error: Illegal zero sized array - c++

Question

I get this error:
error C2229: class 'GenerateRandNum<int [],int>' has an illegal zero-sized array
In my main, I call my random generator function to input into a empty data set
I call the method in my main like so:
//declare small array
const int smallSize = 20;
int smallArray[smallSize];
// call helper function to put random data in small array
GenerateRandNum <int[], int> genData(smallArray, smallSize);
genData.generate();
Header file
template <class T, class B>
class GenerateRandNum
{
public:
T data;
B size;
GenerateRandNum(T list, B length)
{
data = list;
size = length;
}
void generate();
};
File with method definition
template<class T, class B>
void GenerateRandNum<T, B> ::generate()
{
for (B i = 0; i < size; i++)
{
data[0] = 1 + rand() % size;
}
}

Answer 1

Pointers and arrays are not the same in C/C++. They are two very different things. However, arrays decay into pointers. Most notably in function declarations: The declaration
void foo(int array[7]);
is defined to be equivalent to
void foo(int* array);
That said, all the GenerateRandNum constructor gets, is a int* because that's what T = int [] decays to in the function declaration context. The data member of GenerateRandNum, however, is of type int [] (no decay here), which your compiler assumes to be a zero sized array. Consequently, when you try to assign a pointer to the array, your compiler complains.
You have two options to fix this:
You use an std::vector<> instead, as Marco A. suggests.
You declare your GenerateRandNum class as:
template <class T>
class GenerateRandNum {
public:
T* data;
size_t size;
GenerateRandNum(T* list, size_t length) {
data = list;
size = length;
}
void generate();
};
Note:
I have removed the template parameter for the size type: size_t is guaranteed to be suitable for counting anything in memory, so there is absolutely no point in using anything different. Templating this parameter only obfuscates your code.

Answer 2

There are some problems with your approach:
The first array template parameter can't have its dimension deduced from the argument as n.m. noted, you would need to specify it explicitly:
GenerateRandNum<int[20], int>
There no point in doing
data = list
since in your code sample these are two arrays and you can't assign them directly. You can either copy the memory or specialize your routines/template
You should really consider using a vector of integers, e.g.
template <class T, class B>
class GenerateRandNum
{
public:
T data;
B size;
GenerateRandNum(T list, B length) {
data = list;
size = length;
}
void generate();
};
template<class T, class B>
void GenerateRandNum<T, B> ::generate()
{
srand((unsigned int)time(NULL)); // You should initialize with a seed
for (B i = 0; i < size; i++) {
data[i] = 1 + rand() % size; // I believe you wanted data[i] and not data[0]
}
}
int main(){
//declare small array
const int smallSize = 20;
std::vector<int> smallArray(smallSize);
// call helper function to put random data in small array
GenerateRandNum <std::vector<int>, int> genData(smallArray, smallSize);
genData.generate();
}
Example
I fixed two issues in the code above, take a look at the comments.