c++, perform function on every array member

Question

i have a question regaring c++ and arrays.

let say i have a class which is called CustomArray, and is nothing more than an generic array wich has attributes for size and capacity, to make the array dynamic. defined as:

template<typename T>
class CustomArray
{
public:
    int capacity, size;
    T* items;
    //constructor

    //destructor

    //function1

    //function2

    //etc...
};

now i'm kinda stuck, i want to implement a function like:"

void performOnAllItems(/*function?*/)
{
    for(int i = 0; i < size; i++)
    {
        //perform function on element
    }
}

that takes another function as a parameter (if that is possible?) and performs it on all elements. is that possible? and if so... how?

thanks in advance.

Do you really want "c" in the tags? The answers for C++ and C are going to be very different. — David Thornley, Nov 16 '11 at 21:25
C doesn't have templates. The OP is clearly asking for a C++ answer. — Robᵩ, Nov 16 '11 at 21:32
Prefer to use `std::vector` as it already supports `begin(), end()` and *iterators*. — Thomas Matthews, Nov 16 '11 at 21:42

moshbear · Accepted Answer · 2012-05-13T03:17:20.850

9

Add members begin and end like so:

T *begin() { return items; }

T *end() { return items + size; }

Create a functor that derives from std::unary_function.

E.g.

template <typename T>
class MyFunc : std::unary_function<T, void> {
public:
    void operator()(T& t) {
    // ...
    }
};

Then call std::foreach(foo.begin(), foo.end(), MyFunc);

Update

In C++11, you can use a lambda for the foreach:

std::foreach(foo.begin(), foo.end(),
              [/* (1) */](T& t) { /* ... */ }
            );

If (1) isn't empty, then the lambda is a closure; this is known as a capturing lambda, and Visual C++ 10 Lambda Expressions provides a nice example of this.

edited May 13 '12 at 03:17

answered Nov 16 '11 at 21:18

moshbear

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I think his question is what the parameter type of the function should be – Mooing Duck Nov 16 '11 at 21:33
Edited to add std::unary_function example – moshbear Nov 16 '11 at 21:43

score 6 · Answer 2 · answered Nov 16 '11 at 21:22

6

template<class functionptr>
void performOnAllItems(functionptr ptr)
{
    for(int i = 0; i < size; i++)
        ptr(items[i]);
}

or

typedef void (*functionptr)(T&);
void performOnAllItems(functionptr ptr)
{
    for(int i = 0; i < size; i++)
        ptr(items[i]);
}

The second one greatly limits which functions can be used however, the first does not.

answered Nov 16 '11 at 21:22

Mooing Duck

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1

I'll add that, as written, Mooing's first function should also work with functors (see [here](http://stackoverflow.com/questions/356950/c-functors-and-their-uses)) – Vlad Nov 16 '11 at 21:31

Adam Maras · Answer 3 · 2011-11-16T21:34:09.417

1

Sure. You just have to modify performOnAllItems to take a function pointer, which returns void and takes as a parameter either a T or pointer to T (per my solution below) that calls that function pointer for each item in the list.

template <typename T>
void CustomArray::performOnAllItems(void (*action)(T*))
{
    for(int i = 0; i < size; i++)
    {
        action(items + i);
    }
}

edited Nov 16 '11 at 21:34

answered Nov 16 '11 at 21:24

Adam Maras

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Ah, you caught a different error in my code. Let me edit it for you. – Adam Maras Nov 16 '11 at 21:34

c++, perform function on every array member

3 Answers3