Why does default constructor only work with class pointers?

Question

I've been messing around with a default constructor example inspired by this answer. This one works fine:

class Foo {
public:
    int x;
    Foo() = default;
};

int main() {    
    for (int i = 0; i < 100; i++) {
        Foo* b = new Foo();
        std::cout << b->x << std::endl;
    }

But when I try this out with a class instance on the stack it does not! For example, if I instantiate the class like Foo b I get an error saying uninitialised local variable 'b' used.

However, when I change the constructor to Foo() {}, instantiation with Foo b works fine, and I see random garbage from b.x as expected.

Why doesn't a default constructor work when I instantiate a class on the stack?

I compiled the code with MVSC C++17:

Answer 1

这是因为x未初始化。读取未初始化的变量会使您的程序具有未定义的行为，这意味着它可能随时停止“工作”。它也可能在幕后做一些奇怪的事情，而你认为​​一切都很好时却没有意识到。

这些都是零初始化x：

Foo* b = new Foo();
Foo* b = new Foo{};
Foo b{};

虽然这些没有：

Foo *b = new Foo;
Foo b;

MSVC 可能不会捕获并警告您未初始化变量并稍后读取它的所有情况。不需要这样做 - 这就是为什么您可能会在一种情况下收到警告，但在另一种情况下不会收到警告。