What's wrong with this program using template?

[PengYu.UT@gmail.com]

Hi,

I couldn't see what is wrong with the following program. Would you
please help me?

Thanks,
Peng


g++-3.4 -MM -g main.cc .dep
g++-3.4 -g -c -o main.o main.cc
main.cc: In function `int main()':
main.cc:26: error: cannot convert `double' to `function<int(* )()' for
argument `1' to `const interpolator2d< function<int
inter_f(functio n<int(*)())'
make: *** [main.o] Error 1


#include <functional>

template <typename F>
class interpolator2d {
public:
interpolator2d( const F &f) : _f(f) {
}
typename F::result_type operator()(doub le x, double y) const {
return _f(x, y);
}
private:
F _f;
};

template <typename T>
struct function : public std::binary_fun ction<T, T, double{
double operator()(T x, T y) const {
return - (x * x + y * y);
}
};

int main() {
const interpolator2d< function<int inter_f(functio n<int>());
double x = 0;
double y = 0;
inter_f(1.1, 1.1);
}

[Victor Bazarov]

Re: What's wrong with this program using template?

PengYu.UT@gmail .com wrote:

Hi,
>
I couldn't see what is wrong with the following program. Would you
please help me?
>
Thanks,
Peng
>
>
g++-3.4 -MM -g main.cc .dep
g++-3.4 -g -c -o main.o main.cc
main.cc: In function `int main()':
main.cc:26: error: cannot convert `double' to `function<int(* )()'
for argument `1' to `const interpolator2d< function<int
inter_f(functio n<int(*)())'
make: *** [main.o] Error 1
>
>
#include <functional>
>
template <typename F>
class interpolator2d {
public:
interpolator2d( const F &f) : _f(f) {
}
typename F::result_type operator()(doub le x, double y) const {
return _f(x, y);
}
private:
F _f;
};
>
template <typename T>
struct function : public std::binary_fun ction<T, T, double{
double operator()(T x, T y) const {
return - (x * x + y * y);
}
};
>
int main() {
const interpolator2d< function<int inter_f(functio n<int>());

I just wonder how many times people will be posting questions after
walking into this?..

Repeat after me: the statement above is a declaration of a FUNCTION,
not an object as you intended. This is the same as if you wrote

int a(double());

intending to construct an integer from a default-initialised double.
It's a FUNCTION declaration.

To fix it surround each argument (the only argument in your case)
with an extra set of parentheses. Should be

... inter_f((functi on<int>()));

double x = 0;
double y = 0;
inter_f(1.1, 1.1);
}

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Kai-Uwe Bux]

Re: What's wrong with this program using template?

PengYu.UT@gmail .com wrote:

Hi,
>
I couldn't see what is wrong with the following program. Would you
please help me?
>
Thanks,
Peng
>
>
g++-3.4 -MM -g main.cc .dep
g++-3.4 -g -c -o main.o main.cc
main.cc: In function `int main()':
main.cc:26: error: cannot convert `double' to `function<int(* )()' for
argument `1' to `const interpolator2d< function<int
inter_f(functio n<int(*)())'
make: *** [main.o] Error 1
>
>
#include <functional>
>
template <typename F>
class interpolator2d {
public:
interpolator2d( const F &f) : _f(f) {
}
typename F::result_type operator()(doub le x, double y) const {
return _f(x, y);
}
private:
F _f;
};
>
template <typename T>
struct function : public std::binary_fun ction<T, T, double{
double operator()(T x, T y) const {
return - (x * x + y * y);
}
};
>
int main() {
const interpolator2d< function<int inter_f(functio n<int>());

This is parsed as a function declaration. Make that:

const interpolator2d< function<int inter_f((functi on<int>()));

double x = 0;
double y = 0;
inter_f(1.1, 1.1);
}

Best

Kai-Uwe Bux

[PengYu.UT@gmail.com]

Re: What's wrong with this program using template?



On Oct 28, 6:05 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

PengYu...@gmail .com wrote:

Hi,

>

I couldn't see what is wrong with the following program. Would you
please help me?

