使用 lambdas 对 c++ 中的指针数组进行排序

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);
}