CODEWITHSUNDEEP
c++function
user3002386
user3002386

Reputation: 125

How to view standard library functions in C++?

For example, i want to see the code of function toupper() to understand how it works, is there any way? I have searched and opened string.h library, but didn't find anything.

Upvotes: 0

Views: 3288

Answers (2)

Christian Hackl
Christian Hackl

Reputation: 27548

From a strict language point of view, you cannot "see the code" of a standard function, because the C++ language standard only defines functions' prototypes and behaviours, not how they are implemented.

In fact, from a strict language point of view, a standard function like toupper does not even have to have source code, because a standard header, like <string.h> does not even have to be a file!

Of course, in practice, you will probably never encounter a C++ implementation in which standard headers are not files, because files are just a natural and simple implementation of headers. This means that in practice, for the header <string.h>, there is actually a C++ source file called "string.h" somewhere on your computer. Just find it and open it.

I have searched and opened string.h library, but didn't find anything.

Then you have not looked close enough. Hint: This file most likely includes one or more other header files.

Note that if you actually looked for toupper, that function is not in <string.h> anyway. Look in <ctype.h> instead. cppreference.com is a good online reference to tell you which headers contain which functions.

http://en.cppreference.com/w/c/string/byte/toupper

Again, this does not mean that the corresponding header file of your compiler contains that function directly, but it may directly or indirectly include some other file which contains it.

In any case, beware of what you will see inside of your compiler's header files. It will usually be a lot more complicated than you may think, and, more importantly, it will often use constructs you are not allowed to use in your own code; after all, the code in those files is internal to the compiler implementation, and the compiler has a lot of privileges you don't have, for example using otherwise forbidden identifiers like _STD_BEGIN. Also expect a lot of completely non-standard #pragmas and other non-portable stuff.

Another important thing to keep in mind is that you are not supposed to dig through a function's implementation to find out what it does. In badly written software, i.e. software with confusing interfaces and no documentation (which exists everywhere in the real world), you unfortunately have to do this, provided you have access to the source code.

But C++ standard functions are perfectly documented and have, with some arguable exceptions, well-designed interfaces. It may be interesting, and educating, and sometimes even necessary for debugging, to look into their implementation on your system, but don't let this possibility keep you from learning two important software-engineering skills:

  1. Reading documentation.
  2. Programming to interfaces, not to implementations.

Upvotes: 3

awesoon
awesoon

Reputation: 33701

Yes, of course, you could (not all realizations, maybe). For example, the glibc implementation defines toupper function as:

#define __ctype_toupper \
  ((int32_t *) _NL_CURRENT (LC_CTYPE, _NL_CTYPE_TOUPPER) + 128)

int
toupper (int c)
{
  return c >= -128 && c < 256 ? __ctype_toupper[c] : c;
}

Upvotes: 1

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