ARM C++ constructors are different

Programmers who have, like me, come to ARM from other architectures may find one or two things surprising.

For example, the following code is quite simple C++:

class A {
public:
    A() {}
    ~A() {}
};

A a;

int main(void)
{
    return 0;
}

But if we compile it and examine it with gdb, there’s something a bit unexpected:

$ gcc -g constructor.cpp -o constructor
$ gdb ./constructor 
GNU gdb (GDB) 7.5.91.20130417-cvs-ubuntu
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "arm-linux-gnueabihf".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/linaro/constructor...done.
(gdb) ptype A::A
type = class A {
  public:
    A(void);
    ~A(int);
} *(A * const)
(gdb) ptype A::~A
type = void *(A * const)
(gdb)

The types of the constructor and destructor are not quite what we might expect. Traditionally a C++ constructor or destructor does not return a value, however on ARM things are different – the constructor returns a pointer to class A, and the destructor returns a pointer to void.

Why is this the case? On ARM constructors and destructors are specified differently in order to provide scope for optimizing calls to a chain of constructors or destructors while minimizing the pushing stack frames (tail call optimization). There’s a very helpful document available here called the C++ ABI for the ARM Architecture which details the differences between the ARM ABI and the Generic GNU C++ ABI including this little quirk.

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