Class object as template funciton argument - c++

Question

I have an issue with the templates and my knowledge about them is definitely limited.
So I have a class which should store some information:
class Q
{
int integer;
int fractional;
public:
constexpr Q(int i,int f) : integer(i),fractional(f) {}
int get_i() const {return this->integer;}
int get_f() const {return this->fractional;}
constexpr int get_w() {return this->integer + this->fractional;}
friend ostream& operator<<(ostream& os, const Q& q){ os << "Q" << q.integer << "." << q.fractional << " (w:" << q.integer + q.fractional << ")"; return os; }
};
Then I have my templated function. This is just an example but it show the point:
template <Q input_q, Q output_q,const unsigned int X_0_evaluated_bit> void calculate_stuff (const int max_iterations)
{
std::array<Q,input_q.get_w()> input_queue_q;
}
And at the end the main (I'm using the SystemC library) where I generate the constant object Q that I want to use in the function
int sc_main(int argc, char *argv[])
{
constexpr Q Q1_i = Q(1,10);
constexpr Q Q1_o = Q(0,11);
// Number of bits used to address the LUT for the initial value
const unsigned int X_0_evaluated_bit = 5;
// Number of iteration for the Newton-Raphson
const int max_iterations = 2;
calculate_stuff <Q1_i,Q1_o,X_0_evaluated_bit> (max_iterations);
return 0;
}
If I try to compile I get the following error message:
check_ac_one_over.cpp:31:13: error: ‘class Q’ is not a valid type for a template non-type parameter
template <Q input_q, Q output_q,const unsigned int X_0_evaluated_bit> void calculate_stuff (const int max_iterations)
^
check_ac_one_over.cpp:31:24: error: ‘class Q’ is not a valid type for a template non-type parameter
template <Q input_q, Q output_q,const unsigned int X_0_evaluated_bit> void calculate_stuff (const int max_iterations)
^
check_ac_one_over.cpp: In function ‘void calculate_stuff(int)’:
check_ac_one_over.cpp:33:31: error: template argument 2 is invalid
std::array<Q,input_q.get_w()> input_queue_q;
^
check_ac_one_over.cpp:33:46: error: invalid type in declaration before ‘;’ token
std::array<Q,input_q.get_w()> input_queue_q;
^
check_ac_one_over.cpp: In function ‘int sc_main(int, char**)’:
check_ac_one_over.cpp:102:64: error: no matching function for call to ‘calculate_stuff(const int&)’
calculate_stuff <Q1_i,Q1_o,X_0_evaluated_bit> (max_iterations);
^
check_ac_one_over.cpp:102:64: note: candidate is:
check_ac_one_over.cpp:31:76: note: template<<typeprefixerror>input_q, <typeprefixerror>output_q, unsigned int X_0_evaluated_bit> void calculate_stuff(int)
template <Q input_q, Q output_q,const unsigned int X_0_evaluated_bit> void calculate_stuff (const int max_iterations)
^
check_ac_one_over.cpp:31:76: note: template argument deduction/substitution failed:
check_ac_one_over.cpp:102:64: note: invalid template non-type parameter
calculate_stuff <Q1_i,Q1_o,X_0_evaluated_bit> (max_iterations);
^
check_ac_one_over.cpp:102:64: note: invalid template non-type parameter
make: *** [check_ac_one_over.o] Error 1
Now I'm not sure if what I'm trying to do is possible. Does anyone have some ideas how can I make it work ?
Cheers,
Stefano

Answer 1

Does anyone have some ideas how can I make it work ?
Not a great idea, I suppose, but...
The error message is clear:
‘class Q’ is not a valid type for a template non-type parameter
But, in your example, you don't use the full Q object: you use the value of input_q.get_w().
So I suppose that you can pass as template parameter not the full Q object but only the value returned by get_w() that is a int so is a valid as template non-type parameter.
Something (using only the first template parameter; no idea about the use of the other)
template <int input_dim>
void calculate_stuff (const int max_iterations)
{
std::array<Q, input_dim> input_queue_q;
}
that you can call (taking in count that Q1_i and get_w() are constexpr)
calculate_stuff<Q1_i.get_w()> (1);
But observe that
1) get_w() should be also const, not only constexpr
constexpr int get_w() const {return this->integer + this->fractional;}
because a constexpr method isn't automatically const (starting from C++14) but a constexpr object is also const (so can't use get_w() if it isn't defined const)
2) If you want an array of Q as follows
std::array<Q, input_dim> input_queue_q;
the Q type require a constructor without parameters; by example adding default values to your constructor
constexpr Q(int i = 0, int f = 0) : integer(i),fractional(f) {}

Answer 2

I've found a solution. It's not the perfect one but it's quite close. Instead to use the object I'm using the pointer to the object which has become a global variable:
class Q
{
int integer;
int fractional;
public:
constexpr Q(int i = 0,int f = 0) : integer(i),fractional(f) {}
constexpr int get_i() const {return this->integer;}
constexpr int get_f() const {return this->fractional;}
constexpr int get_w() const {return this->integer + this->fractional;}
friend ostream& operator<<(ostream& os, const Q& q){ os << "Q" << q.integer << "." << q.fractional << " (w:" << q.integer + q.fractional << ")"; return os; }
};
template <const Q *input_q,const Q *output_q,const unsigned int X_0_evaluated_bit> void calculate_stuff (const int max_iterations)
{
std::array<Q,input_q->get_w()> input_queue_q;
}
constexpr Q Q1_i = Q(1,10);
constexpr Q Q1_o = Q(0,11);
int sc_main(int argc, char *argv[])
{
// Number of bits used to address the LUT for the initial value
const unsigned int X_0_evaluated_bit = 5;
// Number of iteration for the Newton-Raphson
const int max_iterations = 2;
calculate_stuff <&Q1_i,&Q1_o,X_0_evaluated_bit> (max_iterations);
return 0;
}
Cheers.