Question
What's the syntax / semantics for a "class template"?
Answers
Consider a container class Array that acts like an array of integers:

// This would go into a header file such as "Array.h"
class Array {
public:
Array(int len=10) : len_(len), data_(new int[len]) { }
~Array() { delete[] data_; }
int len() const { return len_; }
const int& operator[](int i) const { return data_[check(i)]; } ← subscript operators often come in pairs
int& operator[](int i) { return data_[check(i)]; } ← subscript operators often come in pairs
Array(const Array&);
Array& operator= (const Array&);
private:
int len_;
int* data_;
int check(int i) const
{ if (i < 0 || i >= len_) throw BoundsViol("Array", i, len_);
return i; }
};

Repeating the above over and over for Array of float, of char, of std::string, of Array-of-std::string, etc, will become tedious.

// This would go into a header file such as "Array.h"
template
class Array {
public:
Array(int len=10) : len_(len), data_(new T[len]) { }
~Array() { delete[] data_; }
int len() const { return len_; }
const T& operator[](int i) const { return data_[check(i)]; }
T& operator[](int i) { return data_[check(i)]; }
Array(const Array&);
Array& operator= (const Array&);
private:
int len_;
T* data_;
int check(int i) const
{ if (i < 0 || i >= len_) throw BoundsViol("Array", i, len_);
return i; }
};

Unlike template functions, template classes (instantiations of class templates) need to be explicit about the parameters over which they are instantiating:

int main()
{
Array ai;
Array af;
Array ac;
Array as;
Array< Array > aai;
...
}

Note the space between the two >'s in the last example. Without this space, the compiler would see a >> (right-shift) token instead of two >'s.   Your Comment




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