Uninitialised char[N]

[scarecrow]

Hi,

After coming across some dubiously safe code I've been scanning the C++ standard but cannot find any definitive reference to what the value of an uninitialised char[N] will be. Is it undefined?

I can see that with gcc 4.6 at least [0] is initialised as unsigned(0) - which leaves methods like strcat safe to use immediately... but if it is undefined then the code works only by fluke of the compiler implementation detail.

Cheers,

[antred]

That depends on where the array is defined. If defined at file scope, its elements will be zero-initialized. If defined locally inside a function, its elements will have random values. But there is always an easy way to make sure an array is properly initialized:

// local array
void func()
{
    char localCharArray[ 100 ] = {};
}

// at file scope
char arrayAtFileScope[ 100 ] = {};

// now a char array that is a class member
class MyClass
{
public:
    MyClass() : m_memberArray()
    {
    }

private:
    char m_memberArray[ 100 ];
};

P.S.: Relevant excerpt from the C++ standard:

3.6.2 Initialization of non-local objects
1) Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization
takes place. Zero-initialization and initialization with a constant expression are collectively called static
initialization; all other initialization is dynamic initialization.

[scarecrow]

If defined at file scope, its elements will be zero-initialized. If defined locally inside a function, its elements will have random values

Can you tell me where to find this within the ISO standard?

The definition is within a class member function in this particular case. I do understand how to correctly initialise it, and I always do, this isn't my code. I was just wanting to make certain that my understanding that it will default (by ISO standards) to undefined is correct before I object to this code.

[edit:]

I found the definition for static storage definitions, if that is what you were meaning by file scope:

The storage for objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place.

[scarecrow]

[edit]: I solved this - my test was broken and you were correct in your assertion about free store and static initialisation values. Thanks!