The C + + object model uses GCC, clang, and VC + + to display memory layouts for C + + classes

Introduction

Various C + + implementations may have different memory layouts for C + + classes/objects, including the order of data members, the structure of virtual function tables (virtual TABLE:VTBL), the processing of inheritance relationships, and so on. Understanding the layout of C + + classes/objects is a great help in understanding the various mechanisms of C + + and designing and developing correctly and rationally.

Support for mainstream compilers

The 3 mainstream compilers described in this article provide the ability to print/export the memory layout of C + + classes/objects, and now use tables to list their characteristics and usage, and the code files for testing are listed in the post (data.cpp)

Compilers and validated versions	Usage	Description
GCC 4.8.4	gcc-- Fdump-class-hierarchy data.cpp	The exported layout diagram is in a. class file in the same directory as the Data.cpp, such as Data.cpp.002t.class, sample content: VTT for A a::_ztt1a:4u entries 0 ( (& a::_ztv1a) + 24u) 8 ((& a::_ztc1a0_1y) + 24u) ((& a::_ztc1a8_1z) + 24u) ((& A::_ZTV1A ) + 48u) Class A size=16 align=8 Base size=16 base align=8 A (0x0x7f9dbc60c540) 0 Vptridx=0u V Ptr= ((& a::_ztv1a) + 24u) Y (0x0x7f9dbc5c11a0) 0 nearly-empty Primary-for A (0x0x7f9dbc60c540) Subvttid x=8u X (0x0x7f9dbc5ccae0) 0 empty virtual vbaseoffset=-24 Z (0x0x7f9dbc5c1208) 8 Nearly-empty Subvttid x=16u vptridx=24u vptr= ((& a::_ztv1a) + 48u) X (0x0x7f9dbc5ccae0) alternative-path
clang 3.4	Clang- Xclang-fdump-record-layouts data.cpp	Data.cpp is required to do a sizeof evaluation of the related classes , output in stdout, similar to * * * Dumping AST Record Layout 0 | class A 0 | class Y (PRI Mary Base) 0 | (Y vtable pointer) 8 | class Z (base) 8 | (Z vtable pointer) 0 | Class X (virtual Base) (empty) | [Sizeof=16, Dsize=16, align=8 | nvsize=16, nvalign=8]
VC + + 2010	Adding compilation options /d1reportsingleclasslayout< class name > : Output a single class-related layout Or /d1reportallclasslayout: Output all class-related layouts	Compile-time output in the Output window, similar to the following: 1> class Asize (8): 1> +--- 1> | +---(base class Y) 1> 0| | {VBPTR} 1> | +--- 1> | +---(base class Z) 1> 4| | {VBPTR} 1> | +--- 1> +--- 1> +---(virtual base X) 1> +--- 1> 1> A::[email protected]@: 1> 0| 0 1> 1| 8 (Ad (y+0) X) 1> 1> A::[email protected]@: 1> 0| 0 1> 1| 4 (Ad (z+0) X)

The code file used for the test

Data.cpp

#include <cstdio>classX {}; classY: Public VirtualX {}; classZ: Public VirtualX {}; classA: PublicY PublicZ {}; intMainintargcChar**argv) {printf ("Ptr:%lu\n",sizeof(void*)); printf ("X:%lu\n",sizeof(X)); printf ("Y:%lu\n",sizeof(Y)); printf ("Z:%lu\n",sizeof(Z)); printf ("A:%lu\n",sizeof(A));

return 0; }   

The C + + object model uses GCC, clang, and VC + + to display memory layouts for C + + classes