What is static block in c or c++?

Question

Hi

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

There is no concept with the name "static block" in C/C++. Java has it however, a "static block" is an initializer code block for a class which runs exactly once, before the first instance of a class is created. The basic concept 'stuff that runs exactly once' can simulated in C/C++ with a static variable, for example:

int some_function(int a, int b)
{
 static bool once=true; 
 if (once)
 {
  // this code path runs only once in the program's lifetime 
  once=false; 
 } 
 ...
}

This is not thread-safe however. Getting this working right in the presence of multiple threads can be difficult and tricky sometimes.

Answer 3

Another alternative is that you might be looking for the analogy of a static block in Java. A block of code that is run when the application is loaded. There is no such thing in C++ but it can be faked by using the constructor of a static object.

foo.cpp:

struct StaticBlock {
    StaticBlock(){
        cout << "hello" << endl;
    }
}


static StaticBlock staticBlock;

void main(int, char * args[]){

}

HOWEVER. I've been bitten by this before as it's a subtle edge case of the C++ standard. If the static object is not reachable by any code called by main the constructor of the static object may or may not be called.

I found that with gcc hello will get output and with visual studio it will not.