为什么 `main` function 中的局部 int 变量会自动初始化？

Question

I'm new to C++ and have that problem while learning C++.

Here is the code

#include <iostream>
using namespace std;

void another_func() {
    int a;
    cout << a << endl;
}

int main() {
    int a;
    cout << a << endl;
    another_func();
}

I'm using g++ (GCC) 10.1.0 and I found that everytime when I run the code, the a inside the main function would be initialize to 0 , while a in another_func would be a random number. As follows,

➤  g++ test.cpp && ./a.out
a in main: 0
a in another_func: 32612

As I know, local variables are storaged in the stack and they don't have auto-initialization mechanism. So the a in another_func is expected. However, can someone tell me why the a in main function was initialized to 0 ?

Thanks in advance!

Answer 1

Uninitialised doesn't mean non-zero, it could have any value. On many OSs freshly allocated memory pages are filled with 0 so in non-debug code uninitialised values are often 0 too.

The behaviour of your program is undefined but what is likely happening is that a in main is either the first use of the stack or you just get lucky and the initialisation code that runs before main leaves that area of the stack 0 .

The call to cout will write to the stack so when you then execute another_func the stack memory wont be all 0 anymore.

Answer 2

tl;dr: You must not read uninitialized values.

Both a s weren't initialized to anything. That means they have an indeterminate value, which could be anything including 0. Any reasoning must stop here because reading from an uninitialized value invokes undefined behaviour (UB).

UB means anything can happen. Your program is invalid and you have no guarantees about anything anymore. You're not allowed to read a , so reasoning about why you might read a specific value in a specific situation is not useful.

That's the point of view of the C++ language. For writing C++ programs that's usually the point of view you need to adopt for yourself. Of course there's a compiler involved. More specifically, a certain compiler in a certain version with a certain configuration compiling this exact piece of code for a specific platform. Including all of that you can do some investigation about why you see a specific result. If you're interested in how compilers work, that's a useful thing to do. But it rarely helps with writing programs. Change a compiler flag or use a slightly different piece of code, and the result might be different.

Answer 3

The standard does not require default zero values for local variables, but some compilers might do zero initialization

Only global variables and static members must be initialized to zero unless explicitly initialized otherwise.