I'm sure this has been asked before but searching "c++" "pointer" "function" "array" gets me nowhere.
I have some stuff declared in main that looks like this:
struct point{
float x;
float y;
int m;
float points[10];
point *next;
};
struct neighbor{
float dist;
point *pt;
};
neighbor **candidate = NULL;
And I want to pass "candidate" to a function such that I can do:
candidate = new neighbor*[10];
for(int i = 0;i<10;i++)
candidate[i] = new neighbor;
Fill it with various data and then exit the function without using a return statement (this is important because I'm using boost threads which can't use a function other than void) and have the main function be able to see the changes the function did.
Sorry this is so basic but I what I think is right isn't working and I can't seem to find what I'm looking for.
Thanks in Advance
Accept a reference:
void fun(neighbor **& candidate);
Or a pointer:
void fun(neighbor *** candidate);
There's almost zero reason to ever need double pointer indirection in C++. You're almost certainly not leveraging the language to its best.
Related
SO im pretty new to c++ and im trying to pass a 2D array of a struct type by reference to a function. As far as i know they are automatically passed by reference. Here is my code.The problem is probably obvious but i cant figure it out. The complier keeps saying variable or field "function" declared void and bArray was not declared in this scope.
void function(balloons bArray[][5]);
int main()
{
struct balloons
{
float totalWeight;
float largestBalloon;
};
balloons balloonsArray[20][5];
function(balloonsArray);
}
void function(balloons bArray[][5])
{
bArray[1][1].totalWeight = 1.0
bArray[1][1].largestBalloon = 1.0
}
You're defining your struct within main, other parts of your code need to use it also. Move the definition outside the function:
struct balloons
{
float totalWeight;
float largestBalloon;
};
void function(balloons bArray[][5]);
int main()
{
// ...
And you haven't terminated the two statements in your function, you'll need semicolons there:
bArray[1][1].totalWeight = 1.0;
bArray[1][1].largestBalloon = 1.0;
I'm a beginner with C++, and this is a pretty basic syntax question, but i can't seem to find an answer elsewhere. Any input would be welcome. Here is a simplified version of the problem.
Say I have a class 'pair'
class pair
{
int a;
int b;
public:
pair(int x,int y)
{
a=x;
b=y;
}
int lookup()
{
return this->a+b;
}
};
Then i instanciate and copy that instance to a spot on the heap.
int func()
{
...
pair test(1,2);
pair *ptr=new pair;
*ptr=test;
}
Now here is the key. I don't destroy this memory allocation after the function ends. I want to use it later in this other function. The problem is, i would prefer to keep it in the heap and NOT have to copy it over to the stack(in the actual program it is very large). I would therefore like to do something like this:
int otherfunc()
{
...
int sum=*ptr.lookup;
}
but I get a compiler error. I end up having to do something like:
int otherfunc()
{
...
point temp=*ptr;
int sum=temp.lookup;
}
While this works, it is redundant, and when dealing with really large things it can even potentially cause an overflow. Anyone know the syntax for calling the method using the pointer while keeping it on the heap? Thanks.
I believe this is what you are trying to do:
int sum = ptr->lookup();
And as an aside, this:
return this->a+b;
Would probably be better as just this:
return a+b;
The expression *ptr.lookup will be interpreted as *(ptr.lookup), which is why you get the syntax error because ptr.lookup does not make sense. You'll need to tell the compiler dereference ptr first by using the parenthesis: (*ptr).lookup.
Because pointers are common in C (and C++), the (*a).b can be written in a simpler form: a->b.
Also, lookup is a function even if it does not take any parameters. You need to call it with ():
int sum=ptr->lookup();
I have the following structure:
struct CountCarrier
{
int *CurrCount;
};
And this is what I want to do:
int main()
{
CountCarrier carrier = CountCarrier();
*(carrier.CurrCount) = 2; // initialize the *(carrier.CurrCount) to 2
IncreaseCount(&carrier); // should increase the *(carrier.CurrCount) to 3
}
void IncreaseCount(CountCarrier *countCarrier)
{
int *currCounts = countCarrier->CurrCount;
(*currCounts)++;
}
So, my intention is specified in the comments.
However, I couldn't get this to work. For starters, the program throws an exception at this line:
*(carrier.CurrCount) = 2;
And I suspect the following line won't work as well. Anything I did wrong?
struct CountCarrier
{
int *CurrCount; //No memory assigned
};
You need to allocate some valid memory to the pointer inside the structure to be able to put data in this.
Unless you do so, What you ar trying to do is attempting to write at some invalid address, which results in an Undefined Behavior, which luckiy in this case shows up as an exception.
Resolution:
struct CountCarrier
{
int *CurrCount; //No memory assigned
CountCarrier():CurrCount(new(int))
{
}
};
Suggestion:
Stay away from dynamic allocations as long as you can.
