Question

Suppose I have a C++ struct that has both POD and non-POD member variables:

struct Struct {
    std::string String;
    int Int;
};

and in order for my program to produce reproduceable behavior I want to have all member variables initialized at construction. I can use an initializer list for that:

 Struct::Struct() : Int() {}

the problem is as soon as I need to change my struct and add a new POD member variable(say bool Bool) I risk forgetting to add it to the initializer list. Then the new member variable will not be value-initialized during struct construction.

Also I can’t use the memset() trick:

Struct::Struct()
{
   memset( this, 0, sizeof( *this ) ); //can break non-POD member variables
}

because calling memset() to overwrite already constructed non-POD member variables can break those.

Is there a way to enforce value-initialization of all POD member variables without explicitly adding their initialization in this case?

Answer 1

The cleanest way would be to write the auto-initialzed template class initialized<T>:

EDIT: I realize now it can be made even more flexible by allowing you to declare initialized<Struct>. This means that you can declare initialization without modifying the original Struct. The default initialization ‘T()’ was inspired on Prasoons answer.

template<class T>  
struct initialized 
{ 
public: 

     initialized() 
        { value = T(); }

    initialized(T t) 
        { value = t; }

    initialized(const initialized<T>& x) 
        { value = x.value; }

    T* operator &() { return &value; } 

     operator T&() { return value; }     

private: 
     T value; 
};


struct PodStruct 
{            
    std::string String;      
    int Int; 
};  


struct GlorifiedPodStruct 
{            
    std::string String;      
    initialized<int> Int; 
};  

void Test()
{
    GlorifiedPodStruct s;
    s.Int = 1;
    int b = s.Int;
    int * pointer = &s.Int;

    initialized<PodStruct> s2;
}

This compiles, but may need more conversion operators, handling of keywords like volatile, etc. But you get the idea.