Question

I am aware this is bad form and that default-values should be specified in the declaration, but if you would please indulge me for a moment.. why does this compile? and what is happening exactly?

#include <iostream>
using namespace std;

class test
{
public:
    test(int n);
};

test::test(int n = 666)
{
    cout << n;
}

int main()
{
    test t;

    cin.sync();
    cin.ignore();

    return 0;
}

Output: 666

.. how do templates affect the same piece of code?

template <class T>
class test
{
public:
    test(int n);
};

template <class T>
test<T>::test(int n = 666)
{
    cout << n;
}

int main()
{
    test<int> t;

    cin.sync();
    cin.ignore();

    return 0;
}

Error: no appropriate default constructor available

Thank you for your time!

Answer 1

It looks like the C++ specification specifically allows the first case and disallows the second!

Quote from the C++ spec (§8.3.6/4):

For non-template functions, default arguments can be added in later declarations of a function in the same scope.

So it looks like for non-template functions, you can indeed introduce the default arguments later on. No idea why this doesn’t work for templates, though!