Zero-initializing an array data member in a constructor

Question

I've got an array of class objects, and inside the class object I've got another array that I'd need to initialize to all zeros. The code compiles and runs, but my output is showing C rather than 0.

From the header file:

class Cache {
private:
    int byte[16];
public:
    Cache();
    int getBytes(int);
    ~Cache();
};

From the cpp file

Cache::Cache()  
{
    byte[16]={0};
}

int Cache::getBytes(int j){
    return byte[j];
}

from the other cpp file

for (int i = 0; i < 16; i++) 
{
    for (int j = 0; j < 16; j++)  //visual check of initializes main memory
    {
        cout << cache[i].getBytes(j) << " ";
}
}

Is this being set up correctly? As I mentioned, getBytes is returning 'C's rather than '0's as expected.

Answer 1

Just use value initialization in the constructor initialization list. That is the idiomatic way of doing this in C++.

Cache::Cache() : byte()
{ 
}

Note that C++11 allows this syntax too:

Cache::Cache() : byte{}
{ 
}

In case you're wondering why this works, from the C++ 11 standard (note this also applies to C++03):

C++11 § 8.5,p10

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

That term value-initialized takes us to:

C++11 § 8.5,p7

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type₉ with a user-provided constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T’s implicitly-declared default constructor is non-trivial, that constructor is called.

• if T is an array type, then each element is value-initialized;

• otherwise, the object is zero-initialized.

The third option in this trips the value-initialization of each element; the fourth applies once we get to each of those elements because they're (a) no class types, so (1) and (2) are gone, and (b) not arrays, so (3) is gone. That leaves only the last one, and your elements are zero-initialized.