CODEWITHSUNDEEP
c++
user3663882
user3663882

Reputation: 7357

Default value of members in a struct

In the struct

struct A
{
    int a;
    A(){ }
};

A a;

Is it specified which value has a.a? Doe we have UB if we try to read a.a?

Upvotes: 0

Views: 85

Answers (2)

vsoftco
vsoftco

Reputation: 56547

If A a; has static storage duration (like defining it outside main()), or thread storage duration (i.e. defined with thread_local in C++11 or later), then a is zero-initialized (thanks @Praetorian for the comment)

3.6.2/2 [basic.start.init] Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place.

In your case, this means that each member of your object A a will be zeroed, hence a.a will be zeroed, then the constructor A() will run (which won't do anything). At the end of the day, a.a will be zero.

If A a has non-static/non-thread storage duration (e.g. having A a; inside a function), then no zero initialization is performed, and again the constructor doesn't do anything. So you have UB (undefined behaviour) if you try to read a.a, since the latter is left un-initialized.

Upvotes: 4

Muhammad Ali
Muhammad Ali

Reputation: 599

If you have used java you might know that variables are assigned a default value in it but that's not the case in c++. Here you only allocate a chunk of memory for your variable. Whatever garbage it contained before it was allocated to you comes with it. So we call it a garbage value. You have to initialize it to your desired value in the constructor of the class or the structure.

Upvotes: 2

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