2

static.h

static int y = 20;
static int& f(){
    static int x = 0;
    return x;
}

general.h

int x = 10;
int& g(){
    static int x = 0;
    return x;
}

outer.h

#include"general.h"
#include"static.h"

void h(){
    x = 20;//static
    y = 20;//non static

    f() = 20;//static
    g() = 20;//non static
}

main.cpp

#include<iostream>
#include"static.h"
#include"general.h"
#include"outer.h"

int main(){
    h();
    std::cout<<"static objects are independent in each file\n";
    std::cout<<"x : "<<x<<", f() : "<<f()<<std::endl;
    std::cout<<"non static objects are only one in every file.\n ";
    std::cout<<"y : "<<y<<", g() : "<<g()<<std::endl;
}

I want to simulate how static keyword changes code behavior.
but, I don't know how to do that...
How do I do?

.....................................................
I split my code.
static.h

static int x = 0;
static int& f(){
    static int k = 0;
    return k;
}
void call_static();

static.cpp

#include"static.h"

void call_static(){
    x = 10;
    f() = 10;
}

general.h

int y = 0;
int& g(){
    static int z = 0;
    return z;
}
void call_general();

general.cpp

#include"general.h"
void call_general(){
    y = 10;
    g() = 10;
}

Static codes are compiled well, but non static codes aren't compiled.

error code

/usr/bin/ld: /tmp/general-539e11.o: in function `g()':
general.cpp:(.text+0x0): multiple definition of `g()'; /tmp/main-5f12cd.o:main.cpp:(.text+0x0): first defined here
/usr/bin/ld: /tmp/general-539e11.o:(.bss+0x0): multiple definition of `y'; /tmp/main-5f12cd.o:(.bss+0x4): first defined here
clang-12: error: linker command failed with exit code 1 (use -v to see invocation)

Is it normal?

  • You need to get different translation units involved to see a difference. Create 2 headers `A.hpp` and `B.hpp` declaring functions `functionA` and `functionB`, create `A.cpp` and `B.cpp` with implementations of those functions using the static symbols, use both functions in your main and link everything together. – fabian Aug 22 '21 at 11:25

1 Answers1

1

In the example you've shown there's only one .cpp file.
.h files don't actually count as "Translation Units", only .c and .cpp files are translation units.
The reason for that is that .h files are added to a program using #include.
#include literally copy-pastes the specified file into the current file, making it just 1 big file.
If you had multiple .c or .cpp files (or a mix of both, that's OK as well), you'll be able to see the effects that the static keyword has on variable/function visibility.

Remember to NEVER #include .c or .cpp files!
Instead, you should add them to your program seperately.
If you are using Visual Studio (not VS Code) simply adding a new source file should automatically add it to the program, without you needing to mention it anywhere.
If you are using another compiler, add the name of your .c or .cpp files to the list of files to be compiled.

I'd reccomend reading about translation units.
you can do that at https://en.wikipedia.org/wiki/Translation_unit_(programming) and What is a "translation unit" in C++?, but I'd also highly recommend just looking up "c and c++ translation units" on google and learning about them in general.

Have fun coding!

Shahar Nacht
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    Thank you! I split my codes and It work. But, non-static codes aren't compile. Why is that? –  Aug 22 '21 at 12:10
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    What files do you have now? What's written in them? Do you wanna maybe upload a `.zip` of all of them for me to look? What do you use to compile the files? Visual Studio/GCC/something else? – Shahar Nacht Aug 22 '21 at 12:17
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    https://drive.google.com/file/d/12dp3eTYyQrP2UzfL-_MP1kzxefC0_nvR/view?usp=sharing –  Aug 22 '21 at 12:27
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    debian, clang-12 –  Aug 22 '21 at 12:28
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    I've found your issue! when you do something like `int y;` at the global scope, you might think that that's a declaration - kinda like declaring the name and args of a function, without the actual code inside it. But variables are not like functions in C, and every time you `int y;` at the global scope, you cause a new allocation of memory. But C doesn't allow 2 variables to have the same name ("`y`" here), because then it wouldn't know which of them you mean when you refer to "`y`". The solution is to replace any declaration to `extern int y;`, and only have one `int y;` in one `.cpp` file. – Shahar Nacht Aug 22 '21 at 13:45
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    I'd highly recommend reading about the `extern` keyword in general! :) – Shahar Nacht Aug 22 '21 at 13:46
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    Thank you for kind answer!! I finally do it! –  Aug 22 '21 at 23:24
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    Happy to help – Shahar Nacht Aug 23 '21 at 07:42