C++ 中的模板/参考

Question

It's 3:30am and I'm working on a exercise about templates in C++. I don't get what I'm doing wrong in this case, could you please help me understand? (I'm not very good at C++, started 2 weeks ago).

Here is the subject:

[...], you must code the function template foreach . This function allows to skim through an array by calling a function for each element of this array. The function accepts as argument the address of the beginning of the array, a reference on function and the size of the array. The reference on function corresponds to the following prototype: void func(const type& elem); Moreover you must provide the function print that is passed to the function foreach and that displays each elements, one per line, whatever their type.

And here is my code:

ex03.hpp

#ifndef __EX03_H_INCLUDED__
# define __EX03_H_INCLUDED__

#include <iostream>
#include <iomanip>

template<typename type>
void foreach(type tab[0], (*)(print(const type& elem)), type size)
{
  int                   i;

  for (i = 0; i != size; i++)
    print(tab[i]);
}

template<typename type>
void print(const type& elem)
{
  std::cout << elem << std::endl;
}

#endif /* !__EX03_H__ */

and:

main.cpp

#include "ex03.hpp"

int main(void)  
{
  int tab[] = { 11, 3, 89, 42 };
  foreach(tab, print<int>, 4);
  std::string tab2[] = { "j’", "aime", "les", "templates", "!" };
  foreach(tab2, print, 5);
  return 0;
}

I'm only allowed to turn in ex03.hpp, this is only a test main. I would like to understand what I'm doing wrong. Thanks!

Answer 1

The problem is in foreach templated function:

• first parameter can be a pointer to the array or it can just be static array array with undefined size
• second parameter can be a function pointer or a reference to the function (which should accept constant element of same type as array elements by reference, as it will not be modified)
• third parameter should be different type as array size is not necessarilly same type as array element type

Fixed ex03.hpp looks like this:

#ifndef __EX03_H_INCLUDED__
#define __EX03_H_INCLUDED__

#include <iostream>

// option 1
template<typename T1, typename T2>
void foreach(T1* arr, void (*fnc)(const T1&), T2 size) {
    for (T2 i = 0; i < size; i++) {
        (*fnc)(arr[i]);
    }
}

// option 2
template<typename T1, typename T2>
void foreach(T1 arr[], void (&fnc)(const T1&), T2 size) {
    for (T2 i = 0; i < size; i++) {
        (*fnc)(arr[i]);
    }
}

template<typename T1>
void print(const T1& element) {
    std::cout << element << std::endl;
}

#endif /* !__EX03_H__ */

And the main.cpp:

#include "ex03.hpp"

#include <string>

int main() {
    const size_t tab1Size = 4;
    int tab1[tab1Size] = { 11, 3, 89, 42 };

    foreach(tab1, print, tab1Size);

    const size_t tab2Size = 5;
    std::string tab2[tab2Size] = { "j’", "aime", "les", "templates", "!" };
    foreach(tab2, print, tab2Size);

    return 0;
}

But i agree as others said before, you should first learn the basics of c++, so it can be easier to understand more complicated stuff.