CODEWITHSUNDEEP
c++templates
pythonic
pythonic

Reputation: 21615

Pass an array as template type

I need to pass an array as a template type. How can achieve it. For example, I want something like this.

Fifo<array, Q_SIZE> f; // This is a queue of arrays (to avoid false sharing)

What should I put in place of array? Assume I need an array of int. Also note that I don't want std::vector or a pointer to an array. I want the whole basic array, something equivalent of say int array[32].

Upvotes: 9

Views: 28951

Answers (4)

Alex Jasmin
Alex Jasmin

Reputation: 39496

The std::end function in C++ 11 has an overload for array types that nicely demonstrate this.
Here's a possible implementation of it:

template< class T, std::size_t N >
T* end(T(&array)[N]) {
    return array + N;
}

If you need an object that wraps an array, a templated factory function will help creating it:

template< class T, std::size_t N >
struct fifo {
    T(&array)[N];
};

template< class T, std::size_t N >
fifo<T,N> make_fifo(T(&array)[N]) {
    return Fifo<T,N>{ array };
}

int main() {
    int arr[] = { 1,2,3,4,5 };

    // Create object from array using template argument deduction
    auto fifo = make_fifo(arr);
}

Upvotes: 3

pythonic
pythonic

Reputation: 21615

Well I've found a solution. I can wrap the array in a structure, such as below. 

typedef struct
{
 int array[32];
} my_array;

Then I can use it as follows.

Fifo<my_array, Q_SIZE> f;

Upvotes: 3

Lol4t0
Lol4t0

Reputation: 12547

If you want to pass array type when creating queue you can write

template <typename Array>
struct Fifo;

template <typename T, int size>
struct Fifo<T[size]>
{

};

or just 

template <typename Array>
struct Fifo
{
};

and use it like 

int main()
{
    Fifo<int[10]> fa;

}

Then, you should understand, that int[10] is completely different type from int[11], and once you created Fifo<int[10]> you cannot store here arrays of size 11 or 9 anymore.

Upvotes: 3

Robᵩ
Robᵩ

Reputation: 168626

Try this:

Fifo<int[32], Q_SIZE> f;

Like this:

#include <iostream>
template <class T, int N>
struct Fifo {
  T t;
};

int main () {
 const int Q_SIZE  = 32;
 Fifo<int[32],Q_SIZE> f;
 std::cout << sizeof f << "\n";
}

Upvotes: 7

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