On 3/2/12 11:26 AM, Luca Cerone wrote:
Dear all,
I'm having a few difficulties in declaring a class with a static
constant member.
This member is a vector of double that has to be created once during
the execution and shared among all the objects.
After being created it can't be modified during the whole execution of
my program.
I write a toy example to show you what I'd like to do (it doesn't work,
but I can't figure out a way to code it well, that's why I'm posting
here).
#include<iostream>
#include<vector>
class QuadFun{
public:
QuadFun(double a, double b) ; // a and b will be used to initialize
// the private vector y to evaluate the values (x-a)*(x-b)
// x has to be a vector that is shared among all the object of the class.
private:
static const std::vector<double> x; // this will be a vector of uniformly
//spaced numbers from 0 to 1. The number of points is determined
//by reading a configuration file. in this example I set the
//number manually in the main function.
std::vector<double> y;
}
int main() {
int numPoints=50; // this value is actually read from a configuration
file
// here is the problem. I don't want the number of points to be a member
// of the class.
QuadFun f1=QuadFun(0,1);
QuadFun f2=QuadFun(0.5,0.75);
// how can I make the vector x mad of numpoints created when I first
create
// an object of class QuadFun?
// how can I make it "dynamic" so that if I change the value of numPoints
// the x vector will be created accordingly? (in an other execution).
return 0 ;
}
Hope I could explain well what I'd like to achieve.
I know this code is WRONG, but I can't understand how to write it
in a good way.
Can you help me?
Thanks a lot in advance,
Cheers, Luca
Luca,
You have three problems here:
1. The C++ concept of const does not strictly allow for run-time
initialization such as you desire; and
2. Initialization in the constructor (which might be different for each
instance) is inconsistent with the static definition (which is shared by
all instances).
3. A static const requires a definition with an initial value.
The third problem is essentially a syntax error. The usual solution for
the second problem would be to add a static member function to
initialize the vector. And here is a reasonably good solution to the
first problem:
In the function you use to initialize the vector, use const_cast to
obtain a non-const pointer to the vector. You can then initialize it as
you require. If it is inconvenient to guarantee that the initialization
function is called only once, you might instead declare the static
member as a pointer to vector, declared with value NULL. If the
initialization function sees NULL, it instantiates the vector,
initializes it as desired, and then sets the pointer (using const_cast).
This is a fairly standard work-around when it is necessary to bend the
concept of const to something similar to (but different from) the C++
concept.
...