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This works:

#include <iostream> using namespace std;

but this fails:

#include <stdio>

When is .h not needed?

About the namespace issue,I didn't find such logic in cstdio:

#pragma once #ifndef _CSTDIO_ #define _CSTDIO_ #include <yvals.h> #ifdef _STD_USING #undef _STD_USING #include <stdio.h> #define _STD_USING #else /* _STD_USING */ #include <stdio.h> #endif /* _STD_USING */ // undef common macro overrides #undef clearerr #undef feof #undef ferror #undef getc #undef getchar #undef putc #undef putchar #define _HAS_CONVENTIONAL_CLIB 1 #define _IOBASE _base #define _IOPTR _ptr #define _IOCNT _cnt #ifndef _FPOSOFF #define _FPOSOFF(fp) ((long)(fp)) #endif /* _FPOSOFF */ typedef FILE _Filet; #ifndef RC_INVOKED #if _GLOBAL_USING _STD_BEGIN using ::_Filet; using ::size_t; using ::fpos_t; using ::FILE; using ::clearerr; using ::fclose; using ::feof; using ::ferror; using ::fflush; using ::fgetc; using ::fgetpos; using ::fgets; using ::fopen; using ::fprintf; using ::fputc; using ::fputs; using ::fread; using ::freopen; using ::fscanf; using ::fseek; using ::fsetpos; using ::ftell; using ::fwrite; using ::getc; using ::getchar; using ::gets; using ::perror; using ::putc; using ::putchar; using ::printf; using ::puts; using ::remove; using ::rename; using ::rewind; using ::scanf; using ::setbuf; using ::setvbuf; using ::sprintf; using ::sscanf; using ::tmpfile; using ::tmpnam; using ::ungetc; using ::vfprintf; using ::vprintf; using ::vsprintf; _STD_END #endif /* _GLOBAL_USING */ #endif /* RC_INVOKED */ #endif /* _CSTDIO_ */
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  • 1
    Maybe take a look the macros _STD_BEGIN and _STD_END for the namespace declaration. Commented Mar 28, 2010 at 9:47
  • I don't know how that macro exactly works,but seems it doesn't wrap stdio.h? Commented Mar 28, 2010 at 9:50
  • Those using statements between _STD_BEGIN and _STD_END import those names into the std namespace. They would now be available both as size_t and std::size_t (don't ask me why they should be kept visible globally). - The C++ versions have other differences. For example, it appears that some things are macros in C libraries, which C++ redefines as normal functions. Also, <cmath> adds various overloads to functions (no overloading in C) etc. Commented Mar 28, 2010 at 10:22

3 Answers 3

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6

Standard C++ headers don't use the .h. Everything else does (or, more accurately, everything else uses whatever extension it wants, .h, .hxx, .hpp, .hh and more).

Standard C headers can be included in one of two ways:

#include <stdio.h> #include <cstdio>

The second form wraps its symbols in the std namespace.

The original intent was that headers could, in principle, be stored in a database in some highly optimised pre-compiled state, in which case the idea of a file extension wouldn't make sense. I don't know that this ever happened in practice.

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6 Comments

What if stdio if it's not Standard C++ headers?
@user: stdio is is standard C headers. Those headers are included as #include <cstdio>.
No, C's stdio.h is wrapped in C++'s cstdio. Likewise for other C headers.
@user198729: they aren't all wrapped into 1 file. string.h > cstring, stdio.h > cstdio, and so on.
@user198729, stdio.h is for backwards compatibility, while cstdio is encouraged for new code.
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5

It's not needed for the header files defined by the C++ Standard, none of which have a .h extension. The C++ version of stdio.h is:

#include <cstdio>

which wraps stdio.h, placing the names in it in the C++ std namespace, but you can still use all the C Standard header files in C++ code, if you wish.

Edit: The macro that places the names in the std namespace in the GCC version of cstdio is:

_GLIBCXX_BEGIN_NAMESPACE(std)

You can check that your own header does what it should do by trying to use something like:

std::printf( "hello" );

in your code.

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5 Comments

I took a look at the cstdio,but didn't find the logic to placing the names in std namespace
@user198729 I don't know which compiler you are using, but that's what the standard says cstdio must do.
The compiler shouldn't matter,it's the header file itself that doesn't mention anywhere about std.You can have a look into it,also I've pasted it in my post above:)
@user198729 The compiler matters a lot - the headers are part of the specific compiler implementation.
I'm using visual c++ 2008 express,seems the magic is done by using ::_Filet; and alike.
1

the .h is not needed, simply when the .h is omitted from the file's name in the filesystem.

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