What is static block in c or c++? [duplicate]

Question

I want to know that what is static block in c or c++ with an example? I know what is static but what is the difference between static and static block?

Answer 1

In C++ there is the concept of an anonymous namespace.

foo.cpp:

namespace {
    int x;
    int y;
}

to get the same effect in C

foo.cpp:

static int x;
static int y;

In simple terms the compiler does not export symbols from translation units when they are either declared static or in an anonymous namespace.

Answer 2

I found this answer on The Code Project. It involves having an extra static variable, but I believe it is more reliable than bradgonesurfing's answer. Basically, it is this:

class Foo
{
public:
    static int __st_init;
private:
    static int static_init(){
        /* do whatever is needed at static init time */
        return 42;
    }
};
int Foo::__st_init = Foo::static_init();

It also means that, like Java's static blocks, you are not required to ever actually have an instance of class Foo, which is useful when the class can take a lot of data, and you simply need to automagically call something before it loads, not instantiate an extra instance of it. You can test that exact code block. I just compiled it (with a little output from static_init(), and had main() print Foo::__st_init, just to make sure), and it worked just fine.

$g++ -v

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-9ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)

---

EDIT:

Sorry that this is so late, but I tested what bradgonesurfing mentioned:

If you test it my accessing the variable in main "just to make sure" you are ensuring the variable is reachable and thus the variable will be initialized and thus static_init will be called. Are you sure it executes if you dont print Foo::__st_init

I used the following inside main.cpp:

#include <iostream>

using namespace std;

class Foo
{
public:
    static int __st_init;
private:
    static int static_init(){
        /* do whatever is needed at static init time */
        cout << "Hello, World!";
        return 42;
    }
};
int Foo::__st_init = Foo::static_init();

int main(int argc, char** argv)
{
        return 0;
}

I compiled with g++ ./main.cpp -o main and ran it and recieved a friendly "Hello, World!" message on my console. Just to be thorough, I also compiled the same version but without the printing and compiled with g++ ./main.cpp -g -o main. I then ran the executable with gdb and had the following result:

(gdb) break Foo::static_init
Breakpoint 1 at 0x400740: file ./main.cpp, line 12.
(gdb) start
Temporary breakpoint 2 at 0x4006d8: file ./main.cpp, line 19.
Starting program: /home/caleb/Development/test/main-c++ 

Breakpoint 1, Foo::static_init () at ./main.cpp:12
12              return 42;
(gdb) 

Here's a more current version output for g++: g++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2