Alignment Specifiers

If we want a local object within a function, but want a different function
to initialise the object, we might try something like this:

#include <vector>

typedef unsigned char uchar;

/* The following functions use placement new internally */
std::vector<int> SomeFunc1(uchar*);
std::vector<int> SomeFunc2(uchar*);
std::vector<int> SomeFunc3(uchar*);
std::vector<int> SomeFunc4(uchar*);

#include <iostream>

int main()
{
    uchar buffer[ sizeof( std::vector<int> ) ];

    std::cout << "Choose method of initialisation: ";

    unsigned choice; std::cin >> choice;

    switch( choice )
    {
    case 1: SomeFunc1( buffer ); break;
    case 2: SomeFunc2( buffer ); break;
    case 3: SomeFunc3( buffer ); break;
    case 4: SomeFunc4( buffer ); break;
    }

    /* Now work with the vector */

}

However, the problem with this is that "buffer" may not be aligned properly
to satisfy the alignment requirements of std::vector<int>. There's two
popular remedies:

(1) Dynamic allocation
    
    uchar (&buffer)[ sizeof( std::vector<int> ) ] =
        *new uchar[1][sizeof( std::vector<int> ) ];

This is guaranteed to be aligned adequately, but it's inefficient because
it uses dynamic memory allocation.

(2) Use a union

     union StrictAlign {
        long double a;
        unsigned long b;
        void *c;
    };

    StrictAlign sa[ sizeof(std::vector<int>) / sizeof(StrictAlign)
                    + bool( sizeof(std::vector<int>) % sizeof(StrictAlign)
) ];

    uchar (&buffer)[ sizeof( std::vector<int> ) ]
        = reinterpret_cast<uchar (&)[sizeof( std::vector<int> )]>(sa);

This is fairly efficient, but it isn't guaranteed to work.

It would be nice to be able to specify alignment requirements for an array
of char's, possibly by extending the functionality of one of the following
keywords:

    uchar buffer[ sizeof(std::vector<int>) ] virtual std::vector<int>;

    uchar buffer[ sizeof(std::vector<int>) ] for std::vector<int>;

    uchar buffer[ sizeof(std::vector<int>) ] explicit std::vector<int>;

 

-Tom1s

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