Is it possible to create an STL-like container, or even just an STL-style iterator, for an existing array of POD-type elements?
For example, suppose I have an array of ints. It would be convenient to be able to call some of the STL functions, such as find_if, count_if, or sort directly on this array.
Non-solution: copying the entire array, or even just references to the elements. The goal is to be very memory- and time-saving while hopefully allowing use of other STL algorithms.
You can call many of the STL algorithms directly on a regular C style array - they were designed for this to work. e.g.,:
int ary[100] = ...;
// init ...
// note: technically, std::begin(ary) can just be ary in this context
std::sort(std::begin(ary), std::end(ary)); // sort the array
std::find(std::begin(ary), std::end(ary), pred); // find some element
// or since C++20:
std::rangs::sort(ary);
std::ranges::find(ary, pred);
I think you'll find that most stuff works just as you would expect.
You can use an inline function template so that you don't have to duplicate the array index
template <typename T, int I>
inline T * array_begin(T (&t)[I])
{
return t;
}
template <typename T, int I>
inline T * array_end(T (&t)[I])
{
return t + I;
}
void foo ()
{
int array[100];
// any standard
std::find(array_begin(array), array_end(array), 10);
// or, since C+11
std::find(std::begin(array), std::end(array), 10);
// or, since C++20
std::ranges::find(array, 10);
}
All the STL algorithms use iterators.
A pointer is a valid iterator into an array of objects.
N.B.The end iterator must be one element past the end of the array. Hence the data+5 in the following code.
#include <algorithm>
#include <iostream>
#include <iterator>
int main()
{
int data[] = {4,3,7,5,8};
std::sort(data,data+5);
std::copy(data,data+5,std::ostream_iterator<int>(std::cout,"\t"));
}
You can use Boost.Array to create a C++ array type with STL semantics.
using arrays:
int a[100];
for (int i = 0; i < 100; ++i)
a[i] = 0;
using boost.arrays:
boost::array<int,100> a;
for (boost::array<int,100>::iterator i = a.begin(); i != a.end(); ++i)
*i = 0;
Update: With C++11, you can now use std::array.
A pointer is a valid model of an iterator:
struct Bob
{ int val; };
bool operator<(const Bob& lhs, const Bob& rhs)
{ return lhs.val < rhs.val; }
// let's do a reverse sort
bool pred(const Bob& lhs, const Bob& rhs)
{ return lhs.val > rhs.val; }
bool isBobNumberTwo(const Bob& bob) { return bob.val == 2; }
int main()
{
Bob bobs[4]; // ok, so we have 4 bobs!
const size_t size = sizeof(bobs)/sizeof(Bob);
bobs[0].val = 1; bobs[1].val = 4; bobs[2].val = 2; bobs[3].val = 3;
// sort using std::less<Bob> wich uses operator <
std::sort(bobs, bobs + size);
std::cout << bobs[0].val << std::endl;
std::cout << bobs[1].val << std::endl;
std::cout << bobs[2].val << std::endl;
std::cout << bobs[3].val << std::endl;
// sort using pred
std::sort(bobs, bobs + size, pred);
std::cout << bobs[0].val << std::endl;
std::cout << bobs[1].val << std::endl;
std::cout << bobs[2].val << std::endl;
std::cout << bobs[3].val << std::endl;
//Let's find Bob number 2
Bob* bob = std::find_if(bobs, bobs + size, isBobNumberTwo);
if (bob->val == 2)
std::cout << "Ok, found the right one!\n";
else
std::cout << "Whoops!\n";
return 0;
}
