Another question from the internet cafe

I can´t believe this, but I just found out that the internet cafe has a copy of vc++6 on it! wow! (not joking)

so anyways, i have another question.

headers include function prototypes for functions in the standard library, correct?

why doesn´t this program work then?

Code:

---


double sin(double);

int main()
{
        sin(5.0);
}

---

i suppose i can´t prototype the function myself... but why?

yes headers do contain prototypes, but they also 'import' the function definition with the keyword "extern" which grabs the functions out of the local library

> why doesn´t this program work then?
How doesn't it work?
To me, all you are doing is calculating the value of sin(5)
You throw the result away!

Ignoring the fact that using a standard name for your own identifier is either a gross error or bad form, what were you expecting it to do? Magically determine that the prorotype you gave for sin actually meant the standard sin and to link with the math library so that everything works like you wanted? C++ isn't quite as lenient as C in this area.

>but they also 'import' the function definition with the keyword
>"extern" which grabs the functions out of the local library
Oh, so adding extern will work? And here I thought extern was the default linkage for functions in global scope, so a function declared with extern and a function declared without extern were equivalent. :rolleyes: I get the feeling you meant something other than what you said.

Quote:

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Originally Posted by Prelude

I get the feeling you meant something other than what you said.

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no, i meant what i said...at the time....
i've been looking at header files since then and realized i was most likely mistaken..

this type of thing is something i've never paid close attention to nor had to implement .

Compile these two programs. Be sure to use the compiler setting that gives you a preprocessed file as well:

Code:

---

double sin(double);

int main()
{
  sin(5.0);
}

---

Code:

---

#include <cmath>

int main()
{
  sin(5.0);
}

---

You may be surprised at what you find. There's more to a standard header than just declarations.

Quote:

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Originally Posted by krygen

why doesn´t this program work then?

---

This works on my compiler, but it is not the recommended way of doing things. The following code is bad (and so on).

Code:

---

extern "C"
{
    double sin(double); //Prototype to the C library
}

int main()
{
    sin(5);
}

---

I supposed I figured that linking with the standard library is the same as linking to another file. e.g when I create my own function in another file and link it, all i need to do is prototype it in the first file. But the standard library is different?

Quote:

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I supposed I figured that linking with the standard library is the same as linking to another file. e.g when I create my own function in another file and link it, all i need to do is prototype it in the first file. But the standard library is different?

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No, they're very similar. No matter what, if you want to use a function, somewhere it has to be defined. Code doesn't just come out of nowhere. When you create your own library's to reuse, you prototype it in a header file, but that header file is linked to the library, in which the functions are then defined. The standard library works the same way. You include a header file, in which there are function prototypes, macros and the like, but that header file is linked to a library in which all those functions are defined. The library becomes part of your program.