Pass code as template parameter

[Edward Jensen]

Hi,

I need to write many instances like the following struct

struct Function191 : public Function {
double operator()(double x) {
return cos(x) - x;
}
friend std::ostream& operator<< (std::ostream& os, const Function191&
f) {
return os << "cos(x) - x";
}
};

Is there a way to generalize this, such that I can write a generic struct,
which takes a parameter which is both interpreted as code for
operator() and as string for operator<<? I was hoping it would be possible
with templates for example:
Func<"cos(x) - x"f1;
Func<"exp(x) - log(x)"f2;

Is this possible?

Best,
Edward

[acehreli]

On Nov 6, 1:37*pm, "Edward Jensen" <[e-mail]:

Hi,

I need to write many instances like the following struct

struct Function191 : public Function {
* * *double operator()(double x) {
* * * * *return cos(x) - x;
* * *}
* * *friend std::ostream& operator<< (std::ostream& os, const Function191&
f) {
* * * * *return os << "cos(x) - x";
* * *}

};

Is there a way to generalize this, such that I can write a generic struct,
which takes a parameter which is both interpreted as code for
operator() and as string for operator<<? I was hoping it would be possible
with templates for example:
Func<"cos(x) - x"f1;
Func<"exp(x) - log(x)"f2;

The preprocessor has two features:

- ## characters concatenate
- # character stringifies

First: macros are known to be messy.

Here is an example

#include <iostream>

struct Function
{};

#define DEFINE_FUNCTION(N,
code) \
struct Function##N : public Function
{ \
double operator()(double x)
{ \
return
(code); \
}
\
friend std::ostream& operator<< (std::ostream& os, const
Function##N&) { \
return os <<
#code; \
}
\
}

DEFINE_FUNCTION(191, x + 1);
DEFINE_FUNCTION(192, x / 2);

int main()
{
Function191 f191;
f191(5);
std::cout << f191 <<'\n';

Function192 f192;
f192(6);
std::cout << f192 << '\n';
}

Ali

[Paavo Helde]

"Edward Jensen" <ed****@jensen.invalidkirjutas:

>
Is there a way to generalize this, such that I can write a generic
struct, which takes a parameter which is both interpreted as code for
operator() and as string for operator<<?

This seems like a task for a preprocessor macro. Introspective support in
C++ is almost non-existant, so one cannot do this kind of things in the
language itself.

hth
Paavo