结合实例详解"pure Virtual function called"

作者：阿波

链接：http://blog.csdn.net/livelylittlefish/article/details/9750593

(4年前的一篇文章，翻出来共享一下。)

本实例即为经典的讲解C++继承、虚函数、运行时多态的实例。今天我们再用它作为讲解"pure virtual functioncalled"的实例。（在某些平台上也可能输出"pure virtual methodcalled"）

1. 实现基类Shape的area函数

file:testVirtualFunc.cpp

#include <stdio.h>
#include <stdlib.h>

#define PI 3.1415926

class Shape
{
private:
    double ValuePerSquareUnit;

protected:
    Shape(double valuePerSquareUnit):
        ValuePerSquareUnit(valuePerSquareUnit)
    {
    }

public:
    virtual double area() const = 0;

    double value() const
    {
    	return ValuePerSquareUnit * area();
    }

    virtual ~Shape()
    {
        printf("Shape::~Shape() is called");
    }

    double getPerSquareUnit()
    {
        return ValuePerSquareUnit;
    }
};

class Rectangle : public Shape
{
private:
    double Width;
    double Height;

public:
    Rectangle(double width, double height, double valuePerSquareUnit):
        Shape(valuePerSquareUnit),Width(width),Height(height)
    {
    }

    virtual ~Rectangle()
    {
    }

    virtual double area() const
    {
        return Width * Height;
    }

};

class Circle: public Shape
{
    double Radius;

public:
    Circle(double radius, double valuePerSquareUnit):
        Shape(valuePerSquareUnit),Radius(radius)
    {
    }

    virtual ~Circle()
    {
    }

    virtual double area() const
    {
        return PI * Radius * Radius;
    }
};

int main()
{
    Rectangle* pr = new Rectangle(30, 20, 10);
    Circle* pc = new Circle(15, 10);

    //invoke Rectangle::area()
    printf("rectangle: area = %.2f, PerSquareUnit = %.2f, value = %.2f\n",
                pr->area(), pr->getPerSquareUnit(), pr->value());
    //invoke Circle::area()
    printf("circle   : area = %.2f, PerSquareUnit = %.2f, value = %.2f\n",
                pc->area(), pc->getPerSquareUnit(), pc->value());

    Shape* shape;
    shape = pr;
    printf("rectangle: area = %.2f, PerSquareUnit = %.2f, value = %.2f\n",
                shape->area(), shape->getPerSquareUnit(), shape->value());

    shape = pc;
    printf("circle   : area = %.2f, PerSquareUnit = %.2f, value = %.2f\n",
                shape->area(), shape->getPerSquareUnit(), shape->value());

    return 0;
}

编译一下看看。

# g++ -o testVirtualFunc testVirtualFunc.cpp
/tmp/ccmGHV0C.o: In function `Shape::Shape(double)':
testVirtualFunc.cpp:(.text._ZN5ShapeC2Ed[Shape::Shape(double)]+0x13): undefined reference to `vtable for Shape'
/tmp/ccmGHV0C.o: In function `Shape::~Shape()':
testVirtualFunc.cpp:(.text._ZN5ShapeD2Ev[Shape::~Shape()]+0x7): undefined reference to `vtable for Shape'
/tmp/ccmGHV0C.o:(.rodata._ZTI9Rectangle[typeinfo for Rectangle]+0x8): undefined reference to `typeinfo for Shape'
/tmp/ccmGHV0C.o:(.rodata._ZTI6Circle[typeinfo for Circle]+0x8): undefined reference to `typeinfo for Shape'
collect2: ld returned 1 exit status

实现基类Shape的area函数即可消除该编译错误。

即将以下area函数

virtual double area() const;

改为

virtual double area() const
{
}

输出结果如下。

# g++ -o testVirtualFunc testVirtualFunc.cpp
# ./testVirtualFunc
rectangle: area = 600.00, PerSquareUnit = 10.00, value = 6000.00
circle   : area = 706.86, PerSquareUnit = 10.00, value = 7068.58
rectangle: area = 600.00, PerSquareUnit = 10.00, value = 6000.00
circle   : area = 706.86, PerSquareUnit = 10.00, value = 7068.58

2. 将基类Shape定义为纯虚类

即将以下area函数

virtual double area() const;

改为

virtual double area() const = 0;

也可以实现该功能，其输出结果与上相同。

3. 对该纯虚类的程序修改

3.1 在Shape类的构造函数中直接调用纯虚函数

修改Shape的构造函数如下。

Shape(double valuePerSquareUnit):
    ValuePerSquareUnit(valuePerSquareUnit)
{
    std::cout << "creating shape, area = " << area() << std::endl;
}

