How to do the equivalent of memset(this, …) without clobbering the vtbl?

Question

I know that memset is frowned upon for class initialization. For example, something like the following:

class X { public: 
X() { memset( this,          


        

Answer 1

Leverage the fact that a static instance is initialised to zero: https://ideone.com/GEFKG0

template 
struct clearable
{
    void clear()
    {
        static T _clear;
        *((T*)this) = _clear;
    };
};

class test : public clearable
{
    public:
        int a;
};

int main()
{
    test _test;
    _test.a=3;
    _test.clear();

    printf("%d", _test.a);

    return 0;
}

However the above will cause the constructor (of the templatised class) to be called a second time.

For a solution that causes no ctor call this can be used instead: https://ideone.com/qTO6ka

template 
struct clearable
{
    void *cleared;
    clearable():cleared(calloc(sizeof(T), 1)) {}

    void clear()
    {
        *((T*)this) = *((T*)cleared);
    };
};

...and if you're using C++11 onwards the following can be used: https://ideone.com/S1ae8G

template 
struct clearable
{
    void clear()
    {
        *((T*)this) = {};
    };
};