CODEWITHSUNDEEP
c++variablesinitialization
Brans
Brans

Reputation: 649

Why variables are initialized with max value instead of zero

I did not work with C++ for many years, and now i wrote c++ project with test.

When i started to debug i find strange thing that variables are not initializing with zero by default. For example when i watch to the my class uninitialized variable (unsigned int), i see its value 3452816845 insted of expected zeo... Thats cause errors in unit-tests. I use such initializattyion:

TEST_METHOD(TestPlus)
        {
            Entity* entity = new Entity();
            entity->mCreateOperator(entity->Plus);
            entity->SetContactValue(1);
            entity->SetContactValue(2);
            entity->mProcessLast();
            Assert::IsTrue(entity->GetContactValue((1+2));
        }

I have default constructor for Entity class :

Entity::Entity(void)    {/*some internal array initialization*/}

I considered that when i using new keyword all class variables will be initialized with 0 by C++ runtime..

Did i miss somethisg?

Upvotes: 4

Views: 2844

Answers (3)

AnT stands with Russia
AnT stands with Russia

Reputation: 320391

The data members of your class are left uninitialized specifically because you explicitly asked the compiler to leave them uninitialized. You did this by writing a default constructor that does nothing to initialize them

Entity::Entity(void) { /*some internal array initialization*/ }

If your class had no user-defined constructor, then this syntax

Entity* entity = new Entity();

would trigger so called value-initialization of the new object, which would indeed set all immediate scalar data members of Entity to zero.

However, the very moment you wrote your own default constructor Entity::Entity() you basically told the compiler that you want to suppress value-initialization for Entity class and that you want to initialize such members manually. So now you have to to exactly that: manually initialize all immediate scalar data members of Entity. Since you did not do that in your constructor, these members were left uninitialized.

Upvotes: 5

Mats Petersson
Mats Petersson

Reputation: 129314

Actually the value 3452816845 (0xCDCDCDCD) is a special fill-pattern used by Microsoft's runtime to IDENTIFY uninitialized variables. The compiler does this so that you can detect when you have forgotten to initialize a variable, and this value was picked as a "good" value because it isn't a valid address and is a large number as unsigned, and a "large" negative number in the signed range, so it's typically easy to notice that something is wrong when your code uses one of these values - it's nearly always WAY outside the range you expect it to be in.

Upvotes: 6

Luchian Grigore
Luchian Grigore

Reputation: 258568

Unless the type has a default constructor or is a static variable, it won't be initialized at all. Reading the value is thus undefined behavior.

int a; //namespace scope (static storage) - value-initialized (0)
void foo()
{
    int x;  
    //reading x is illegal, you can only assign to it
    static int y;
    //guaranteed to be value-initialized (0)
}

Upvotes: 3

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