error: passing `const Equal4' as `this' argument of `bool Equal4::operato r()(int)' discards qualifiers

[Lycan. Mao..]

Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.

template <typename _Predicate>
struct Unary_negate {
typedef typename _Predicate::arg ument_type argument_type;
typedef typename _Predicate::ret urn_type return_type;

_Predicate pred_;

explicit
Unary_negate(co nst _Predicate& p) : pred_(p) {};

bool operator() (const argument_type& x) const
{return !pred_(x);}
};

and then:

struct Equal4
: Unary_function< int, bool>
{
bool operator() (const int i)
{
return (i == 4);
}
};

then I use them like this:

STLite::find_if (p.begin(), p.end(), Unary_negate<Eq ual4>(Equal4()) );

You see, I rewrite the find_if and iterators and Unary_function, but
they are almost the same like the STL ones.

then, as a result of compiling, I get:

functor.hpp: In member function `typename
_Adaptable_pred icate::return_t ype
STLite::Unary_n egate<_Adaptabl e_predicate>::o perator()(int) const
[with _Adaptable_pred icate = Equal4]':
algorithm.hpp:2 0: instantiated from `_Input_iterato r
STLite::find_if (_Input_iterato r, _Input_iterator , _Predicate) [with
_Input_iterator = Node_list_itera tor<int, Node<int>, int*, int&>,
_Predicate = STLite::Unary_n egate<Equal4>]'
main.cpp:100: instantiated from here
functor.hpp:50: error: passing `const Equal4' as `this' argument of
`bool Equal4::operato r()(int)' discards qualifiers

At first, I think it's my fault when copying the STL, but I test like
this:

struct Equal4_wrap {
typedef int arg;
typedef bool ret;
Equal4 pred_;
explicit Equal4_wrap(con st Equal4& p) : pred_(p) {}
ret operator() (const int& i) {return !pred_(i);}
};

which I think exactly the same as the instantialized one of
Unary_negate, and it works well, really.

I'm confused, I hope some one can help. Thank you very much!

Mao..

[John Harrison]

Lycan. Mao.. wrote:

Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.

template <typename _Predicate>
struct Unary_negate {
typedef typename _Predicate::arg ument_type argument_type;
typedef typename _Predicate::ret urn_type return_type;

_Predicate pred_;

explicit
Unary_negate(co nst _Predicate& p) : pred_(p) {};

bool operator() (const argument_type& x) const
{return !pred_(x);}
};

and then:

struct Equal4
: Unary_function< int, bool>
{
bool operator() (const int i)
{
return (i == 4);
}
};

const in the wrong place, you don't need 'const int' but you do need to
make operator() a const method.

struct Equal4 : Unary_function< int, bool>
{
bool operator() (int i) const
{
return (i == 4);
}
};

[Lycan. Mao..]

On 3ÔÂ4ÈÕ, ÏÂÎç8ʱ53·Ö, John Harrison <john_androni.. [e-mail] rote:

Lycan. Mao.. wrote:

Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.

template <typename _Predicate>
struct Unary_negate {
typedef typename _Predicate::arg ument_type argument_type;
typedef typename _Predicate::ret urn_type return_type;

_Predicate pred_;

explicit
Unary_negate(co nst _Predicate& p) : pred_(p) {};

bool operator() (const argument_type& x) const
{return !pred_(x);}
};

and then:

struct Equal4
: Unary_function< int, bool>
{
bool operator() (const int i)
{
return (i == 4);
}
};

const in the wrong place, you don't need 'const int' but you do need to
make operator() a const method.

struct Equal4 : Unary_function< int, bool>
{
bool operator() (int i) const
{
return (i == 4);
}

};

Yes, thank you very much! I have tested it, and yes, it's right! Thank
you again.

But can you show me why? Why one is all right and the other fails?(I
mean both Equal4_wrap/Unary_negate and operator() (const int)/
operator() (int) const)

Thank you!

