Problem when using template as the base class?

Problem when using template as the base class?

[Peng Yu]

Hi,

I have the following program. It also includes the non-template
equivalent class, which works fine. If you uncomment the comments, you
will see the compiler errors. The errors are also shown below. Do you
know what is wrong? Is it a bug of the compiler?

BTW, I use g++-3.4.

Thanks,
Peng


//////////////////////////////////////////////////
#include <iostream>
#include <boost/program_options.hpp>

class base_arguments {
  public:
    base_arguments() {}
  protected:
    boost::program_options::variables_map _vm;

};

/*template <typename T>
class plot_arguments : public T {
  public:
    plot_arguments() {}
    int boundary_pad() const {
      return T::_vm["boundary_pad"].as<int>();
    }

};*/

class plot_arguments1 : public base_arguments {
  public:
    plot_arguments1(){}
    int boundary_pad() const {
      return base_arguments::_vm["boundary_pad"].as<int>();
    }

};

int main(int argc, char *argv[])
{
  //plot_arguments<base_arguments> arg();
  plot_arguments1 arg1();
}

////////////////////////////////////////////////

The error that I get is:

main_1mod_bim_plot.cc: In member function `int
plot_arguments<T>::boundary_pad() const':
main_1mod_bim_plot.cc:16: error: expected primary-expression before
"int"
main_1mod_bim_plot.cc:16: error: expected `;' before "int"
main_1mod_bim_plot.cc:16: error: expected primary-expression before
"int"
main_1mod_bim_plot.cc:16: error: expected `;' before "int"
main_1mod_bim_plot.cc: At global scope:
main_1mod_bim_plot.cc:29: warning: unused parameter 'argc'
main_1mod_bim_plot.cc:29: warning: unused parameter 'argv'
make: *** [main_1mod_bim_plot-g.o] Error 1

Re: Problem when using template as the base class?

[Peng Yu]

> I have the following program. It also includes the non-template
> equivalent class, which works fine. If you uncomment the comments, you
> will see the compiler errors. The errors are also shown below. Do you
> know what is wrong? Is it a bug of the compiler?
>
> BTW, I use g++-3.4.

I corrected the error by adding template keyword. But I don't
understand why "template" keyword is need. Would you please help me?

#include <iostream>
#include <boost/program_options.hpp>

class base_arguments {
  public:
    base_arguments() {}
  protected:
    boost::program_options::variables_map _vm;
};

template <typename T>
class plot_arguments : public T {
  public:
    plot_arguments() {}
    int boundary_pad() const {
      return T::_vm["boundary_pad"].template as<int>();
    }
};

class plot_arguments1 : public base_arguments {
  public:
    plot_arguments1(){}
    int boundary_pad() const {
      return base_arguments::_vm["boundary_pad"].as<int>();
    }
};

int main(int argc, char *argv[])
{
  plot_arguments<base_arguments> arg();
  plot_arguments1 arg1();
}

Re: Problem when using template as the base class?

[Daniel Llorens del Río]

On 23 Sep, 2006, at 1:47, Peng Yu wrote:

>> I have the following program. It also includes the non-template
>> equivalent class, which works fine. If you uncomment the comments,  
>> you
>> will see the compiler errors. The errors are also shown below. Do you
>> know what is wrong? Is it a bug of the compiler?
>>
>> BTW, I use g++-3.4.
>
> I corrected the error by adding template keyword. But I don't
> understand why "template" keyword is need. Would you please help me?

Because when the compiler first parses your function, it doesn't know  
what type T is, and so it cannot know that 'T::(something).as' is a  
template and 'T::(something)/as<int>' is valid syntax.

This is standard C++, go google up 'templates two-phase name lookup'.

	Daniel.