C++ - [OOP] Initialization

Default Initialization

Default Initialization is when a variable is initialized without explicit value. They could be in an uninitialized state and accessing them could be an undefined behavior.

• POD types like int, float are initialized to an intermediate value. Accessing them without initialization is an undefined behavior,
• User defined types should either have no constructor provided, or have a default constructor present.

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class A{
    A(){};  // This is NOT default initialization
    A() = default;  // This IS default initialization
};

class B{
    int value;  // There's a synthesized default initializer as well.
};

• static storage duration always default initializes to 0, whereas automatic storage variables are default initialized to intermediate values

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int x;          // automatic storage, intermediate value
static int y;   // default initialized to 0
int *j;         // automatic storage, intermediate value
static int *p   // static storage, default initialized to nullptr

int arr[5];        // Elements have indeterminate values
static int arr[5]; // All elements default-initialized to 0

Best Practices:

• Use brace initialization {} (C++ 11) to explicitly initialize variables

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int i{};    // explicitly initialized to 0;

Initialization Order

We need to make sure the vairable order is consisntent in both the initializer list and the variable definition. One common warning we see is warning: <VAR> will be initialized after [-Wreorder]

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class MyClass{
    /*
        * Below public functions are defined in the order of operation
    */
    public:
        // Step 0: Create this object when first powered on
        MyClass(): previous_time_(millis()), current_status_(Status::UNINITIALIZED)
        {}

        Status current_status_;
        unsigned long previous_time_;
};

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