[John Harrison]

Lycan. Mao.. wrote:

On 3ÔÂ4ÈÕ, ÏÂÎç8ʱ53·Ö, John Harrison <john_androni.. [e-mail] ote:

>>Lycan. Mao.. wrote:

>>>Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.

>>>template <typename _Predicate>
struct Unary_negate {
typedef typename _Predicate::arg ument_type argument_type;
typedef typename _Predicate::ret urn_type return_type;

>> _Predicate pred_;

>> explicit
Unary_negate(co nst _Predicate& p) : pred_(p) {};

>> bool operator() (const argument_type& x) const
{return !pred_(x);}
};

>>>and then:

>>>struct Equal4
: Unary_function< int, bool>
{
bool operator() (const int i)
{
return (i == 4);
}
};

const in the wrong place, you don't need 'const int' but you do need to
make operator() a const method.

struct Equal4 : Unary_function< int, bool>
{
bool operator() (int i) const
{
return (i == 4);
}

};

Yes, thank you very much! I have tested it, and yes, it's right! Thank
you again.

But can you show me why? Why one is all right and the other fails?(I
mean both Equal4_wrap/Unary_negate and operator() (const int)/
operator() (int) const)

Thank you!

Look at the error message

functor.hpp: In member function `typename
_Adaptable_pred icate::return_t ype
STLite::Unary_n egate<_Adaptabl e_predicate>::o perator()(int) const
[with _Adaptable_pred icate = Equal4]':

STLite::Unary_n egate::operator () is a const member function, so when it
tries to call Equal4::operato r() is expects that to also be a const
membe function.

Now look at your Equal4_wrap, it's operator() is not a const member
function, so it has no problem calling Equal4::operato r().

If you had written

struct Equal4_wrap {
...
ret operator() const (const int& i) {return !pred_(i);}
};

you would have got the same error in your test.

john

[Lycan. Mao..]

On 3ÔÂ4ÈÕ, ÏÂÎç9ʱ24·Ö, John Harrison <john_androni.. [e-mail] rote:

Lycan. Mao.. wrote:

On 3ÔÂ4ÈÕ, ÏÂÎç8ʱ53·Ö, John Harrison <john_androni.. [e-mail] ote:

>Lycan. Mao.. wrote:

>>Hello, I'm trying to write a function adapter object, but it fails
with the above information. Can you help me.

>>template <typename _Predicate>
struct Unary_negate {
typedef typename _Predicate::arg ument_type argument_type;
typedef typename _Predicate::ret urn_type return_type;

> _Predicate pred_;

> explicit
Unary_negate(co nst _Predicate& p) : pred_(p) {};

> bool operator() (const argument_type& x) const
{return !pred_(x);}
};

>>and then:

>>struct Equal4
: Unary_function< int, bool>
{
bool operator() (const int i)
{
return (i == 4);
}
};

>const in the wrong place, you don't need 'const int' but you do need to
make operator() a const method.

>struct Equal4 : Unary_function< int, bool>
{
bool operator() (int i) const
{
return (i == 4);
}

>};

Yes, thank you very much! I have tested it, and yes, it's right! Thank
you again.

But can you show me why? Why one is all right and the other fails?(I
mean both Equal4_wrap/Unary_negate and operator() (const int)/
operator() (int) const)

Thank you!

Look at the error message

functor.hpp: In member function `typename
_Adaptable_pred icate::return_t ype
STLite::Unary_n egate<_Adaptabl e_predicate>::o perator()(int) const
[with _Adaptable_pred icate = Equal4]':

STLite::Unary_n egate::operator () is a const member function, so when it
tries to call Equal4::operato r() is expects that to also be a const
membe function.

Now look at your Equal4_wrap, it's operator() is not a const member
function, so it has no problem calling Equal4::operato r().

If you had written

struct Equal4_wrap {
...
ret operator() const (const int& i) {return !pred_(i);}

};

you would have got the same error in your test.

john

Thank you very much! I understand it now.

Thank you!