Notes from talking with a friend about C++, and member initialization in a class declaration.
In C++98 you could:
- Initialize a static const member. That's it.
C++11 is more permissive on this. See
this document.
A simple test-code:
class A {
public:
int a = 5;
};
int func() {
A x;
return x.a;
}
> g++ asdf.cpp -std=c++0x
asdf.cpp:3:11: sorry, unimplemented: non-static data member initializers
asdf.cpp:3:11: error: ISO C++ forbids in-class initialization of non-const static member ‘a’
Too old.
Compiler: GCC g++ 4.7.2
> g++ -std=c++11 ./asdf.cpp -c
> objdump -d asdf.o
asdf.o: file format elf32-i386
Disassembly of section .text:
00000000 <_Z4funcv>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 10 sub $0x10,%esp
6: c7 45 fc 05 00 00 00 movl $0x5,-0x4(%ebp)
d: 8b 45 fc mov -0x4(%ebp),%eax
10: c9 leave
11: c3 ret
Quite interesting - the "implicit constructor" was inlined in the code. I was expecting a function to be created for this, though this seems better (faster), but it might lead to a code bloat (unless the behavior changes if more fields are present, which would make sense).
Compiler: clang 3.2
> clang ./asdf.cpp -std=c++11 -c
> objdump -d asdf.o
asdf.o: file format elf32-i386
Disassembly of section .text:
00000000 <_Z4funcv>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 18 sub $0x18,%esp
6: 8d 45 f8 lea -0x8(%ebp),%eax
9: 89 04 24 mov %eax,(%esp)
c: e8 fc ff ff ff call d <_Z4funcv+0xd>
11: 8b 45 f8 mov -0x8(%ebp),%eax
14: 83 c4 18 add $0x18,%esp
17: 5d pop %ebp
18: c3 ret
Disassembly of section .text._ZN1AC1Ev:
00000000 <_ZN1AC1Ev>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 08 sub $0x8,%esp
6: 8b 45 08 mov 0x8(%ebp),%eax
9: 89 45 fc mov %eax,-0x4(%ebp)
c: 8b 45 fc mov -0x4(%ebp),%eax
f: 89 04 24 mov %eax,(%esp)
12: e8 fc ff ff ff call 13 <_ZN1AC1Ev+0x13>
17: 83 c4 08 add $0x8,%esp
1a: 5d pop %ebp
1b: c3 ret
Disassembly of section .text._ZN1AC2Ev:
00000000 <_ZN1AC2Ev>:
0: 50 push %eax
1: 8b 44 24 08 mov 0x8(%esp),%eax
5: 89 04 24 mov %eax,(%esp)
8: 8b 04 24 mov (%esp),%eax
b: c7 00 05 00 00 00 movl $0x5,(%eax)
11: 58 pop %eax
12: c3 ret
>
This one created the implicit constructor I was expecting. Well, actually two constructors, one in the other. Interesting.
TODO: Visual C++?
