and it's not crashing anymore, it's probably is because that lua_state pointer got copied to other places, so now after i made it private it's not copyable anymore!
so first in header, private section declare lua_State *G;
and use G = luaL_newstate(); in the constructer
and some how it won't work in destructor, so i had this in the loop
for(int i = effects->size()-1; i > -1 ; i--)
{
effects->at(i).act();
if(effects->at(i).should_remove())
{
effects->at(i).close_lua();
effects->erase(effects->begin()+i);
}
}
that manually run the close_lua() witch is
void Effect::close_lua()
{
lua_close(G);
//std::cout << "closed"; a chick line
}
these lines worked perfectly as i watching the memory of the program in task manager
I've had to completely rewrite this problem as I've found out a lot more about it now.
Background:
My programme is drawing some 3d objects under directx11. I have a class that contains the data, pointers, and functions needed to draw the required 3d objects. Everything was working well. I could create many different 3d objects and draw them wherever I wanted. Great!
Then I needed to put them in a container and into a vector so I didn't have to create each object manually, this was where the trouble started; it would crash 1 time in 5 or so.
Unhandled exception at 0x00C308C1 in SpritesNTextN3D.exe: 0xC0000005: Access violation reading location 0xFFFFFFFF.
It crashed when using vectors and maps. I continued this line of enquiry and tried using a pointer and new:
ThreeD_Cube* threed_cube_p;
threed_cube_p = new ThreeD_Cube;
This also caused it to crash when I ran its draw function.
threed_cube_p->draw(threeD, camera, d3dContext_mp);
However if created as a standard object:
ThreeD_Cube threed_cube_;
The draw function never crashes.
threed_cube_-.draw(threeD, camera, d3dContext_mp);
Likewise, creating a pointer to threed_cube_ works as expected.
Question:
What is new doing that the default constructor isn't. Is there anything I should be looking at to resolve this problem?
It seems you have a good constructor, but bad/insufficient (default) assignment operator and bad/insufficient (default) copy constructor.
Let's see why some parts of your code works but some not:
//threed_cube_vec[0].draw(threeD, camera, d3dContext_); // doesnt work!!!
It tells you what's in threed_cube_vec[0] is a bad/corrupted object.
ThreeD_Cube test = threed_cube_vec[0]; // But, if I copy it...
In this line, (for some reason) firstly the constructor is called, which gives you a good object. Then the "=" is called, partially the object is modified, but the object is still good since it was already good before the "="
ThreeD_Cube* test = &threed_cube_vec[0];
As for a pointer, it is the essentially object threed_cube_vec[0] itself, so still corrupted.
ThreeD_Cube test = threed_cube_vec[0];
vector<ThreeD_Cube> test2;
test2.push_back(test);
test2[0].draw(threeD, camera, d3dContext_);
This does not fixed the problem as you said. "test" is a good object, but when you push_back(test) into test2, a copy is pushed back [if you change it to test2.push_back(std::move(test) , the problem could be gone]. Since the copy constructor is incomplete, the object in test2[0] is corrupted. Similar scenario happens with your map.
Conclusion: if an object is originated from the constructor, you get a good object; if an object is originated from a copy constructor, it is corrupted.
A quick test you can do: resize your vector after you declare it, the error should be gone temporarily.
Crash after m = XMMatrixIdentity() - aligment memory in classes?
This topic covers the answer to my problem. I eventually tracked it down to XMMATRIX causing crashes with new due to memory alignment.
Here's a shortened version of the problem:
void matrix_test_pointer()
{
XMMATRIX* xmmatrix_p;
xmmatrix_p = new XMMATRIX;
*xmmatrix_p = XMMatrixIdentity(); // this is where it crashes
}
void matrix_test()
{
XMMATRIX xmmatrix;
xmmatrix = XMMatrixIdentity();
}
int main()
{
string wait;
matrix_test();
cout << "matrix_test() completed.\n";
matrix_test_pointer();
cout << "matrix_test_pointer() completed.\n"; // If it does you are lucky :)
cin >> wait;
return 0;
}
Chances are matrix_test will complete but it will crash before pointer will complete.
