Template error

[JBIRD304]

Hey guys I am getting an error I don't understand when I compile this code. This what I get in my error window. Any suggestions? Thanks in advance.

1>------ Build started: Project: Assign 20, Configuration: Debug Win32 ------
1> Assgn20-1.cpp
1>c:\users\j-bird\documents\visual studio 2010\projects\assign 20\assign 20\assgn20-1.cpp(25): error C2664: 'std::basic_string<_Elem,_Traits,_Ax>::basic_string(const std::basic_string<_Elem,_Traits,_Ax> &)' : cannot convert parameter 1 from 'const int' to 'const std::basic_string<_Elem,_Traits,_Ax> &'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> Reason: cannot convert from 'const int' to 'const std::basic_string<_Elem,_Traits,_Ax>'
1> with
1> [
1> _Elem=char,
1> _Traits=std::char_traits<char>,
1> _Ax=std::allocator<char>
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
1> c:\users\j-bird\documents\visual studio 2010\projects\assign 20\assign 20\assgn20-1.cpp(25) : while compiling class template member function 'std::string Array<T>::IndexOutOfBounds::getSubscript(void) const'
1> with
1> [
1> T=std::string
1> ]
1> c:\users\j-bird\documents\visual studio 2010\projects\assign 20\assign 20\assgn20-1.cpp(50) : see reference to class template instantiation 'Array<T>::IndexOutOfBounds' being compiled
1> with
1> [
1> T=std::string
1> ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

This is my code.

 #include <iostream>
#include <string>
using namespace std;

template <class T>
class Array
{
private:
    T * aptr;                     // pointer to array
    int arySize;					// size of array 
public:
    class IndexOutOfBounds			// exception class
    { 
    private:
        int subscript;				// bad subscript    
    public:
        IndexOutOfBounds(int s) : subscript(s)
        { }
        T getSubscript() const    	// accessor
        { return subscript; }
    };
    Array(int);                  // ctor
    Array(const Array & ary);    // copy ctor
    // overloaded assignment operator needed
    ~Array();                    // dtor
    int size() const             // returns array size
    { return arySize; }    
    // parameter is a subscrip; operator[]() returns a reference 
    // to element indexed by the subscript
    T & operator[](int sub);  // overloaded [] operator
};

int main()
{
    string boo = "boo";
    string hoo = "hoo";
    Array<string> str(3);	// Array object ary can hold three string elements
	  try {  	// store strings in Array and attempt to store at invalid index
       str[0] = boo;
		for (int num = 1; num <= 3; num++) {
			cout << "Storing \"boo\" + hoo at index " << num << endl;
			str[num] = str[num-1] + hoo;
		} // endfor
	  } // endtry
	  catch(Array<string>::IndexOutOfBounds e)
	  {
		   cout << " ** Index " << e.getSubscript() << " is invalid\n\n";
	  } // endcatch*/
    
	  Array<int> ary(10);		// Array object ary can hold ten elements
	  try {     // store values in Array and attempt to store at invalid index
		   for (int num = 0; num <= 10; num++) {
          ary[num] = 1 + num;
			cout << "Stored " << ary[num] << " at index " << num << endl;
		   } // endfor
	  } // endtry
	  catch(Array<int>::IndexOutOfBounds e)
	  {
		   cout << " ** Index " << e.getSubscript() << " is invalid\n\n";
	  } // endcatch
    
    Array<double> aryD(8);	// Array object ary can hold eight elements
	  try {
        // store values in Array and attempt to store at invalid index
		   for (int num = 0; num <= 8; num++) {
          aryD[num] = 1.1 + num;
			cout << "Stored " << aryD[num] << " at index " << num << endl;
		   } // endfor
	  } // endtry
	  catch(Array<double>::IndexOutOfBounds e)
	  {
		   cout << " ** Index " << e.getSubscript() << " is invalid\n\n";
	  } // endcatch
    
	  return 0;
}
   
// function implementation for the Array class

template <class T>
Array<T>::Array(int s) : arySize(s)        // ctor
{
    aptr = new int[s];     // allocate space for elements
    for (int k = 0; k < arySize; k++)
        *(aptr + k) = 0;    // initialize elements to 0
}

template <class T>
Array<T>::Array(const Array & obj)    // copy ctor
{
    arySize = obj.arySize;      // set arySize
    aptr = new T[arySize];    // allocate space to copy Array
    for(int k = 0; k < arySize; k++)    // copy Array
        *(aptr + k) = *(obj.aptr + k);
}

template <class T>
Array<T>::~Array()
{
    if (arySize > 0)
        delete [] aptr;         // free dynamic memory
}

template <class T>
T & Array<T>::operator[](int sub) 
{
    if (sub < 0 || sub >= arySize)
        throw IndexOutOfBounds(sub);
    return aptr[sub];
}

[TwilightSpectre]

Try either changing line 15 to T subscript; and change line 17 to reflect that, or change line 19 to int getSubscript() const. Probably the latter.

[JBIRD304]

I tried both neither one worked though. Thanks for the suggestion though. Any other suggestions?

[Hippogriff]

NT was right, that is "one" of your problems.
Working on the errors VS shows me.

T getSubscript() const    	// accessor
      { return subscript; } 

Subscript is always an int, no need to try and return T from this function.


aptr = new int[s]; // allocate space for elements
Aptr can be any variable type (in this case a string). You can't use new int here. Use new T[s];

for (int k = 0; k < arySize; k++)
      *(aptr + k) = 0;    // initialize elements to 0 

This is a harder one...From what I can see it will complain "operator = is ambiguous" because it can't see if it is working on a string or a char*. Changing it to *(char*)(aptr + k) = 0; seems to fix the error but other people could tell you better than me.

[Daleth]

template <class T>
Array<T>::Array(int s) : arySize(s)        // ctor
{
    aptr = new int[s];     // allocate space for elements
    for (int k = 0; k < arySize; k++)
        *(aptr + k) = 0;    // initialize elements to 0
}

James2250 wrote:

This is a harder one...From what I can see it will complain "operator = is ambiguous" because it can't see if it is working on a string or a char*. Changing it to *(char*)(aptr + k) = 0; seems to fix the error but other people could tell you better than me.

The problem is not because the function does not know if a std::string or a char* is being worked on. In fact, prior to instantiation, std::string and char* have no meaning with respect to T. The caveat with writing template definitions is that there is no implicit conversion. Everything must be explicit.

template <class T>
Array<T>::Array(int s) : arySize(s)        // ctor
{
    aptr = new T[s];     // Do not assume the type to be int
    for (int k = 0; k < arySize; k++)
        *(aptr + k) = T();    // Default construct an object of type T
                      //Avoid using an explicit value if you can (even 0). This is because
                      //    you don't know if type T can be constructed from such a value.
}

*(char*)(aptr + k) = 0;
Casting like this is dangerous, especially considering the main function uses instantiations with std::string and double. For future reference:
static_cast<yourType>(variable) will help the compiler spit out errors when you are attempting to make incorrect casts (such as attempting to cast std::string* to char*).

[JBIRD304]

OK I see the logic behind that. I changed it and its trying to run, but now it's crashing saying I have an Invalid NULL pointer. It works if I comment out the first try catch in main(). My teacher even had this problem with his demo today in class. Thanks for the tips guys!

[TwilightSpectre]

Try changing your for loop to num < 3 rather than num <= 3, otherwise you will go out of bounds.