Question

While working with some templates and writing myself a basic container class with iterators, I found myself needing to move the body of member functions from a template class into a separate file to conform to style guidelines. However, I’ve run into an interesting compile error:

runtimearray.cpp:17: error: expected
constructor, destructor, or type
conversion before ‘&’ token
runtimearray.cpp:24: error: expected
constructor, destructor, or type
conversion before ‘&’ token
runtimearray.cpp:32: error: expected
constructor, destructor, or type
conversion before ‘&’ token
runtimearray.cpp:39: error: expected
constructor, destructor, or type
conversion before ‘&’ token
runtimearray.cpp:85: error: expected
constructor, destructor, or type
conversion before ‘RuntimeArray’
runtimearray.cpp:91: error: expected
constructor, destructor, or type
conversion before ‘RuntimeArray’

runtimearray.h:

#ifndef RUNTIMEARRAY_H_
#define RUNTIMEARRAY_H_

template<typename T>
class RuntimeArray
{
 public:
  class Iterator
  {
    friend class RuntimeArray;
   public:
    Iterator(const Iterator& other);

    T& operator*();
    Iterator& operator++();
    Iterator& operator++(int);
    Iterator& operator--();
    Iterator& operator--(int);
    bool operator==(Iterator other);
    bool operator!=(Iterator other);

   private:
    Iterator(T* location);

    T* value_;
  };

  RuntimeArray(int size);
  ~RuntimeArray();

  T& operator[](int index);

  Iterator Begin();
  Iterator End();

 private:
  int size_;
  T* contents_;
};

#endif  // RUNTIMEARRAY_H_

runtimearray.cpp:

#include "runtimearray.h"

template<typename T>
RuntimeArray<T>::Iterator::Iterator(const Iterator& other)
    : value_(other.value_)
{
}

template<typename T>
T& RuntimeArray<T>::Iterator::operator*()
{
  return *value_;
}

template<typename T>
RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator++()
{
  ++value_;
  return *this;
}

template<typename T>
RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator++(int)
{
  Iterator old = *this;
  ++value_;
  return old;
}

template<typename T>
RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator--()
{
  --value_;
  return *this;
}

template<typename T>
RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator--(int)
{
  Iterator old = *this;
  --value_;
  return old;
}

template<typename T>
bool RuntimeArray<T>::Iterator::operator==(Iterator other)
{
  return value_ == other.value_;
}

template<typename T>
bool RuntimeArray<T>::Iterator::operator!=(Iterator other)
{
  return value_ != other.value_;
}

template<typename T>
RuntimeArray<T>::Iterator::Iterator(T* location)
    : value_(location)
{
}

template<typename T>
RuntimeArray<T>::RuntimeArray(int size)
    : size_(size),
      contents_(new T[size])
{
}

template<typename T>
RuntimeArray<T>::~RuntimeArray()
{
  if(contents_)
    delete[] contents_;
}

template<typename T>
T& RuntimeArray<T>::operator[](int index)
{
  return contents_[index];
}

template<typename T>
RuntimeArray<T>::Iterator RuntimeArray<T>::Begin()
{
  return Iterator(contents_);
}

template<typename T>
RuntimeArray<T>::Iterator RuntimeArray<T>::End()
{
  return Iterator(contents_ + size_);
}

How can I make these errors go away? The files make sense to me, but alas, it’s the compiler’s say that matters.

Answer 1

I think that you are missing the typename keyword.

e.g.

template<typename T>
RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator++()

should be

template<typename T>
typename RuntimeArray<T>::Iterator& RuntimeArray<T>::Iterator::operator++()

‘Nested’ types which are dependent on a template parameter need the typename keyword to tell the compiler that they should be types where this otherwise would be ambiguous.