I have a very simple class that looks as follows:
class CHeader
{
public:
CHeader();
~CHeader();
void SetCommand( const unsigned char cmd );
void SetFlag( const unsigned char flag );
public:
unsigned char iHeader[32];
};
void CHeader::SetCommand( const unsigned char cmd )
{
iHeader[0] = cmd;
}
void CHeader::SetFlag( const unsigned char flag )
{
iHeader[1] = flag;
}
Then, I have a method which takes a pointer to CHeader as input and looks
as follows:
void updateHeader(CHeader *Hdr)
{
unsigned char cmd = 'A';
unsigned char flag = 'B';
Hdr->SetCommand(cmd);
Hdr->SetFlag(flag);
...
}
Basically, this method simply sets some array values to a certain value.
Afterwards, I create then a pointer to an object of class CHeader and pass it to
the updateHeader function:
CHeader* hdr = new CHeader();
updateHeader(hdr);
In doing this, the program crashes as soon as it executes the Hdr->SetCommand(cmd)
line. Anyone sees the problem, any input would be really appreciated
When you run into a crash, act like a crime investigator: investigate the crime scene.
what is the information you get from your environment (access violation? any debug messages? what does the memory at *Hdr look like? ...)
Is the passed-in Hdr pointer valid?
Then use logical deduction, e.g.:
the dereferencing of Hdr causes an access violation
=> passed in Hdr points to invalid memory
=> either memory wasn't valid to start with (wrong pointer passed in), or memory was invalidated (object was deleted before passing in the pointer, or someone painted over the memory)
...
It's probably SEGFAULTing. Check the pointers.
After
your adding some source code
your comment that the thing runs on another machine
the fact that you use the term 'flag' and 'cmd' and some very small datatypes
making me assume the target machine is quite limited in capacity, I suggest testing the result of the new CHeader for validity: if the system runs out of resources, the resulting pointer will not refer to valid memory.
There is nothing wrong with the code you've provided.
Are you sure the pointer you've created is the same same address once you enter the 'updateHeader' function? Just to be sure, after new() note the address, fill the memory, sizeof(CHeader), with something you know is unique like 0XDEAD, then trace into the updateHeader function, making sure everything is equal.
Other than that, I wonder if it is an alignment issues. I know you're using 8 bit values, but try changing your array to unsigned ints or longs and see if you get the same issue. What architecture are you running this on?
Your code looks fine. The only potential issue I can see is that you have declared a CHeader constructor and destructor in your class, but do not show the implementation of either. I guess you have just omitted to show these, else the linker should have complained (if I duplicate this project in VC++6 it comes up with an 'unresolved external' error for the constructor. It should also have shown the same error for the destructor if you had a... delete hdr; ...statement in your code).
But it is actually not necessary to have an implementation for every method declared in a class unless the methods are actually going to get called (any unimplemented methods are simply ignored by the compiler/linker if never called). Of course, in the case of an object one of the constructor(s) has to be called when the object is instantiated - which is the reason the compiler will create a default constructor for you if you omit to add any constructors to your class. But it will be a serious error for your compiler to compile/link the above code without the implementation of your declared constructor, so I will really be surprised if this is the reason for your problem.
But the symptoms you describe definitely sounds like the 'hdr' pointer you are passing to the updateHeader function is invalid. The reason being that the 1st time you are dereferencing this pointer after the updateHeader function call is in the... Hdr->SetCommand(cmd); ...call (which you say crashes).
I can only think of 2 possible scenarios for this invalid pointer:
a.) You have some problem with your heap and the allocation of memory with the 'new' operator failed on creation of the 'hdr' object. Maybe you have insufficient heap space. On some embedded environments you may also need to provide 'custom' versions of the 'new' and 'delete' operator. The easiest way to check this (and you should always do) is to check the validity of the pointer after the allocation:
CHeader* hdr = new CHeader();
if(hdr) {
updateHeader(hdr);
}
else
//handle or throw exception...
The normal behaviour when 'new' fails should actually be to throw an exception - so the following code will cater for that as well:
try{
CHeader* hdr = new CHeader();
} catch(...) {
//handle or throw specific exception i.e. AfxThrowMemoryException() for MFC
}
if(hdr) {
updateHeader(hdr);
}
else
//handle or throw exception...
}
b.) You are using some older (possibly 16 bit and/or embedded) environment, where you may need to use a FAR pointer (which includes the SEGMENT address) for objects created on the heap.
I suspect that you will need to provide more details of your environment plus compiler to get any useful feedback on this problem.