c++,function,templates,arguments,parameter-passing , C++ , Member function passed as template argument
C++ , Member function passed as template argument
Question:
Tag: c++,function,templates,arguments,parameter-passing
I would like to pass the function eval2(T c, T &d), which is the member function in the class Algo1
Algo1.h
#ifndef ALGO1_H
#define ALGO1_H
#include "Algo2.h"
template <typename T>
class Algo1
{
private: T a, b;
public:
Algo1() : a(0), b(0) {}
Algo1(T a_, T b_) : a(a_), b(b_) {}
void anal(T &c);
void eval1(T c);
void eval2(T c, T &d);
friend void get(Algo1 &al, T &a, T &b);
};
#endif
as the template argument in anal(T &c) function.
Algo1.hpp
#ifndef ALGO1_HPP
#define ALGO1_HPP
template <typename T>
void Algo1<T>::anal(T &c) {
Algo2<T>::process(eval2<T>, b, c);} //Pass the member function, wrong
template <typename T>
void Algo1<T>::eval1(T c) { a += c; }
template <typename T>
void Algo1<T>::eval2(T c, T &d) { d = a + b + c;}
#endif
In practice, eval2() represents some cost function working with member data. The "destination" class containing method process() looks like
Algo2.h
#ifndef ALGO2_H
#define ALGO2_H
template <typename T>
class Algo2
{
public:
template <typename Function>
static void process(Function f, T &x, T &res);
};
#endif
Algo2.hpp
#ifndef ALGO2_HPP
#define ALGO2_HPP
template <typename T>
template <typename Function>
void Algo2<T>::process(Function f, T &x, T &res) { f(x, res); } //Call passed function as static
#endif
Unfortunately, eval2(T c, T &d) is the member function processing member data, it can not be declared as static. However, outside the class it can not be called without the object. Hence, the function process() is unable to call the passed function as the static function. To fix the problem and extract the data, the friend function get(Algo1 &al, T &a, T &b ) was declared and defined
template <typename T>
inline void get(Algo1 <T> &al, T &a, T &b )
{
a = al.a;
b = a1.b;
}
It was "built" into the non-member function eval3()
template <typename T>
inline void eval3(T c, T &d)
{
Algo1 <T> alg;
T a, b;
get(alg, a, b);
}
and the function anal() was refined to call eval3 instead of eval 2 to the form of
template <typename T>
void Algo1<T>::anal(T &c)
{
Algo2<T>::process(eval3<T>, b, c); //Pass the function OK
}
I am having two questions:
Is there any more comfortable way how to pass a member function while keeping the call it as static?
where to declare and define get() and eval3() functions to avoid the error
Error 1 error LNK2019: unresolved external symbol "void __cdecl get(class Algo1 &,double &,double &)" ([email protected]@[email protected]@@[email protected]) referenced in function "void __cdecl eval3(double,double &)"
Thank you very much for your help.
_____________Comment________________
The second point has been solved. Instead of the declaration
friend void get(Algo1 &al, T &a, T &b);
it needs to be declared like this
template <typename T>
friend void get(Algo1 &al, T &a, T &b);
Answer:
- Is there any more comfortable way how to pass a member function while keeping the call it as static?
With C++11 and lambda, you may do
template <typename T>
void Algo1<T>::anal(T &c) {
Algo2<T>::process([=](T a1, T& a2) { return this->eval2(a1, a2); }, this->b, c);
}
Before, you have to create your functor manually, something like:
template<typename T>
class MyFunctor
{
public:
explicit(Algo1<T>* algo1) : algo1(algo1) {}
void operator () (T a1, T& a2) const
{
algo1->eval2(a1, a2);
}
private:
Algo1<T>* algo1;
};
And then:
template <typename T>
void Algo1<T>::anal(T &c) {
Algo2<T>::process(MyFunctor<T>(this), this->b, c);
}
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