Why is a FILE* pointer zero-initialized in C++'s default initialization?

Question

Consider

#include <iostream>
#include <cstdio>

using namespace std;

struct Foo {
    FILE* fp;
    Foo* mp;
};

int main()
{
    Foo f1;   // default initialization
    cout << f1.fp << '\t' << f1.mp << '\n';

    Foo f2{}; // zero initialization / direct-list-initialization
    //Foo f2 = {}; // zero initialization / copy-list-initialization
    cout << f2.fp << '\t' << f2.mp << '\n';

    FILE* fp; // local POD variable, undefined
    cout << fp << '\n';
}

Compilation and execution,

g++ tp.cc && ./a.out

gives something like:

0       0x5639eb8f00c0
0       0
0x5639eb8f02f0

Both FILE* fp and Foo* mp are of pointer types, thus also of POD types. So, in the default initialization (line Foo f1;), both should be not initialized. But the result, however, appears to suggest the FILE* fp is zero-initialized. How come? Is FILE* a special pointer type that the compiler loves to (zero) initialize, but refuses to zero initialize other pointers?

Answer 1

You didn't initialize the pointers. They get random (undefined) values (specifically, whatever the runtime happened to leave on the stack during initialization before main was called). Sometimes those values happen to be NULL. C++ didn't initialize them, you just got "lucky". As noted in the comments on your question, on different compilers, or with different variable declaration orders, the FILE* often ends up non-NULL, because it's not being initialized, just inheriting whatever happened to already be on the stack.