如何制作引用局部变量的函数指针？

Question

I'm trying to add all items in one list to another using the first list's forEach method that takes a function pointer and calls the function on each element, but I'm having trouble creating a function pointer that can call the second list's member functions. I've tried using lambdas:

LinearLinkedList<bar> test;
//<add items to test>
LinearLinkedList<bar> sorted;
auto addToSorted = [&](bar * b) { sorted.addSorted(b, &barLessThan); };
test.forEach(&addToSorted);

and using an inner struct :

LinearLinkedList<bar> test;
//<add items to test>
LinearLinkedList<bar> sorted;
struct Sorter {
    static LinearLinkedList<bar> * list = &sorted;
    static void addToSorted(bar * b) { list->addSorted(b, &barLessThan); }
};
test.forEach(&Sorter::addToSorted);

but both are rejected by the compiler because they reference sorted in an illegal way. How can I reference sorted in a call to forEach ?

Answer 1

Lambdas can decay to function pointers only if they don't capture anything. Consider the following snippet:

#include <iostream>

void run(int(*f)())
{
    std::cout << f();
}

struct S {
    static int something;
    static int nothing()
    {
        return something;
    }
};
int S::something = 0;

int main()
{
    auto simpleLambda = [](){ return 1; };
    run(simpleLambda);

    // auto capturingLambda = [&](){ return 1; };
    // run(capturingLambda);

    S::something = 2;
    run(S::nothing);
}

The commented out section won't compile because the capturingLambda has a non-empty capture list. You can use the workaround I put in the code, or better yet make your forEach function templated on the functor type as all of the STL algorithms do.