using unary_function

[curious2007]

Hello,

I have the following code, but does not compile:

#include <iostream>
using namespace std;
 
template<class Arg, class Result>
struct unary_function
{
       typedef Arg argument_type;
       typedef Result result_type;
};
 
class Greater: public unary_function<int, bool>
{
      public:
             result_type operator () (argument_type k)
             {
              if (k>10)
              return true;
              else
              return false;
             }
};
 

I get the following errors:

expected template-name before '<' token
expected `{' before '<' token
expected unqualified-id before '<' token

All of these errors are related to the line:
class Greater: public unary_function<int, bool>

I would appreciate any insights. Thank you.

[weaknessforcats]

What compiler are you using?

This compiled without error using Visual Studio.NET 2005.

[curious2007]

DEV C++. It seems like Visual Studio.NET 2005 is much more flexible.

[curious2007]

OK, so when I replace:

class Greater: public unary_function<int, bool>

with

class Greater: public std::unary_function<int, bool>

I get rid of the error. However, I don't know why this works because I have had using namespace std at the first place.

[weaknessforcats]

It's because you also have a struct named unary_function. Now the compiler can't tell whether to use yours or the one in std. So it bails on you.

This is the reason for namespaces and why your unary_function should be in your namespace. Had it been so, this error would never have cropped up.

[curious2007]

how can I put my unary_function into namespace?

[weaknessforcats]

[code=cpp]
namespace curious2007
{
template<class Arg, class Result>
struct unary_function
{
typedef Arg argument_type;
typedef Result result_type;
};

class Greater: public unary_function<int, bool>
{
public:
result_type operator () (argument_type k)
{
if (k>10)
return true;
else
return false;
}
};


} // end of namespace

Now it's called: curious2007::unary_function.

[SFaX]

unary_function is declared in <functional>... try to include it explicitly