Default Arguments for Class Templates
Class templates can have default arguments for type or value parameters. Specify default arguments with the equal (=) sign followed by the type name or value. For multiple template arguments, all arguments after the first default argument must have default arguments. When declaring a template class object with default arguments, omit the arguments to accept the default argument. If there are no nondefault arguments, do not omit the empty angle brackets.
A template that is multiply declared cannot specify a default argument more than once. The following code demonstrates an error:
template <class T = long> class A;
template <class T = long> class A { /* . . . */ }; // Generates C4348.
Example
In the following example, an array class template is defined with a default type int for the array element and a default value for the template parameter specifying the size.
// template_default_arg.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;
template <class T = int, int size = 10> class Array
{
T* array;
public:
Array()
{
array = new T[size];
memset(array, 0, size * sizeof(T));
}
T& operator[](int i)
{
return *(array + i);
}
const int Length() { return size; }
void print()
{
for (int i = 0; i < size; i++)
{
cout << (*this)[i] << " ";
}
cout << endl;
}
};
int main()
{
// Explicitly specify the template arguments:
Array<char, 26> ac;
for (int i = 0; i < ac.Length(); i++)
{
ac[i] = 'A' + i;
}
ac.print();
// Accept the default template arguments:
Array<> a; // You must include the angle brackets.
for (int i = 0; i < a.Length(); i++)
{
a[i] = i*10;
}
a.print();
}
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