Have C++ standard library ifdef or ifndef preprocessor instructions?

Question

I'm building my own terminal app project in C++ and I'm asking myself if standard library has ifdef or ifndef preprocessors instructions. I want to know that because I need to create different header files which need some standard library headers such as "string" and some others, i don't want to include the same library 3 or more times because it makes the program heavier.
For example i wrote on my header files something like this to prevent the .h file to be included more than once:

#ifndef myheader_h
#define myheader_h
    // my file code here
#endif

I tried compiling but the compiler say me nothing about errors or warnings.
I also tried to read the standard-library source code ( https://en.cppreference.com/w/cpp/header ) and I haven't found any preprocessor rule like ifdef or ifndef.
Should i include standard library headers like this?

#ifndef string_h
#define string_h
    #include <string>
#endif

I hope my question isn't already asked because I haven't found it while searching it.

Updates

To some who said "you're not in the position where you need to worry about" and who said "it costs very little if it has proper include guards" , I meant: program's heaviness is important, I want to make it slighter so I don't want to entirely include the same file multiple times. Have std lib files proper include guards? (my header files have them, didn't know std lib files)

Answer 1

There is no requirement for the standard header files to #define any specific pre-processor symbols to make sure they can be #include d multiple times.

Having said that, any sane implementation would make sure that they can be #include d multiple times without adversely affecting application code.

Turns out, that is a requirement by the standard for most headers (Thanks, @Rakete1111).

From the C++ standard

A translation unit may include library headers in any order ( [lex] ). Each may be included more than once, with no effect different from being included exactly once, except that the effect of including either <cassert> or <assert.h> depends each time on the lexically current definition of NDEBUG .

Not only that, they are very likely to be using the #pragma once directive. Hence, even if you use #include multiple times for the same header, they are going to be read only once.

In summary, don't worry about standard header files. If your header files are implemented correctly, your application would be just fine.

Answer 2

I'm asking myself [sic] if standard library has ifdef or ifndef preprocessors instructions

The standard doesn't specify whether there are ifdef -style header guards, although it does require that multiple inclusion is protected in some manner. I took a look at a random header of stdlibc++ standard library implementation. It does have header guards.

i don't want to include the same library 3 or more times because it makes the program heavier

Including a header file multiple times does not make a program "heavier".

Should i include standard library headers like this?

 #ifndef string_h #define string_h #include <string> #endif 

That is not necessary, or particularly useful.

Answer 3

Those preprocessor directives you're talking about are called "header guards", and the standard library headers definitely have them (or some other mechanism that does the same thing) like all other proper header files. Including them multiple times shouldn't cause any problems, and you only need to worry about these when you're writing your own header files.

The "source code" that you're reading is just the documentation which says how the header files should work, but it doesn't provide the actual code. To see the code, you can look in the header files provided by your compiler. For example, the <iostream> header in Visual Studio has both #pragma once and header guards:

#pragma once
#ifndef _IOSTREAM_
#define _IOSTREAM_
//...
#endif /* _IOSTREAM_ */

The headers provided by the GCC compiler also has header guards:

#ifndef _GLIBCXX_IOSTREAM
#define _GLIBCXX_IOSTREAM 1
//...
#endif /* _GLIBCXX_IOSTREAM */