>

Thanks,
Peng

>

g++-3.4 -MM -g main.cc .dep
g++-3.4 -g -c -o main.o main.cc
main.cc: In function `int main()':
main.cc:26: error: cannot convert `double' to `function<int(* )()'
for argument `1' to `const interpolator2d< function<int
inter_f(functio n<int(*)())'
make: *** [main.o] Error 1

>

#include <functional>

>

template <typename F>
class interpolator2d {
public:
interpolator2d( const F &f) : _f(f) {
}
typename F::result_type operator()(doub le x, double y) const {
return _f(x, y);
}
private:
F _f;
};

>

template <typename T>
struct function : public std::binary_fun ction<T, T, double{
double operator()(T x, T y) const {
return - (x * x + y * y);
}
};

>

int main() {
const interpolator2d< function<int inter_f(functio n<int>());I just wonder how many times people will be posting questions after

walking into this?..
>
Repeat after me: the statement above is a declaration of a FUNCTION,
not an object as you intended. This is the same as if you wrote
>
int a(double());
>
intending to construct an integer from a default-initialised double.
It's a FUNCTION declaration.
>
To fix it surround each argument (the only argument in your case)
with an extra set of parentheses. Should be
>
... inter_f((functi on<int>()));
>

double x = 0;
double y = 0;
inter_f(1.1, 1.1);
}V

main.cc:26: error: cannot convert `double' to `function<int(* )()' for
argument `1' to `const interpolator2d< function<int

I still don't understand the error message. Where is the `double'? Does
`function<int(* )()' mean a function? Can I declare function in
arguments of a class constructor?

Thanks,
Peng

[Victor Bazarov]

Re: What's wrong with this program using template?

PengYu.UT@gmail .com wrote:

On Oct 28, 6:05 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>PengYu...@gmai l.com wrote:

>>Hi,

>>

>>I couldn't see what is wrong with the following program. Would you
>>please help me?

>>

>>Thanks,
>>Peng

>>

>>g++-3.4 -MM -g main.cc .dep
>>g++-3.4 -g -c -o main.o main.cc
>>main.cc: In function `int main()':
>>main.cc:26: error: cannot convert `double' to `function<int(* )()'
>>for argument `1' to `const interpolator2d< function<int
>>inter_f(funct ion<int(*)())'
>>make: *** [main.o] Error 1

>>

>>#include <functional>

>>

>>template <typename F>
>>class interpolator2d {
>> public:
>> interpolator2d( const F &f) : _f(f) {
>> }
>> typename F::result_type operator()(doub le x, double y) const {
>> return _f(x, y);
>> }
>> private:
>> F _f;
>>};

>>

>>template <typename T>
>>struct function : public std::binary_fun ction<T, T, double{
>> double operator()(T x, T y) const {
>> return - (x * x + y * y);
>> }
>>};

>>

>>int main() {
>> const interpolator2d< function<int inter_f(functio n<int>());I
>>just wonder how many times people will be posting questions after
>>walking into this?..

>>
>Repeat after me: the statement above is a declaration of a FUNCTION,
>not an object as you intended. This is the same as if you wrote
>>
> int a(double());
>>
>intending to construct an integer from a default-initialised double.
>It's a FUNCTION declaration.
>>
>To fix it surround each argument (the only argument in your case)
>with an extra set of parentheses. Should be
>>
> ... inter_f((functi on<int>()));
>>

>> double x = 0;
>> double y = 0;
>> inter_f(1.1, 1.1);
>>}V

>
main.cc:26: error: cannot convert `double' to `function<int(* )()'
for argument `1' to `const interpolator2d< function<int
>
I still don't understand the error message. Where is the `double'?

1.1 is the double.

Does `function<int(* )()' mean a function?

Your 'inter_f' *function* takes one argument, a pointer to a function.

Can I declare function in
arguments of a class constructor?

Not sure what you're asking here, sorry. Please rephrase.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[John Carson]

Re: What's wrong with this program using template?

