On Sat, 2009-02-28 at 12:06 -0500, me22 wrote:
>
> GCC allows the Variable Length Arrays feature from C99 in C++ as an extension.
>
> Consider compiling with -ansi -pedantic
>
> ~ Scott
You can see what the compiler is doing for you if you look at the
assembly language. Here is the part where the array gets allocated on
the stack (with my comments added):
call _ZNSirsERi # cin >> array_size
movl -12(%rbp), %eax # load array_size
cltq # convert long to quad
subq $1, %rax # make sure the new stack
addq $1, %rax # pointer meets all the
salq $2, %rax # alignment specs.
addq $15, %rax
addq $15, %rax
shrq $4, %rax
salq $4, %rax
subq %rax, %rsp # allocate the array
movq %rsp, -48(%rbp) # and save pointer to it
I did this on an x86-64 system in 64-bit mode, and I did not worry
through the alignment code to see exactly what's going on. In
particular,
subq $1, %rax
addq $1, %rax
is pretty weird. But the real point is where the array gets allocated on
the stack.
- Bob
