是 <math.h> 对于C或C ++？

Question

Im needing the natural logarithm function for use in a .cpp (c++) source file. Now, of course I can do this with a quick google search and a simple library solution. But Im a bit confused...

On the cplusplus dot com website under reference/cmath/log/ they have an example of how to use the log function, as follows

/* log example */
#include <stdio.h>      /* printf */
#include <math.h>       /* log */

int main ()
{
   double param, result;
   param = 5.5;
   result = log (param);
   printf ("log(%f) = %f\n", param, result );
   return 0;
}

some questions i have:

1) Why are they using

<stdio.h>

I thought this was for C and not really for C++ ?

2) Why are they using

<math.h>

I though the .h represented C header files rather than the .hpp C++ header files?

Forgetting about the use of stdio (i'll use iostream anyway) but even so by using

<math.h>

It feels like I'm writing C code and not C++ code. Im learning C++ through a taught course and the instructor covered C in the first week and then said we wont be using C again but will be using C++ from now on. I feel like I wont be able to explain myself if the teacher asks "why did you use a C header file? You are supposed to be working in C++".

Any explanations much appreciated.

Answer 1

<math.h> is a header specified in the C standard. Its usage is supported in C++, but formally deprecated (which means, approximately, slated for potential removal from a future standard) by all C++ standards. I would suggest it is unlikely to be removed from a future C++ standard, for as long as backward compatibility to C is considered important or desirable.

<cmath> is a header specified in the C++ standard. It provides essentially the same functionality as in C's <math.h> , except that names (other than a couple of macros) reside in namespace std .

A similar story goes for <stdio.h> (C) and <cstdio> (C++), except that usage of stream I/O (eg <iostream> ) is encouraged in C++.

Standard C++ headers never have a .hpp extension. That naming convention for headers is a convention encouraged by some, but is not formally required.

Answer 2

The C++11 Standard says:

D.5 C standard library headers

1 For compatibility with the C standard library and the C Unicode TR, the C++ standard library provides the 25 C headers, ...

The inclusion of these headers is stated as deprecated, meaning:

Normative for the current edition of the Standard, but not guaranteed to be part of the Standard in future revisions.

So they are still (just) part of C++ .

They are provided for compatibility which is to allow the programmer to compile programs originally written for C with a standard conforming C++ compiler with little or no modification. That means things like not having to change the #include statements from <stdio.h> to <ctsdio> .

So the example given in cplusplus.com is actually standards conforming C++ that just happens to be compatible with a C90 and a C99 conforming C compiler. Presumably they do this because the page describing the math library gives information for both C and C++ languages following the standards for C90 , C99 , C++98 and C++11 .

So to answer the specific questions:

1) Why are they using

 <stdio.h> 

I thought this was for C and not really for C++ ?

It's for C++ compatibility with C . Presumably they use it so the code will also compile on a C90/C99 conforming C compiler for which the page gives specifications.

1) Why are they using

 <math.h> 

I though the .h represented C header files rather than the .hpp C++ header files?

No. The standard does not specify what extensions files should use. In practice many C++ projects use .h as an extension for their header files.

I feel like I wont be able to explain myself if the teacher asks "why did you use a C header file?

Given that the C compatibility headers are deprecated (though probably not going anywhere) I would suggest it better to use the <cstdio> and <cmath> versions. However the idea that you are somehow writing C code simply because of your choice of library function is wrong. If it is legal C++ code being fed through a C++ compiler then it is C++ . It may be more procedural in character and less object oriented in philosophy but it is nevertheless fully C++ . Many, many, many C++ programs use libraries written in other languages, especially C . That doesn't make those programs somehow C .

Answer 3

About your first qustion, stdio.h is required for the printf function used
About your second question, math.h can be used by both C and C++, but cmath will define the methods in std namespace while math.h will define those in the global namespace

Generally put, you can use C code within C++ code, there usually not going to be any problem with that, especially when dealing with well known libraries like math.h

Answer 4

<math.h> (for C) and <cmath> (for C++) are very similar in basic usage of functions. <cmath> begins to differ in more advanced stages, involving templates, STL, and Object-Oriented Programming in general.

Whether you use C or C++, <math.h> can be used, but I don't think the opposite ( <cmath> for C) works. I recommend <cmath> for C++ as it's the same as , but has more powerful OOP features.