Static keyword does not make function internal linkage?

[ivanov ivan ivanovich]

In book "C Primer Plus" there is a paragraph about linkage of functions:
By default, C++ functions have external linkage, so they can be shared across files. But functions qualified with the keyword static have internal linkage and are confined to the defining file.

Then I try to check it by following codes

file ex1.h

#ifndef EX1_H
#define EX1_H
#include <iostream>
static int addadd(const int &x, const int &y){return x+y;} // static keyword make this function only internal linkage
#endif 

file run1.cc

#include <iostream>
#include "ex1.h"
int main(){
	int var1{2},var2{3};
	std::cout<<addadd(var1,var2)<<std::endl;
}

I anticipate an error displayed but it works properly. I wonder the static keyword in file ex1.h has to make function addadd internal linkage (can not be used in other files) and when I call it in file run1.cc, an error about not declaring must be. But it is not.

Would you explain it to me, please! Thanks.

[jlb]

The reason you're not seeing your expected error is because you have executable code inside a header file. Remember the #include copies the text from the header file into the source file. If addadd() was in a separate source file then you would not be able to use the function in another source file. Also if you had more than one source file #including that header you would probably have problems as well.

By the way instead of using the static keyword in C++ you can use an anonymous namespace instead.

file fun2.cc

namespace
{
    int addadd(int x, int y)
    {
        return x + y;
    }
}

// Just to show that you can only use addadd() in this file.
int doaddition(int x, int y)
{
    return(addadd(x, y);
}

main.cc

#include <iostream>

using namespace std;

int doaddition(int x, int y);

int main()
{
    cout << doaddition(1, 3) << endl;

    // Error.
    cout << addadd(1, 6) << endl;
    return(0);
}