[Solved] Why compilers put zeros into arrays while they do not have to?

Question

A structValue{}; is aggregate initialization, so 0 are guaranteed.

As A has no user provided constructor because explicitly defaulted constructors do not count as such, the same applies for value initialization as in A* psstructValue = new A();.

For the default initialization cases: Reading uninitialized variables is UB, and Undefined behavior is undefined. The compiler can do with that whatever it wants. Showing you 0 is just as legal as crashing. Maybe there even were 0 in the memory you read by chance. Maybe the compilers felt like 0 initializing. Both equally fine from the standard’s point of view.

That being said, you have a better chance of seeing garbage when testing with Release / optimized builds. Debug builds tend to do extra stuff to help diagnosing problems, including doing some extra initialization.

(For the record: gcc and clang with -O3 appear to do no unnecessary initialization on my Linux system at first glance. Nevertheless, I got “all zeroes” for every case. That appears to be by chance.)

Accepted Answer

A structValue{}; is aggregate initialization, so 0 are guaranteed.

As A has no user provided constructor because explicitly defaulted constructors do not count as such, the same applies for value initialization as in A* psstructValue = new A();.

For the default initialization cases: Reading uninitialized variables is UB, and Undefined behavior is undefined. The compiler can do with that whatever it wants. Showing you 0 is just as legal as crashing. Maybe there even were 0 in the memory you read by chance. Maybe the compilers felt like 0 initializing. Both equally fine from the standard’s point of view.

That being said, you have a better chance of seeing garbage when testing with Release / optimized builds. Debug builds tend to do extra stuff to help diagnosing problems, including doing some extra initialization.

(For the record: gcc and clang with -O3 appear to do no unnecessary initialization on my Linux system at first glance. Nevertheless, I got “all zeroes” for every case. That appears to be by chance.)