operator<< overload for inner enum of a template

[Doctor Smith]

Hello!

I would like to have an output streaming operator for an enum that is
inside a templated class, but I just can't seem to make it work! I
suspect that I have a template ambiguity issue, but I haven't pinned
down what change will make the code work.

The following example illustrates the problem. I would like the
program to print 'C', but it prints '2'.

#include <iostream>

template <typename AX_FLOAT>
class M
{
public:
enum T
{
A,
B,
C
};
};

template <typename AX_FLOAT>
std:stream& operator<<(std:stream& os, typename M<AX_FLOAT>::T t)
{
switch (t)
{
case M<AX_FLOAT>::A: os << 'A'; break;
case M<AX_FLOAT>::B: os << 'B'; break;
case M<AX_FLOAT>::C: os << 'C'; break;
}

return os;
}

int main()
{
M<float>::T t = M<float>::C;

std::cout << "t = " << t << std::endl;
}

Any insights on how to make this work, or why it will never work, is
muchly appreciated.

 

[Ian Collins]

Hello!

I would like to have an output streaming operator for an enum that is
inside a templated class, but I just can't seem to make it work! I
suspect that I have a template ambiguity issue, but I haven't pinned
down what change will make the code work.

The following example illustrates the problem. I would like the
program to print 'C', but it prints '2'.

Try:

template <typename AX_FLOAT>
class M
{
public:
enum T
{
A,
B,
C
};

friend std:stream& operator<<(std:stream& os, T t)
{
switch (t)
{
case M<AX_FLOAT>::A: os << 'A'; break;
case M<AX_FLOAT>::B: os << 'B'; break;
case M<AX_FLOAT>::C: os << 'C'; break;
}

return os;
}
};

 

[Doctor Smith]

Thanks! That little change fixed the problem neatly in my test
program. Presumably it will work equally well in our project code.

Though, it would be interesting to get the technical explanation as to
why the first works and the second doesn't.

 

[Jerry Coffin]

Hello!

I would like to have an output streaming operator for an enum that is
inside a templated class, but I just can't seem to make it work! I
suspect that I have a template ambiguity issue, but I haven't pinned
down what change will make the code work.

The following example illustrates the problem. I would like the
program to print 'C', but it prints '2'.

One possibility would be like this:

#include <iostream>

template <typename F>
struct M {
enum Ts {A, B, C};

class T {
typename M::Ts t;
public:
T(typename M::Ts val) : t(val) {}

friend std:stream &operator<<(
std:stream &os,
typename M<F>::T const &m)
{
static char const names[] = "ABC";
return os << names[m.t];
}
};
};

int main()
{
M<float>::T t = M<float>::C;

std::cout << "t = " << t << std::endl;
}