How to override ostream insertion for nested class

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This line from the code below does not compile

C++

cout << cfoobar<int>::cgoobar{};

However this line from the code below does compile

C++

cout << cfoobar<int>{};

Mysteriously it also compiles the line below if cfoobar is not templated

C++

cout << cfoobar::cgoobar{};

May I please inquire how to override the ostream insertion operator for the case shown here

import std.core;
using namespace std;

template<typename T>
struct cfoobar
{
	struct cgoobar {};
};
template<typename charType, typename T>
basic_ostream<charType>& operator << (basic_ostream<charType>& ostream, const cfoobar<T>&) { ostream << __FUNCSIG__; return ostream; }

template<typename charType, typename T>
basic_ostream<charType>& operator << (basic_ostream<charType>& ostream, const typename cfoobar<T>::cgoobar&) { ostream << __FUNCSIG__; return ostream; }

int main()
{
	cout << cfoobar<int>{};
	cout << cfoobar<int>::cgoobar{};
}

What I have tried:

Have tried placing override in class

Posted 28-Aug-21 19:02pm

BernardIE5317

Updated 30-Aug-21 1:13am

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armagedescu · Answer 1 · 2021-08-30T01:13:00

This might be not as flexible as would want. Use typedef with explicit type instanciation. It might require some extra work for using it, but it is usually small indeed:

C++

using namespace std;
template<typename T> struct cfoobar
{
	struct cgoobar {};
};

template<typename T, typename charType = char>
basic_ostream<charType>& operator <<
                            (basic_ostream<charType>& os, const cfoobar<T>&)
{
	os << "overload 1:" << __FUNCSIG__; return os;
}

//do it in exact place you need such an overload
typedef cfoobar<int>::cgoobar int_cgoobar;
template<typename charType = char>
basic_ostream<charType>& operator <<
                           (basic_ostream<charType>& os, const int_cgoobar& c)
{
	os << "overload 2:"<< __FUNCSIG__; return os; 
}

int main()
{
	cout << cfoobar<int>{} << endl;
	cout << int_cgoobar{} << endl;
	return 0;
}

Potential problem are expected with the way you are using using ostream which is already defined in iostream. Use a different name, for instance os

C++

template<typename T, typename charType = char>
basic_ostream<charType>& operator << (basic_ostream<charType>& os, const cfoobar<T>&) { os<< __FUNCSIG__; return os; }

template< typename T, typename charType = char>
basic_ostream<charType>& operator << (basic_ostream<charType>& os, const int_cgoobar& c) { os<< __FUNCSIG__; return os; }