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Implicit declaration of function with header file

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Join Date: Jan 2008

Posts: 19

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Suraine

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Implicit declaration of function with header file

Apr 6th, 2008

Dear c++ gurus,

I'm new in creating a header file. never try before.

ERm... i have write a header file with a program:

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Header file: MySensor.H
 
extern short int DetectSensor(int sensorvalue[10])
{
      ....;
}
 
Header file: MySensor.H

extern short int DetectSensor(int sensorvalue[10])
{
      ....;
}

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Program file: MyFile.C
 
#include <MySensor.H>
 
short int i;
int p, sensorvalue[10];
 
int getsensorvalue()
{
       ...;
}
 
int main()
{
      for( p = 0; p < 10; p++)
      {
            sensorvalue[p] = getsensorvalue();;
      }
 
      i = DetectSensor(sensorvalue);
      return 0;
}
 
Program file: MyFile.C

#include <MySensor.H>

short int i;
int p, sensorvalue[10];

int getsensorvalue()
{
       ...;
}

int main()
{
      for( p = 0; p < 10; p++)
      {
            sensorvalue[p] = getsensorvalue();;
      }

      i = DetectSensor(sensorvalue);
      return 0;
}

But it comes out saying that:
Implicit declaration of function 'short int DetectSensor(...)'

What happen? is it relate to my header file format is wrong?

Thanks.

Best regards,
Suraine

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Ancient Dragon Ancient Dragon is offline

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Re: Implicit declaration of function with header file

Apr 6th, 2008

1. The header file should not have any code -- just function prototype
extern short int DetectSensor(int sensorvalue[10]); Notice the semicolon at the end and there are no ( and )

2. In the *.cpp file, enclose the name of your header file in quotes
#include "MySensor.H"; Angle bracks are for header files that are were supplied by your compiler, such as <string>. The ones you create are in your project's folder and should be surrounded by quotes.

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.

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Join Date: Jan 2008

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Suraine

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Re: Implicit declaration of function with header file

Apr 6th, 2008

Thanks, Ancient Dragon.

So, that means in header file. We can just declare the function prototype, but cant have its function definition.

only:

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extern short int DetectSensor(int sensorvalue[10]);
extern short int DetectSensor(int sensorvalue[10]);

but not

 Help with Code Tags 
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extern short int DetectSensor(int sensorvalue[10]);
 
short int DetectSensor(int sensorvalue[10])
{
    function definition block;
}
extern short int DetectSensor(int sensorvalue[10]);

short int DetectSensor(int sensorvalue[10])
{
    function definition block;
}

function definition should be done in *.cpp file, right?

If it is, what is header file for? not for shorten the file length, but for?

Thanks again.

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Ancient Dragon Ancient Dragon is offline

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Re: Implicit declaration of function with header file

Apr 6th, 2008

Header files for mostly useful in multi-file projects where there are two or more *.cpp files which need to know about the same functions and definitions. Look in your compiler's include directory, it contains many header files. The ones you write are no different than those.

The more you write programs and header files the more useful they will appear to you. Especially when you begin to write multi-file programs. Many large projects consist of hundreds of *.cpp files, so common header files are absolutely essential.

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.