Fork#40686: std::string custom operator new/delete function
Open Date: 2020-08-30 10:32
Last Update: 2020-09-04 11:57
URL for this Ticket:
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Last Changes/Comment on this Ticket:
2020-09-04 11:57 Updated by: keith
Comment:
Reply To marvol
So, you think it is because the implementation of the std::string is in an
external dll?
Yes ... in fact, I know it to be so.
It may explain that my operator new was not called.
It does. There is a fundamental, and significant, difference between the
properties of ELF shared objects, as used on Linux, and the equivalent PE-coff
DLLs, as used on Windows. Do you know what it is?
Indeed, If I compile it with the "-static" linker option my operator new/
delete is called.
Actually, -static-libstdc++ is sufficient.
But why was my operator delete called? Is it because the compiler optimized
the std::string destroy function to be inlined?
Yes. You can study the effect of differing optimization levels, by compiling
with -S -o-, and examining the assembly output. At -O1 (and above), your Obj
class object is optimized away, and your test function includes exactly one
reference to a std::string method — likely a copy constructor — and then your
delete operator code is called; at -O2 (and -O3) even the delete call is
optimized away, and your code is in-lined. At no optimization level, is your
new operator ever called, but it is likely that whatever new operator
implementation std::string knows about is called, (by what I assume to be its
cryptically named copy constructor). That, in itself, is disturbing, because
you have a potentially incompatible new and delete pairing, at any optimization
level of -O1 or above.
I am thinking about the experience that I had with std::vector - maybe it
is because the std::vector is a template that is compiled with the source.
Maybe. You haven't shown me anything, which I might evaluate.
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Ticket Status:
Reporter: marvol
Owner: (None)
Type: Issues
Status: Open
Priority: 1 - Lowest
MileStone: (None)
Component: GCC
Severity: 1 - Lowest
Resolution: Works For Me
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Ticket details:
Hello,
I am writing a custom wrapper for new/delete allocations to get an aligned
memory. I noticed that if I use std::string, operator new/delete are not called
as it is in a linux version of g++ (operators new/delete are called). With the
default optimization level, operator new/delete is not called and with
optimization levels O1, O2, O3 only operator delete is called.
Source:
1. #include <string>
2. #include <cstdio>
3. #include <cstdlib>
4.
5. void* operator new(size_t size)
6. {
7. printf("Calling new: %zu", size);
8. void* address = malloc(size);
9.
10. printf(" address: 0x%p\n", address);
11. return address;
12. }
13.
14. void* operator new[](size_t size)
15. {
16. printf("Calling new[]: %zu", size);
17. void* address = malloc(size);
18.
19. printf(" address: 0x%p\n", address);
20. return address;
21. }
22.
23. void operator delete(void* address)
24. {
25. printf("Calling delete, address: 0x%p\n", address);
26. free(address);
27. }
28.
29. void operator delete[](void* address)
30. {
31. printf("Calling delete[], address: 0x%p\n", address);
32. free(address);
33. }
34.
35. struct Obj
36. {
37. std::string m_str;
38. };
39.
40. void test()
41. {
42. Obj obj;
43. obj.m_str = "long string ......................";
44. }
45.
46. int main()
47. {
48. test();
49.
50. return 0;
51. }
Compiling with the default optimization level:
• g++ main.cpp -std=c++11 -o test.exe
Output:
• NONE
Compiling with the O2 optimization level:
• g++ main.cpp -std=c++11 -O2 -o test.exe
Output:
• Calling delete, address: 0x00542098
I am using:
• OS: Windows 7
• Mingw: 5.4.1
• Shell: cmd MINGW32_NT-6.1 MARTIN-LENOVO 1.0.18(0.48/3/2) 2012-11-21 22:34
i686 Msys
• Uname -a:
• G++: g++ (MinGW.org GCC Build-2) 9.2.0
• GCC: gcc (MinGW.org GCC Build-2) 9.2.0
• ld: GNU ld (GNU Binutils) 2.32
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