When you think of using pointers always think whether you really need one. In this case it doesn't really seem that you need one, A simple int member would be just fine.
You need to create the pointer. ie. carrier->CurrCount = new int;
*(carrier.CurrCount)
This is dereferencing the pointer carrier.CurrCount, but you never initialized it. I suspect this is what you want:
carrier.CurrCount = new int(2);
I seriously doubt that your program throws an exception at the line:
*(carrier.CurrCount) = 2;
While throwing an exception is certainly allowed behaviour, it seems much more likely that you encountered an access violation that caused the process to be killed by the operating system.
The problem is that you are using a pointer, but your pointer is not initialised to point at anything. This means that the result of the pointer dereference is undefined.
In this situation there does not seem to be any advantage to using a pointer at all. Your CurrCount member would work just as well if it was just a plain int.
If you are using C++, then you should encash its facilities. Instead of correcting your code, I am showing here that how the code should look like:
struct CountCarrier
{
int CurrCount; // simple data member
CountCarrier(int count) : CurrCount(count) {} // constructor
CountCarrier& operator ++ () // overloaded operator
{
++ CurrCount;
return *this;
}
};
We are overloading operator ++, because you have only one data member. You can replace with some named method also, like void IncrementCount().
CountCarrier carrier(2);
++ carrier;
As Als said, you need to provide some memory for the code to work.
But why make it so complicated? You don't need any pointers for the code you have to work. The "modern C++" way looks more like this:
struct CountCarrier
{
public:
CountCarrier(int currCount) : currCount(currCount) {}
void IncreaseCount() { ++currCount; }
int GetCount() const { return currCount; }
private:
int currCount;
};
int main()
{
CountCarrier carrier(2); // Initialize carrier.currCount to 2
carrier.IncreaseCount(); // Increment carrier.currCount to 3
}
Note how much cleaner and less error prone that is. Like I said, pick up a good introductory C++ book and read through it.
My question is how to access and modify a 2D array defined in one class that is friends with another class. Below are some details on my question:
In class A I declare and allocate the appropriate space for my 2D array (pointer-to-pointer) u.
Class A
{
public:
friend class B;
long double **u;
int fun;
void make();
};
void A::make()
{
long double **u = new long double *[nx];
for (int i=0;i<nx;i++)
u[i] = new long double [ny];
int fun = 9;
}
Class A is friends with Class B; I need to use the array I declared in Class A in a function defined in class B. Below is my Class B:
class B
{
public:
void get(A*);
};
void B::get(A *pt)
{
using namespace std;
cout << pt->fun;
cout << pt->u[0][0];
}
I get a Bus error on my second cout pt->u[0][0]. Is there a simple way to use this setup I have to access my u[][] array? I think that I get the error because the pointer points to the 1st entry of my array, thus my whole 2D array is saved in memory as a single row (thinking aloud here). I'm a Fortran guy so this stuff is a little new to me.
Any help or "pointers" to other helpful threads would be appreciated.
Thank you !
Alberto
I think you get error because A::u is not initialized ( in method A::make you initialize a local variable u, not member. You need to change
void A::make()
{
long double **u = new long double *[nx]; // should be just u, or this->u.
There are some problems with your code: nx and ny don't seem to be defined anywhere, and in make you don't initialize A::fun at all, you instead set a local variable named fun which goes out of scope immediately.
As for your error, it sounds like the error stems from the fact that make() has not been called on pt. Ensure that make() is called on the instance you pass to get, otherwise the array u will not be allocated.
I am having problem with passing a pointer by reference.
This is the method
void set_range(double **measu)
{
if ((*measu)[0] < 0) //or i is used for a loop
return ;
}
int main()
{
double *mes;
set_range(&mes[1]);
}
I have allocated memory and required values are set. But this program gives me "Unhandled exception Access violation reading location" error.
So my question is,how to pass the pointer of mes[1] instead of mes[0] (which normally passed when (&mes) is given) to the set_range method?
One problem is that &mes[1] is of type double *, not the double ** required of your function.
Another problem is that mes doesn't point to anything - it's uninitialized. So dereferencing it will access junk (which is why you get an access violation).
I'm trying to come up with some code to help clarify, but honestly I have no idea what you're trying to do. Some more code would help us figure out what your goal is, but just given the above I have no idea why you need a double ** or whether you need dynamic memory or just a single double variable.
Change your function to take a double* instead of a double**, eg:
void set_range(double *measu)
{
if (*measu < 0) //or i is used for a loop
return;
}
int main()
{
double *mes;
...
set_range(&mes[1]);
}
Alternatively, use a real reference instead:
void set_range(double &measu)
{
if (measu < 0) //or i is used for a loop
return;
}
int main()
{
double *mes;
...
set_range(mes[1]);
}