The question
I am writing a software in c++17 for which performances are absolutely critical. I would like available in a few key functions constants in arrays themselves in arrays. It matters that both these array are accessible by a integer value in such (or similar) manner :int main()
{
for (int i = 0; i < size_of_A ; i++)
{
for (int j = 0; j < size_of_B_in_A(i); j++)
{
std::cout << A[i][j];
}
}
}
This would be the kind of array we would like to create assuming some function int f(a, b)
A
{
// B1
{
f(1, 1),
f(1, 2),
f(1, 3),
...
f(1, large number)
},
// B2
{
f(2, 1),
...
f(2, some other large number)
},
... etc
}
The Twist
Each inner array may be of different size which we have will stored elsewhere, we have to find the size at compile time. I would rather not use std::vector for they are assumed slightly slower . Also an I suppose a std::vector would be stored on the heap which would be a performance issue in my specific case. Furthermore, std::vector cannot be used as "inline constexpr" which would be necessary as I expect to have a large amount of value in those array never going to change. I am fine with recompiling all those values each time but not keeping them in an external file by policy as I am to follow a strict coding style.What I Have Tried
brace initializer
// A.hh
#pragma once
#include <iostream>
void test1();
void test2();
inline constexpr int B1[1] = {1};
inline constexpr int B2[2] = {2, 3};
inline constexpr int B3[3] = {4, 5, 6};
inline constexpr const int *A[3] = {B1, B2, B3};
// main.cc
#include "A.hh"
int main()
{
std::cout << "values : ";
for (int i = 0; i < 3; i++)
{
for (int j = 0; j <= i; j++)
{
std::cout << A[i][j];
}
}
std::cout << "\n\naddress test : \n";
std::cout << &A << '\n';
test1();
test2();
}
// somewhere.cc
#include "A.hh"
void test1()
{
std::cout << &A << '\n';
}
// elsewhere.cc
#include "A.hh"
void test2()
{
std::cout << &A << '\n';
}
which prints :
./a.out
values : 123456
address test :
0x56180505cd70
0x56180505cd70
0x56180505cd70
Therefore A has not been copied in main.cc, somewhere.cc and elsewhere.cc which is good. I would like to go further and be able to create a huge amount of values.
struct with constexpr
using tips found here , I do this to be able to perform operations during array construction.// B.hh
#pragma once
#include <iostream>
template <int N>
struct X
{
int arr[N];
constexpr X(): arr()
{
for (int i = 0; i < N; i++)
{
arr[i] = i % 3;
}
}
};
inline constexpr auto A = X<500>();
// main.cc
#include "B.hh"
int main()
{
for (int i = 0; i < 500; i++)
{
std::cout << A.arr[i];
}
}
Which unsuspectingly prints out
012012 (etc)...
Finally an array of array
And this where I am stuck#pragma once
#include <iostream>
template <int N>
struct sub_array
{
int arr[N];
constexpr sub_array() : arr()
{
for (int i = 0; i < N; i++)
{
arr[i] = i;
}
}
};
struct array
{
sub_array</*what here ?*/> arr[100];
constexpr array() : arr()
{
for (int i = 0; i < 100; i++)
{
int size = i * 2; // a very large number
// the value of 'size' is not usable in a constant expression
//
// I see why it is, but I can't think of any other way
arr[i] = sub_array<size>;
}
}
};
inline constexpr array A = array();
How can I build such kind of array ?
Thank you for your time and consideration.
