Private static member in base class

Question

#include <iostream>
#include <string>

class Base
{
    static std::string s;
};

template<typename T>
class Derived
    : Base
{
public:
    Derived()
    {
        std::cout << s << std::endl;
    }
};

std::string Base::s = "some_text";    

int main()
{
    Derived<int> obj;
}

This programs compiles and runs normally. static variable s is private in base class that is inherited privately. How is Derived class accessing it?

If Derived class is not template, compiler complains about accessing private variable.

[aminasya@amy-aminasya-lnx c++]$ g++ --version
g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Reading this question, could someone explain why the compiler doesn't think the line 'std::string Base::s = "some_text";' is global access, so prohibited for a private variable? Has been some time since I used C++. — Jacques de Hooge, Jul 08 '15 at 16:43
That's a definition, not a declaration. It's just initializing the string that was previous declared inside of Base; since it was declared inside Base it is a class static variable, and it is private since that is the default access level for class. — Nir Friedman, Jul 08 '15 at 16:44
I think this is a compiler bug, as this compiles with gcc but not clang, for me. — Nir Friedman, Jul 08 '15 at 16:46
GCC bug, clang and ICC reject this code. Note that GCC has many bugs when it comes to access specification control and templates... — peppe, Jul 08 '15 at 16:47

score 17 · Accepted Answer · answered Jul 08 '15 at 16:49

This is definitely a GCC bug, equivalent to GCC Bug 58740:

class A {
    static int p;
};
int A::p = 0;
template<int=0>
struct B : A {
    B() {(void)p;}
};
int main() {
    B<>();
}

The bug is still open, and this code still compiles on 5.1. GCC has issues with template member access, this is just another such example.

Nir Friedman · Answer 2 · 2015-07-08T20:25:25.087

5

I think this is a compiler bug, as this compiles with gcc but not clang, for me.

Edit: as an additional data point, this bug does not seem to have been fixed, as I can reproduce in gcc 4.9.2 and 5.1.

edited Jul 08 '15 at 20:25

answered Jul 08 '15 at 16:39

Nir Friedman

17,108
2
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score 3 · Answer 3 · answered Jul 08 '15 at 16:47

It's a bug in g++ 4.8.4. The code compiles, only if Derived is a template and the member is static.

Tests:

template static compiles
no template static fails
template not static fails
no template not static fails

Private static member in base class

3 Answers3