Re: std::vector padding behavior and alignment

"Alf P. Steinbach" <>
Thu, 30 Apr 2009 04:00:08 +0200
* Andrew Tomazos:

Please consider the following:

We construct a std::vector, push_back 1000 items and then destroy it.
Each normal constructor. copy constructor and destructor is counted...

#include <vector>
#include <cstdio>

class A
    A(int x) : m_x(x) { s_iConstructs++; }
    A(const A& a) { s_iCopies++; }
    ~A() { s_iDestructs++; }

    static void dump()
        std::printf("s_iConstructs == %d\n", s_iConstructs);
        std::printf("s_iCopies == %d\n", s_iCopies);
        std::printf("s_iDestructs == %d\n", s_iDestructs);

    int m_x;

    static int s_iConstructs;
    static int s_iCopies;
    static int s_iDestructs;

int A::s_iConstructs(0);
int A::s_iCopies(0);
int A::s_iDestructs(0);

int main(int argc, char** argv)
        std::vector<A> v;
        for (int i = 0; i < 1000; i++)


The output from gcc version 4.3.2:

    s_iConstructs == 1000
    s_iCopies == 2023
    s_iDestructs == 3023

The output from Microsoft C/C++ Compiler v15.x:

    s_iConstructs == 1000
    s_iCopies == 3137
    s_iDestructs == 4137

Clearly when a new item is pushed onto the vector, the underlying
memory is not reallocated each time. If it were, the number of copy
constructor calls would be closer to 500,000. Instead they are closer
to 3,000.

Furthermore as type A has no default constructor, we must conclude
that std::vector is heap allocating an uninitialized buffer beyond the
size that it requires, and leaving that extra area uninitialized.

It must allocate its buffer with malloc and not with new[], as new[]
requires that the type be default constructable. (Is this correct?)

No. The internal buffer will in practice be allocated as a buffer of 'char' or
'unsigned char', via 'std::allocator::allocate'. It's a buffer of bytes
(although at the 'std::vector' code level accessed as a buffer of uninitialized
T), and it can be allocated using just about any allocation mechanism.

When a new item is pushed, it must be using placement new to copy
construct the items in-place over this uninitialized memory.

Yes, effectively. It does that using 'std::allocator::construct', which in turn
does the placement new.

Now some types have certain alignment restrictions (such as being 4-
byte aligned or 16-byte aligned to the memory address space).

When new SomeAlignedType[1000] is called, the new[] memory allocator
must be aware of these alignment requirements, and select an
appropriately aligned section of memory for the type.

Yes, but only implicitly. In practice it doesn't use different alignments for
different types. It just uses some suitably large alignment, say 8.

My question is this: How can std::vector use malloc and still respect
these alignment restrictions?

Both 'malloc' and 'new' return a pointer suitably aligned for any type.

 By what method does std::vector detect
the alignment requirements of a certain type,

In practice it doesn't. In principle it could. C++0x introduces some support for
alignment issues.

and then by what method
does it allocate its array?

A std::vector uses the allocator that it's been instantiated with. By default
that's 'std::allocator<T>'. Whose 'allocate' routine in turn uses '::operator
new', the global byte level allocation function (which in practice probably just
forwards to 'malloc', but this is implementation-dependent).

If the answer is implementation dependent than specifically how does
gcc and msvc do it?

Oh, check the standard library implementation source code.

For MSVC that should be particularly easy, at least if you have Visual Studio
which has an excellent debugger: just step into the push_back call.

Is this facility documented and available to user-defined container

Yes, look up 'std::allocator'.

However, it's not the best design ever made.

Cheers & hth.,

- Alf

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