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"）
- 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
- 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。
- 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
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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) +++
参考：
https://www.artima.com/cppsource/pure_virtual.html
https://blog.csdn.net/livelylittlefish/article/details/9750593
