Uninitialised char[N]

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,

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:

Code:

// 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.

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.

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