sorting array of pointers in c++ using lambdas - c++

Question

I am trying to write sorting template function to make it work with custom classes.
My code is:
#include <iostream>
#include <vector>
struct test
{
int value;
test(int a) : value(a){};
void print() { printf("the value is : %d", value); };
};
template <class T>
void bubblesort(T *m_data, size_t size, bool (*cmp)(const T &l, const T &r))
{
for (uint32_t i = 0; i < size; i++)
for (uint32_t j = 1; j < size - i; j++)
if (cmp(m_data[j - 1], m_data[j]))
{
T temp = m_data[j];
m_data[j] = m_data[j - 1];
m_data[j - 1] = temp;
}
}
int main()
{
std::vector<test> arr;
for (size_t i = 0; i < 10; i++)
arr.emplace_back(i);
std::vector<test *> arr1;
for (auto &i : arr)
arr1.emplace_back(&i);
bubblesort<test>(&arr[0], arr.size(), [](const test &l, const test &r) { return l.value < r.value; });
// bubblesort<test*>(&arr1[0], arr1.size(), [](const test *&l, const test *&r) { return l->value < r->value; });
for (auto i : arr)
printf("%d\n", i.value);
}
My question is how do you sort arr1 using the bubblesort function above? What kind of modification do I have to make in my code to be able to do so?
uncommenting the bubblesort line gives error
error: invalid user-defined conversion from 'main()::<lambda(const test*&, const test*&)>' to 'bool (*)(test* const&, test* const&)' [-fpermissive]
[build] 48 | bubblesort<test *>(&arr1[0], arr1.size(), [](const test *&l, const test *&r) { return l->value < r->value; });

Answer 1

Your function has the wrong type; T is test*, so you need test* const& - "reference to const pointer to test" - as the error message says.
(const test*& is "reference to pointer to const test.)

Answer 2

Your solution cannot work with templates...it cannot deduce the parameters type.
Consider modifying your code as follow:
struct test
{
int value;
explicit test(int a) : value(a) {};
void print() { printf("the value is : %d", value); };
};
template <class T, class _cmp>
void bubblesort(T *m_data, size_t size, _cmp cmp)
{
for (uint32_t i = 0; i < size; i++)
for (uint32_t j = 1; j < size - i; j++)
if (cmp(m_data[j - 1], m_data[j]))
{
T temp = m_data[j];
m_data[j] = m_data[j - 1];
m_data[j - 1] = temp;
}
}
int main()
{
std::vector<test> arr;
for (int i = 0; i < 10; i++)
arr.emplace_back(i);
std::vector<test *> arr1;
for (auto i : arr)
arr1.emplace_back(&i);
bubblesort<test>(&arr[0], arr.size(), [](const test &l, const test &r) -> bool { return l.value < r.value; });
bubblesort<test*>(&arr1[0], arr1.size(), [](const test* l, const test* r) -> bool { return l->value < r->value; });
for (auto i : arr)
printf("%d\n", i.value);
}