How exactly are Destructor calls made - constructor

Question

I wonder how exactly a constructor or destructor is being called for eg in c++? Im especially interested in the OS point of view. Im also interested in the case where we run android app written in java and we want to get info about user session. Can we use conatructor to set time of beginning of session and destr to set time of ending the session and save the data in database? Does actually OS handle destructors calls or something else? Thanks in advance!

Answer 1

I'm not familiar with how Java handles constructor and destructor (Java involves virtual machine layer), but I'll try to answer this from a cpp point of view.
The short answer to your question: OS does not participate in constructor or destructor (unless there's heap allocation, system call...). Compiler will insert calls to constructor and destructor in the right place when it generates machine code.
For a simple program as follows:
class A{
int* i;
public:
A() { i = new int; }
~A() { delete i; }
};
int main() {
A a;
}
Let's examine the assembly code emitted by compiler using objdump:
00000000004006a6 <main>:
4006a6: 55 push %rbp
4006a7: 48 89 e5 mov %rsp,%rbp
4006aa: 48 83 ec 10 sub $0x10,%rsp
4006ae: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
4006b5: 00 00
4006b7: 48 89 45 f8 mov %rax,-0x8(%rbp)
4006bb: 31 c0 xor %eax,%eax
4006bd: 48 8d 45 f0 lea -0x10(%rbp),%rax
4006c1: 48 89 c7 mov %rax,%rdi
4006c4: e8 27 00 00 00 callq 4006f0 <_ZN1AC1Ev>
4006c9: 48 8d 45 f0 lea -0x10(%rbp),%rax
4006cd: 48 89 c7 mov %rax,%rdi
4006d0: e8 3f 00 00 00 callq 400714 <_ZN1AD1Ev>
4006d5: b8 00 00 00 00 mov $0x0,%eax
4006da: 48 8b 55 f8 mov -0x8(%rbp),%rdx
4006de: 64 48 33 14 25 28 00 xor %fs:0x28,%rdx
4006e5: 00 00
4006e7: 74 05 je 4006ee <main+0x48>
4006e9: e8 92 fe ff ff callq 400580 <__stack_chk_fail#plt>
4006ee: c9 leaveq
4006ef: c3 retq
Note that depending on the underlying architecture and compiler, your output might not be the same as mine, but the structure should generally be the same.
You can see compiler automatically generates calls to constructor callq 400714 <_ZN1AD1Ev> and destructor callq 400714 <_ZN1AD1Ev>. The assembly code for constructor is:
00000000004006f0 <_ZN1AC1Ev>:
4006f0: 55 push %rbp
4006f1: 48 89 e5 mov %rsp,%rbp
4006f4: 48 83 ec 10 sub $0x10,%rsp
4006f8: 48 89 7d f8 mov %rdi,-0x8(%rbp)
4006fc: bf 04 00 00 00 mov $0x4,%edi
400701: e8 8a fe ff ff callq 400590 <_Znwm#plt>
400706: 48 89 c2 mov %rax,%rdx
400709: 48 8b 45 f8 mov -0x8(%rbp),%rax
40070d: 48 89 10 mov %rdx,(%rax)
400710: 90 nop
400711: c9 leaveq
400712: c3 retq
400713: 90 nop
Assembly for destructor:
0000000000400714 <_ZN1AD1Ev>:
400714: 55 push %rbp
400715: 48 89 e5 mov %rsp,%rbp
400718: 48 83 ec 10 sub $0x10,%rsp
40071c: 48 89 7d f8 mov %rdi,-0x8(%rbp)
400720: 48 8b 45 f8 mov -0x8(%rbp),%rax
400724: 48 8b 00 mov (%rax),%rax
400727: 48 89 c7 mov %rax,%rdi
40072a: e8 31 fe ff ff callq 400560 <_ZdlPv#plt>
40072f: 90 nop
400730: c9 leaveq
400731: c3 retq
400732: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
400739: 00 00 00
40073c: 0f 1f 40 00 nopl 0x0(%rax)