Why is zero-initialization not the default for non-initialized variables in c++? Is there a compiler option to force it?

Question

In most languages I know, scalar-type variables are zero-initialized by default, if they were not initialized in the code. Why doesn't this happen in c/c++? The only reason I could think of is performance, but

• is it really that performance-consuming, if I do initialize it?
• is it preferable to have undefined behavior?
• if I want to avoid undefined behavior, I have to initialize it anyway, so what did I win?

Wouldn't it be easier to explicitly tell the compiler somehow not to zero-initialize a variable, if this might be a performance issue?

An finally my question: Is there a gcc option to tell the compiler to zero-init by default?

Answer 1

One of the founding principals of c++ is to not force developers to pay for what they don't use. If you write something like int x; x = 1; then you shouldn't have to pay for the zero initialization of x, even if that cost happens to be very tiny.

Edit : Regarding your other two points

is it preferable to have undefined behavior?

Undefined behavior is not necessarily a bad thing to have in the language (you can argue both ways). It's definitely a bad thing if you write code that causes it. Notably it gives more freedom to implementers and enables important optimizations.

if I want to avoid undefined behavior, I have to initialize it anyway, so what did I win?

It's not undefined behavior to have an uninitialized variable. It's undefined behavior to try to read from one.