则会在编译时出现如下错误：

# g++ -o testPureVirtualFunc testPureVirtualFunc.cpp
testPureVirtualFunc.cpp: In constructor ‘Shape::Shape(double)’:
testPureVirtualFunc.cpp:18: warning: abstract virtual ‘virtual double Shape::area() const’ called from constructor
/tmp/cc2WbWlw.o: In function `Shape::Shape(double)':
testPureVirtualFunc.cpp:(.text._ZN5ShapeC2Ed[Shape::Shape(double)]+0x2c): undefined reference to `Shape::area() const'
collect2: ld returned 1 exit status

即不能在构造函数中直接调用纯虚函数。

(Meyers, 3rd edition, Item 9: "Never call virtual functionsduring construction or destruction.") --in the third edition of Scott Meyers's "Effective C++"

3.2 在Shape类的构造函数中间接调用纯虚函数

修改Shape的构造函数如下。

Shape(double valuePerSquareUnit):
    ValuePerSquareUnit(valuePerSquareUnit)
{
    //std::cout << "creating shape, area = " << area() << std::endl;
    std::cout << "creating shape, value = " << value() << std::endl;
}

其执行结果如下。

# g++ -o testPureVirtualFunc testPureVirtualFunc.cpp
# ./testPureVirtualFunc
pure virtual method called
terminate called without an active exception
Aborted (core dumped)

进入core dump文件调试，可以发现，是在value函数中调用area时出现该问题的。

(gdb) bt
#0  0x00110402 in __kernel_vsyscall ()
#1  0x00a8e690 in raise () from /lib/libc.so.6
#2  0x00a8ff91 in abort () from /lib/libc.so.6
#3  0x06894ba0 in __gnu_cxx::__verbose_terminate_handler () from /usr/lib/libstdc++.so.6
#4  0x06892685 in ?? () from /usr/lib/libstdc++.so.6
#5  0x068926c2 in std::terminate () from /usr/lib/libstdc++.so.6
#6  0x06892d65 in __cxa_pure_virtual () from /usr/lib/libstdc++.so.6
#7  0x08048b00 in Shape::value (this=0x8261008) at testPureVirtualFunc.cpp:27
#8  0x08048cd0 in Shape (this=0x8261008, valuePerSquareUnit=10) at testPureVirtualFunc.cpp:19
#9  0x08048d44 in Rectangle (this=0x8261008, width=30, height=20, valuePerSquareUnit=10) at testPureVirtualFunc.cpp:49
#10 0x08048920 in main () at testPureVirtualFunc.cpp:87

3.3 为什么会出现"pure virtual functincalled"

要想搞清楚这个问题，要思考这两件事情：

a. 具体是在什么时刻调用了纯虚函数？

b. 是谁打印的"pure virtual functioncalled"？

带着这两个问题，我们要深入调用内部，找到根本原因。调试过程如下。

注意，编译时要加上"-g"选项，为调试准备符号。

# g++ -g -o testPureVirtualFunc testPureVirtualFunc.cpp

# gdb ./testPureVirtualFunc
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) l
78          virtual double area() const
79          {
80              return PI * Radius * Radius;
81          }
82      };
83
84
85      int main()
86      {
87          Rectangle* pr = new Rectangle(30, 20, 10);
(gdb) b 87
Breakpoint 1 at 0x80488e8: file testPureVirtualFunc.cpp, line 87.
(gdb) r
Starting program: /home/abo/linux/2009-12-28 testPureVirtualFuncCalled/testPureVirtualFunc 

warning: Missing the separate debug info file: /usr/lib/debug/.build-id/ac/2eeb206486bb7315d6ac4cd64de0cb50838ff6.debug

warning: Missing the separate debug info file: /usr/lib/debug/.build-id/88/27308433e33aeefb560f42fb133577c8936f20.debug

warning: Missing the separate debug info file: /usr/lib/debug/.build-id/92/8ab51a53627c59877a85dd9afecc1619ca866c.debug

warning: Missing the separate debug info file: /usr/lib/debug/.build-id/db/8cb95645d5df469d4aece301cdb5e60087be21.debug

warning: Missing the separate debug info file: /usr/lib/debug/.build-id/ba/4ea1118691c826426e9410cafb798f25cefad5.debug

