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[Q] Generating compile time error when sizeof(int) != 4

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[Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

I was recently given a list of possible C++ interview questions by a recruiter for a firm on Wall St. One of the questions was:

How would you generate a compile time error if the sizeof(int) is not equal to 4?

I was thinking along the lines of template specialization or some such thing, but I'm not sure. Any ideas?

Thanks.

-Marc

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Re: [Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

You can do it with templates, but it really isn't a better approach complexity-wise. The common method is to use the preprocessor with <climits>:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <climits>
#include <iostream>
 
#if INT_MAX != 2147483647
#  error integers must be 32 bits long
#endif
 
int main ( void )
{
  std::cout<< INT_MAX <<std::endl;
 
  return 0;
}
#include <climits>
#include <iostream>

#if INT_MAX != 2147483647
#  error integers must be 32 bits long
#endif

int main ( void )
{
  std::cout<< INT_MAX <<std::endl;

  return 0;
}

Though you need to be careful because the calculation might make an unwarranted assumption as above, such as CHAR_BIT being 8. Sometimes the assumption is safe, other times it constitutes an error.

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Re: [Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

How about this?

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(Toggle Plain Text)

#include <stdio.h>

char dummy [ sizeof(int) == 4 ];

int main(void)
{
   return 0;
}

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Re: [Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

>How about this?
Cute, but it's not very informative, is it?

Though you do meet the requirements for the question. I wish I had remembered that trick for my reply, it's cleaner.

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Re: [Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

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Originally Posted by Narue

You can do it with templates, but it really isn't a better approach complexity-wise.

Could you demonstrate the template based way to do this? I can't seem to figure this out. Thanks!

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Re: [Q] Generating compile time error when sizeof(int) != 4

Nov 5th, 2004

>Could you demonstrate the template based way to do this?

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 C++ Syntax (Toggle Plain Text) 
template <bool>
struct static_assert;
 
template <>
struct static_assert<true> {
};
 
int main()
{
  static_assert<sizeof ( int ) == 4>();
}
template <bool>
struct static_assert;

template <>
struct static_assert<true> {
};

int main()
{
  static_assert<sizeof ( int ) == 4>();
}