Question

OK first and foremost, performance is most important here so I doubt a map would work. I have a list of structs (about 16 of them) like

struct A { ... };
struct B { ... }; 
...

each are different and each are of different sizes.

I’m wondering what elegant way we might be able to do something along the lines of:

char BufferA[sizeof(struct A)];
char BufferB[sizeof(struct B)];

then write some method or mapping to return BufferA if you are working with struct A. Speed is definitely the most important, I imagine using templates would help but I’m not sure it the whole thing can be templatized.

Update*** Sorry for not being clear, the buffers are all pre-allocated. I just need a very fast way to get the proper Buffer given a struct type.

Update 2*** Sorry for not specifying, alignment is an important trait here and I do in fact byte-align each struct with #pragma pack(push, 1)

Answer 1

template<typename X>
struct Buffer
{
    static char *ptr()
    {
        // Note if no alignment is needed for your use then
        // just a simple "static char buf[sizeof(X)]; return buf;"
        // would be sufficient instead of the following.
        union Aligner {
            X x;
            char buf[sizeof(X)];
        };

        static Aligner a;

        return a.buf;
    }
};

struct B
{
    int x, y, z;
};

void foo()
{
    Buffer<B>::ptr()[2] = 12;
}

With g++ -O2 the code above generates just a fixed memory write operation in foo.

.globl _Z3foov
    .type   _Z3foov, @function
_Z3foov:
.LFB1:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5

    movb    $12, _ZZN6BufferI1BE3ptrEvE1a+2   <=== this is the assignment

    popl    %ebp
    ret
    .cfi_endproc