Breakpoint 1, main () at testPureVirtualFunc.cpp:87
87          Rectangle* pr = new Rectangle(30, 20, 10);
(gdb) step
Rectangle (this=0x909d008, width=30, height=20, valuePerSquareUnit=10) at testPureVirtualFunc.cpp:49
49              Shape(valuePerSquareUnit),Width(width),Height(height)
(gdb) p this
$1 = (Rectangle * const) 0x909d008
(gdb) p *this
$2 = {<Shape> = {_vptr.Shape = 0x0, ValuePerSquareUnit = 0}, Width = 0, Height = 0}
(gdb) info line Rectangle
Line 42 of "testPureVirtualFunc.cpp" is at address 0x8048d08 <Rectangle> but contains no code.
(gdb) step
Shape (this=0x909d008, valuePerSquareUnit=10) at testPureVirtualFunc.cpp:16
16              ValuePerSquareUnit(valuePerSquareUnit)
(gdb) p this
$3 = (Shape * const) 0x909d008
(gdb) p *this
$4 = {_vptr.Shape = 0x0, ValuePerSquareUnit = 0}
(gdb) x/4 0x909d008
0x909d008:      0x00000000      0x00000000      0x00000000      0x00000000
(gdb) step
19              std::cout << "creating shape, value = " << value() << std::endl;
(gdb) x/4 0x909d008
0x909d008:      0x08048fa8      0x00000000      0x40240000      0x00000000
(gdb) p sizeof(ValuePerSquareUnit)
$5 = 8
(gdb) p sizeof(*this)
$6 = 12
(gdb) p *this
$7 = {_vptr.Shape = 0x8048fa8, ValuePerSquareUnit = 10}
(gdb) x/15i 0x8048fa8
0x8048fa8 <_ZTV5Shape+8>:       xor    %al,0x8c6e0804(%edi)
0x8048fae <_ZTV5Shape+14>:      add    $0x8,%al
0x8048fb0 <_ZTV5Shape+16>:      cmp    $0x8c,%al
0x8048fb2 <_ZTV5Shape+18>:      add    $0x8,%al
0x8048fb4:      add    %al,(%eax)
0x8048fb6:      add    %al,(%eax)
0x8048fb8 <_ZTV6Circle>:        add    %al,(%eax)
0x8048fba <_ZTV6Circle+2>:      add    %al,(%eax)
0x8048fbc <_ZTV6Circle+4>:      int3
0x8048fbd <_ZTV6Circle+5>:      popl   (%eax,%ecx,1)
0x8048fc0 <_ZTV6Circle+8>:      mov    %es:(%eax,%ecx,1),%eax
0x8048fc4 <_ZTV6Circle+12>:     or     0x48bd808(%esp,%eax,1),%cl
0x8048fcb <_ZTV6Circle+19>:     or     %ch,(%eax)
0x8048fcd <_ZTI6Circle+1>:      movsl  %ds:(%esi),%es:(%edi)
0x8048fce <_ZTI6Circle+2>:      add    $0x8,%al
(gdb) step
Shape::value (this=0x909d008) at testPureVirtualFunc.cpp:27
27              return ValuePerSquareUnit * area();
(gdb) x/4 0x909d008
0x909d008:      test   $0x8f,%al
0x909d00a:      add    $0x8,%al
0x909d00c:      add    %al,(%eax)
0x909d00e:      add    %al,(%eax)
(gdb) x/4w 0x909d008
0x909d008:      0x08048fa8      0x00000000      0x40240000      0x00000000
(gdb) step
pure virtual method called
terminate called without an active exception

Program received signal SIGABRT, Aborted.
0x00110402 in __kernel_vsyscall ()
(gdb) 

我们可以通过C++filt命令对那些看不太懂的符号进行解析，

#c++filt _ZTV5Shape
vtablefor Shape
#c++filt _ZTV6Circle
vtablefor Circle

具体的解释，我们可以查看 c++filt 的 man 手册页。

通过strace命令，我们可以清楚地看到程序在core dump之前调用write在标准输出中打印出"pure virtual method called"，如下。

# ./testPureVirtualFunc
pure virtual method called
terminate called without an active exception
Aborted (core dumped)
# strace ./testPureVirtualFunc

其部分strace结果如下。

mmap2(0xa65000, 1410608, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xa65000
mmap2(0xbb8000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153) = 0xbb8000
mmap2(0xbbb000, 9776, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xbbb000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f28000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7f28af0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xbb8000, 8192, PROT_READ)     = 0
mprotect(0xbe7000, 4096, PROT_READ)     = 0
mprotect(0x68be000, 16384, PROT_READ)   = 0
mprotect(0xa61000, 4096, PROT_READ)     = 0
munmap(0xb7f29000, 71683)               = 0
brk(0)                                  = 0x8410000
brk(0x8431000)                          = 0x8431000
write(2, "pure virtual method called\n", 27pure virtual method called
) = 27
write(2, "terminate called without an acti"..., 45terminate called without an active exception
) = 45
rt_sigprocmask(SIG_UNBLOCK, [ABRT], NULL, 8) = 0
gettid()                                = 25398
tgkill(25398, 25398, SIGABRT)           = 0
--- SIGABRT (Aborted) @ 0 (0) ---
+++ killed by SIGABRT (core dumped) +++

Reference

http://www.artima.com/cppsource/pure_virtual.html

http://www.codeproject.com/KB/cpp/PureVirtualFunctionCall.aspx

http://blog.csdn.net/kikikind/archive/2008/07/13/2645316.aspx

http://www.cnblogs.com/whjiang/archive/2007/10/22/932880.html

http://support.microsoft.com/kb/120919/zh-cn

http://support.microsoft.com/kb/120919/en-us/