<PengYu.UT@gmai l.comwrote in message
news:1162080786 .785018.105720@ i42g2000cwa.goo glegroups.com

On Oct 28, 6:05 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:

>PengYu...@gmai l.com wrote:

>>Hi,

>>

>>I couldn't see what is wrong with the following program. Would you
>>please help me?

>>

>>Thanks,
>>Peng

>>

>>g++-3.4 -MM -g main.cc .dep
>>g++-3.4 -g -c -o main.o main.cc
>>main.cc: In function `int main()':
>>main.cc:26: error: cannot convert `double' to `function<int(* )()'
>>for argument `1' to `const interpolator2d< function<int
>>inter_f(funct ion<int(*)())'
>>make: *** [main.o] Error 1

>>

>>#include <functional>

>>

>>template <typename F>
>>class interpolator2d {
>> public:
>> interpolator2d( const F &f) : _f(f) {
>> }
>> typename F::result_type operator()(doub le x, double y) const {
>> return _f(x, y);
>> }
>> private:
>> F _f;
>>};

>>

>>template <typename T>
>>struct function : public std::binary_fun ction<T, T, double{
>> double operator()(T x, T y) const {
>> return - (x * x + y * y);
>> }
>>};

>>

>>int main() {
>> const interpolator2d< function<int inter_f(functio n<int>());I
>>just wonder how many times people will be posting questions after
>>walking into this?..

>>
>Repeat after me: the statement above is a declaration of a FUNCTION,
>not an object as you intended. This is the same as if you wrote
>>
> int a(double());
>>
>intending to construct an integer from a default-initialised double.
>It's a FUNCTION declaration.
>>
>To fix it surround each argument (the only argument in your case)
>with an extra set of parentheses. Should be
>>
> ... inter_f((functi on<int>()));
>>

>> double x = 0;
>> double y = 0;
>> inter_f(1.1, 1.1);
>>}V

>
main.cc:26: error: cannot convert `double' to `function<int(* )()'
for argument `1' to `const interpolator2d< function<int
>
I still don't understand the error message. Where is the `double'?
Does `function<int(* )()' mean a function? Can I declare function in
arguments of a class constructor?

The point is that the compiler is not interpreting your code as a call to
the class constructor at all.

const interpolator2d< function<int inter_f(functio n<int>());

is interpreted as the declaration of a function called inter_f which takes
an argument of type function<intand returns an object of type const
interpolator2d< function<int.


--
John Carson

[VJ]

Re: What's wrong with this program using template?

Kai-Uwe Bux wrote:

PengYu.UT@gmail .com wrote:
>
>

>>Hi,
>>
>>I couldn't see what is wrong with the following program. Would you
>>please help me?
>>
>>Thanks,
>>Peng
>>
>>
>>g++-3.4 -MM -g main.cc .dep
>>g++-3.4 -g -c -o main.o main.cc
>>main.cc: In function `int main()':
>>main.cc:26: error: cannot convert `double' to `function<int(* )()' for
>>argument `1' to `const interpolator2d< function<int
>>inter_f(funct ion<int(*)())'
>>make: *** [main.o] Error 1
>>
>>
>>#include <functional>
>>
>>template <typename F>
>>class interpolator2d {
> public:
> interpolator2d( const F &f) : _f(f) {
> }
> typename F::result_type operator()(doub le x, double y) const {
> return _f(x, y);
> }
> private:
> F _f;
>>};
>>
>>template <typename T>
>>struct function : public std::binary_fun ction<T, T, double{
> double operator()(T x, T y) const {
> return - (x * x + y * y);
> }
>>};
>>
>>int main() {
> const interpolator2d< function<int inter_f(functio n<int>());

>
>
This is parsed as a function declaration. Make that:
>
const interpolator2d< function<int inter_f((functi on<int>()));
>
>

> double x = 0;
> double y = 0;
> inter_f(1.1, 1.1);
>>}

>
>
>
Best
>
Kai-Uwe Bux

still does not work. that line needs to be:

const interpolator2d< function<double inter_f((functi on<double>()));

or not to use double numbers in the last line