Clang compilation error on MacOS [closed] - c++

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This problem seems to only be present on MacOS, compilation is fine on linux also using clang.
the following code is a simplification but demonstrates the issue,
#include<iostream>
int index = 0;
int main()
{
std::cout << index << std::endl;
}
throws this error on compilation:
main.cpp:2:5: error: redefinition of 'index' as different kind of symbol
int index = 0;
^
/usr/include/strings.h:73:7: note: previous definition is here
char *index(const char *, int) __POSIX_C_DEPRECATED(200112L);
^
main.cpp:5:18: warning: address of function 'index' will always evaluate to
'true' [-Wpointer-bool-conversion]
std::cout << index << std::endl;
~~ ^~~~~
main.cpp:5:18: note: prefix with the address-of operator to silence this warning
std::cout << index << std::endl;
^
&
1 warning and 1 error generated.
These were the compiler arguments used:
clang++ -std=c++11 main.cpp -o test
when removing iostream with stdio or nothing the code compiles as expected. Is their a way to fix this or will I have to rename my variable to avoid this?
I did find this but I am already using the C++11 flag and the -std=c11 flag doesn't seem to be valid for C++ code.

Answer 1

The specific version of clang/xcode you're using happens to include the <strings.h> header when you include <iostream>. <strings.h> provides a function called index() at global scope. Thus, you cannot declare a variable also at global scope with the same name.
Either rename the variable, or move it into main():
#include <iostream>
int main()
{
int index = 0;
std::cout << index << std::endl;
}
This works because when a variable has the same identifier as something else but is in a different scope, it is considered a different entity altogether.
To give you an example on how that works, consider this code:
#include <iostream>
int myVar = 0;
int main()
{
int myVar = 1;
std::cout << myVar << '\n';
std::cout << ::myVar << '\n';
}
This will print:
1
0
because myVar refers to the local variable, but ::myVar to the one at global scope.

Answer 2

Is their a way to fix this or will I have to rename my variable to avoid this?
C++ provides namespaces specifically to avoid collisions between names. You can create one for your variable:
#include<iostream>
namespace MyGlobals {
int index = 0;
}
int main()
{
std::cout << MyGlobals::index << std::endl;
}