使用编译器查看类的内存布局

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代码

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// main.cpp
class Base
{
protected:
    int n;

public:
    int foo(int p)
    {
        return n + p;
    }

    virtual int bar(int p){
        return n + p;
    }
};

struct Point
{
    double cx, cy;
};

class Derived : public Base
{
public:
    int foo(int p)
    {
        return n + a + p;
    }

protected:
    int a, b;
    Point a_point;
    char c;
};

int main(int argc, char **argv)
{
    return sizeof(Derived);
}

msvc

命令格式：

  • 查看单个类的内存布局： cl <FileName> /d1reportSingleClassLayout[ClassName] 。
  • 查看所有类的内存布局： cl <FileName> /d1reportAllClassLayout 。

使用 cl 查看单个类的内存布局：

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cl main.cpp /d1reportSingleClassLayoutDerived > msvc.txt

输出内容：

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main.cpp

class Derived	size(40):
	+---
 0	| +--- (base class Base)
 0	| | {vfptr}
 4	| | n
	| +---
 8	| a
12	| b
16	| Point a_point
32	| c
  	| <alignment member> (size=7)
	+---

Derived::$vftable@:
	| &Derived_meta
	|  0
 0	| &Base::bar
Microsoft (R) Incremental Linker Version 14.29.30151.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:main.exe
main.obj

参考：https://www.gonwan.com/2010/09/20/c-class-layout-using-msvc/

gcc

命令格式：

  • gcc8 之前：gcc -fdump-class-hierarchy <FileName>。
  • gcc8 之后：gcc -fdump-lang-class <FileName>。

使用例子：

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gcc -fdump-lang-class main.cpp

输出的内容:

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Vtable for Base
Base::_ZTV4Base: 3 entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI4Base)
16    (int (*)(...))Base::bar

Class Base
   size=16 align=8
   base size=12 base align=8
Base (0x0x7fcb184c8420) 0
    vptr=((& Base::_ZTV4Base) + 16)

Class Point
   size=16 align=8
   base size=16 base align=8
Point (0x0x7fcb184c8540) 0

Vtable for Derived
Derived::_ZTV7Derived: 3 entries
0     (int (*)(...))0
8     (int (*)(...))(& _ZTI7Derived)
16    (int (*)(...))Base::bar

Class Derived
   size=48 align=8
   base size=41 base align=8
Derived (0x0x7fcb18374208) 0
    vptr=((& Derived::_ZTV7Derived) + 16)
  Base (0x0x7fcb184c85a0) 0
      primary-for Derived (0x0x7fcb18374208)

参考：

https://stackoverflow.com/questions/2979606/print-layout-of-c-object-with-g-compiler

https://gcc.gnu.org/onlinedocs/gcc-13.2.0/gcc/Developer-Options.html

clang

命令格式：clang -cc1 -fdump-record-layouts <FileName>

使用例子:

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clang -cc1 -fdump-record-layouts main.cpp > clang.txt

输出内容：

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*** Dumping AST Record Layout
         0 | class Base
         0 |   (Base vtable pointer)
         8 |   int n
           | [sizeof=16, dsize=12, align=8,
           |  nvsize=12, nvalign=8]

*** Dumping AST Record Layout
         0 | struct Point
         0 |   double cx
         8 |   double cy
           | [sizeof=16, dsize=16, align=8,
           |  nvsize=16, nvalign=8]

*** Dumping AST Record Layout
         0 | class Derived
         0 |   class Base (primary base)
         0 |     (Base vtable pointer)
         8 |     int n
        12 |   int a
        16 |   int b
        24 |   struct Point a_point
        24 |     double cx
        32 |     double cy
        40 |   char c
           | [sizeof=48, dsize=41, align=8,
           |  nvsize=41, nvalign=8]

参考：
https://eli.thegreenplace.net/2012/12/17/dumping-a-c-objects-memory-layout-with-clang/