Why a C-style typecasting is mandatory while initializing a POD data in initializer list?

Actually the following syntax is not allowed by C++ Standard (both C++03, and C++11):

A() : m_pod((POD) {1,2}) {} // ok!

Since GCC compiles this, it is a GCC extension.

If you compile it with -pedantic option, it gives this warning:

pod.cpp:8:29: warning: ISO C++ forbids compound-literals

---

In C++11, you can write this:

A() : m_pod{1,2} {}

Demo : http://ideone.com/XaO4y

Or simply this:

class A {
  POD m_pod {1,2}; //in-place initialization
public:
  A() {}
};

Ideone doesn't support this though.

The syntax you're using isn't just for initializer lists, it's for any initialization of class types outside of their declarations. For example:

POD p;
p = (POD) {1, 2};

These are called compound literals; they were added to C in C99. They aren't actually supported in C++; GCC allows them in C++ (and C89) as an extension. C++11 adds the syntax:

p = POD({1, 2});

Or in your case:

A() : m_pod(POD({1,2})) {}