模板和函数对象 - c ++

Question

i have a problem with this class.
the goal is to make the main function work properly. we were supposed to implement the "And" function object so that the code will work. i can't find what is the problem with our solution.
(the solution start and end are marked in comments in the code before the "main" function)
can you please help?
thanks

#include <iostream>
#include <algorithm>

using namespace std;

class NotNull
{
    public:
    bool operator()(const char* str) {return str != NULL;}
};

class BeginsWith
{
    char c;
    public:
    BeginsWith(char c) : c(c) {}
    bool operator()(const char* str) {return str[0] == c;}
};

class DividesBy {
    int mod;
    public:
    DividesBy(int mod) : mod(mod) {}
    bool operator()(int n) {return n%mod == 0;}
};

//***** This is where my sulotion starts ******

template <typename Function1, typename Function2, typename T>
class AndFunction
{
    Function1 f1;
    Function2 f2;
    public:
    AndFunction(Function1 g1, Function2 g2) : f1(g1), f2(g2) {}
    bool operator()(T t)
    {
        return (f1(t) && f2(t));
    }
};

template <typename Function1, typename Function2, typename T>
AndFunction <Function1, Function2, T>

bool And(Function1 f1, Function2 f2)
{
    return AndFunction<Function1, Function2, T>(f1, f2);
}

//***** This is where my sulotion ends ******

int main(int argc, char** argv)
{
    int array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    char* strings[4] = {"aba", NULL, "air", "boom"};
    cout << count_if(array,array+10,And(DividesBy(2),DividesBy(4))) << endl;
    // prints 2, since 4 and 8 are the only numbers which can be divided by
    // both 2 and 4.
    cout << count_if(strings,strings+4,And(NotNull(),BeginsWith('a'))) <<endl;
    // prints 2, since only "aba" and "air" are both not NULL and begin
    // with the character 'a'.
    return 0;
}

Answer 1

Obviously, you don't know the T parameter when creating your functor. Did you consider delaying the introduction of T to the actual call (ie. making operator() a member-template)?

Answer 2

your not calling your overloaded () operator when you create the object here: return AndFunction<Function1, Function2, T>(f1, f2); (you need a () before the ; ) this code shouldn't even compile, actually, as currently it returns an object, not a bool.

---

EDIT: As pointed out, the function ( bool And(Function1 f1, Function2 f2) ) must not return bool but rather a functional object for count_if to call via the overloaded () operator

Answer 3

Technically speaking you should be using the unary_function and binary_function classes as parents if you'd like them to place nice with STL algorithms. Here:

template<typename Func1, typename Func2,typename T>
struct AndFunction : public unary_function<T,bool>{
    AndFunction(Func1 _func1, Func2 _func2) 
        : _myFunc1(_func1),
        _myFunc2(_func2){}

    bool operator()(T _t){
        return _myFunc1(_t) && _myFunc2(_2);
    }

private:
    Func1 _myFunc1;
    Func2 _myFunc2;
};

In your case you need to do

template<typename Func1, typename Func2, typename T> 
AndFunction<Func1, Func2, T> And(Func1 _func1, Func2 _func2){
    return AndFunction<Func1,Func2,T>(_func1,_func2);
};

so that you don't confuse the operator with object creation and that you specify how you are to receive the function instructions.

On the flip side, the way your main works I think you really just want

struct And : public binary_function<bool, bool, bool>{
    bool operator()(bool _1, bool _2){
        return _1 && _2;
    }
};

Hope that helps.

Answer 4

The template parameter T can't be inferred, it must be specified explicitly:

template <typename T, typename Function1, typename Function2>
AndFunction <Function1, Function2, T>
And(Function1 f1, Function2 f2)
{
    return AndFunction<Function1, Function2, T>(f1, f2);
}

//***** This is where my sulotion ends ******

int main(int argc, char** argv)
{
    int array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    char* strings[4] = {"aba", NULL, "air", "boom"};
    cout << count_if(array,array+10,And<int>(DividesBy(2),DividesBy(4))) << endl;
    // prints 2, since 4 and 8 are the only numbers which can be divided by
    // both 2 and 4.
    cout << count_if(strings,strings+4,And<const char*>(NotNull(),BeginsWith('a'))) <<endl;
    // prints 2, since only "aba" and "air" are both not NULL and begin
    // with the character 'a'.
    return 0;
}

jpalecek's solution is better and works as follows:

//***** This is where my sulotion starts ******

template <typename Function1, typename Function2>
class AndFunction
{
    Function1 f1;
    Function2 f2;
    public:
    AndFunction(Function1 g1, Function2 g2) : f1(g1), f2(g2) {}
  template<typename T> bool operator()(T)
    {
        return (f1(t) && f2(t));
    }
};

template <typename Function1, typename Function2>
AndFunction <Function1, Function2>
And(Function1 f1, Function2 f2)
{
    return AndFunction<Function1, Function2>(f1, f2);
}

//***** This is where my sulotion ends ******