将基于 Object 的 Lambda 转换为 Function 指针

Question

say I have this program situation overhere:

struct A
{
    int(*func_ptr)();
};

struct B
{
    int b = 0;
    void SetFunctionPointerToLambda(A& a)
    {
        a.func_ptr = [=] () { return b; };
    }
};

int main()
{
    A a;
    B b;
    b.SetFunctionPointerToLambda(a);
}

I want to set a function pointer of an object A to a lambda, which is based of the values of an object B. So, how is this going to be done?

Current error message:

loeschen5.cc: In member function 'void B::SetFunctionPointerToLambda(A&)':
loeschen5.cc:14:41: error: cannot convert 'B::SetFunctionPointerToLambda(A&)::<lambda()>' to 'int (*)()' in assignment
         a.func_ptr = [=] () { return b; };
                                         ^

Thanks for any help!

Answer 1

Ok, so I solved this with a functional object:

So not only the function, also the necesarry variables are stored:

struct FunctionObject
{
    const int *b;
    int operator ()() const
    {
        return *b;
    }
};

struct A
{
    FunctionObject func_ptr;
};

struct B
{
    int b = 56;
    void SetFunctionPointerToLambda(A& a) const
    {
        a.func_ptr = FunctionObject { &b };
    }
};

int main()
{
    A a;
    B b;
    b.SetFunctionPointerToLambda(a);
}

And it works fine.