What's wrong with this program using template?

[PengYu.UT]

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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

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

[Victor Bazarov]

Pe*******@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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

int main() {
const interpolator2d<function<int inter_f(function<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((function<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]

Pe*******@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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

int main() {
const interpolator2d<function<int inter_f(function<int>());

This is parsed as a function declaration. Make that:

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

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

Best

Kai-Uwe Bux

[PengYu.UT]

On Oct 28, 6:05 pm, "Victor Bazarov" <[e-mail]:

[e-mail] 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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

int main() {
const interpolator2d<function<int inter_f(function<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((function<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]

Pe*******@gmail.com wrote:

On Oct 28, 6:05 pm, "Victor Bazarov" <[e-mail]:

>[e-mail] 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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

>>int main() {
const interpolator2d<function<int inter_f(function<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((function<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]

<Pe*******@gmail.comwrote in message
news:11**********************@i42g2000cwa.googlegr oups.com

On Oct 28, 6:05 pm, "Victor Bazarov" <[e-mail]:

>[e-mail] 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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

>>int main() {
const interpolator2d<function<int inter_f(function<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((function<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(function<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]

Kai-Uwe Bux wrote:

Pe*******@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(function<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()(double x, double y) const {
return _f(x, y);
}
private:
F _f;
};

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

int main() {
const interpolator2d<function<int inter_f(function<int>());

This is parsed as a function declaration. Make that:

const interpolator2d<function<int inter_f((function<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((function<double>()));

or not to use double numbers in the last line