templated class can't redefine operator[]

Question

I've this class

namespace baseUtils {

template<typename AT>
class growVector {

     int size;
     AT **arr;
     AT* defaultVal;

    public:

     growVector(int size, AT* defaultVal );   //Expects number of elements (5) and default value (NULL)
     AT*& operator[](unsigned pos);
     int length();
     void reset(int pos); //Resets an element to default value
     void reset();   //Resets all elements to default value
     ~growVector();
};

}

and this is the implementation for operator[]

template<typename AT>
AT*& growVector<AT>::operator [](unsigned pos){
    if (pos >= size){
     int newSize = size*2;
     AT** newArr = new AT*[newSize];
     memcpy(newArr, arr, sizeof(AT)*size);
     for (int i = size; i<newSize; i++)
      newArr[i] = defaultVal;
     size = newSize;
     delete arr;
     arr = newArr;
    }
    return arr[pos];
}

(yes I do realize i don't check if size*2 >= pos... but that's not the point now) if I use it in code like:

int main() {

    growVector<char> gv();
    char* x = NULL;
    for (int i = 0; i< 50; i++){
     gv[i] = x;
    }
    gv.reset();
    return 0;
}

the compiler says

../src/base.cpp:98: warning: pointer to a function used in arithmetic
../src/base.cpp:98: error: assignment of read-only location ‘*(gv + ((unsigned int)i))’
../src/base.cpp:98: error: cannot convert ‘char*’ to ‘baseUtils::growVector<char>()’ in assignment

referring to the line gv[i] = x; (seems like it doesn't see the redefinition of [])

Why???? What am I missing?

Answer 1

The problem is your declaration

growVector<char> gv();

The compiler interprets this as declaring a function called gv which returns a growVector<char>, not as an object as you indend. Since there isn't a default constructor, this wouldn't compile anyway. Change it to:

growVector<char> gv(0,0);

Answer 2

The compiler thinks this line

growVector<char> gv();

is declaring a function, rather than a variable. Drop the () and things should work.

Answer 3

After correcting the constructor problem I've the linker sayng:

/home/dario/workspace/base/Debug/../src/base.cpp:95: undefined reference to `baseUtils::growVector<char>::growVector(int, char*)'
/home/dario/workspace/base/Debug/../src/base.cpp:98: undefined reference to `baseUtils::growVector<char>::operator[](unsigned int)'
/home/dario/workspace/base/Debug/../src/base.cpp:100: undefined reference to `baseUtils::growVector<char>::reset()'
/home/dario/workspace/base/Debug/../src/base.cpp:101: undefined reference to `baseUtils::growVector<char>::~growVector()'
/home/dario/workspace/base/Debug/../src/base.cpp:101: undefined reference to `baseUtils::growVector<char>::~growVector()'

like it cannot link... why??? :O

Answer 4

I just would like to point out that it is a good practice to have two versions of subscript [] operator in the class: const (which will be used for r-value) and non-const. You have implemented non-const version but it can not be used in const functions or in any function that receive instance of your class as const reference or pointer pointer to const.