I have a struc like this:
struct process {int PID;int myMemory[];};
however, when I try to use it
process p;
int memory[2];
p.myMemory = memory;
I get an criptic error from eclipse saying int[0] is not compatible with int[2];
what am i doing wrong?
Thanks!
Don't use static arrays, malloc, or even new if you're using C++. Use std::vector which will ensure correct memory management.
#include <vector>
struct Process {
int pid;
std::vector<int> myMemory;
};
Process p;
p.reserve(2); // allocates enough space on the heap to store 2 ints
p.myMemory.push_back( 4815 ); // add an index-zero element of 4815
p.myMemory.push_back( 162342 ); // add an index-one element of 162342
I might also suggest creating a constructor so that pid does not initially have an undefined value:
struct Process {
Process() : pid(-1), myMemory() {
}
int pid;
std::vector<int> myMemory;
};
I think you should declare myMemory as an int* then malloc() when you know the size of it. After this it can be used like a normal array. Int[0] seems to mean "array with no dimension specified".
EXAMPLE:
int *a; // suppose you'd like to have an array with user specified length
// get dimension (int d)
a = (int *) malloc(d * sizeof(int));
// now you can forget a is a pointer:
a[0] = 5;
a[2] = 1;
free((void *) a); // don't forget this!
All these answers about vector or whatever are confused :) using a dynamically allocated pointer opens up a memory management problem, using vector opens up a performance problem as well as making the data type a non-POD and also preventing memcpy() working.
The right answer is to use
Array<int,2>
where Array is a template the C++ committee didn't bother to put in C++99 but which is in C++0x (although I'm not sure of the name). This is an inline (no memory management or performance issues) first class array which is a wrapper around a C array. I guess Boost has something already.
memory a priori. Otherwise he would have chosen int myMemory[2] in his struct declaration.In C++, array definition is almost equal to pointer constants, meaning that their address cannot be changed, while the values which they point to can be changed. That said, you cannot copy elements of an array into another by the assignment operator. You have to go through the arrays and copy the elements one by one and check for the boundary conditions yourself.
The syntax ...
struct process {int PID;int myMemory[];};
... is not valid C++, but it may be accepted by some compilers as a language extension. In particular, as I recall g++ accepts it. It's in support for the C "struct hack", which is unnecessary in C++.
In C++, if you want a variable length array in a struct, use std::vector or some other array-like class, like
#include <vector>
struct Process
{
int pid;
std::vector<int> memory;
};
By the way, it's a good idea to reserve use of UPPERCASE IDENTIFIERS for macros, so as to reduce the probability of name collisions with macros, and not make people reading the code deaf (it's shouting).
Cheers & hth.,
You cannot make the array (defined using []) to point to another array. Because the array identifier is a const pointer. You can change the value pointed by the pointer but you cannot change the pointer itself. Think of "int array[]" as "int* const array".
The only time you can do that is during initialization.
// OK
int array[] = {1, 2, 3};
// NOT OK
int array[];
array = [1, 2, 3]; // this is no good.
int x[] is normally understood as int * x.
In this case, it is not, so if you want a vector of integers of an undetermined number of positions, change your declaration to:
struct process {int PID;int * myMemory;};
You should change your initialization to:
int memory[2];
p.myMemory = new int[ 10 ];
p.myMemory = new int[2] int[0] and int[2] are not compatible types.