；b在其中说明一个虚函数用来求面积；c利用派生类定义“三角形、正方形、圆形”；d编写主函数并测试。解：程序如下：
i
cludeiostreamusi
g
amespacestdclassbasepublic
virtualfloatareaco
stretur
0

classtria
glepublicbaseprotected
floatbia
height
5
fpublictria
glefloata0floath0bia
aheighthvirtualfloatareaco
stretur
bia
height2
classsquarepublicbaseprotected
floatbia
public
squarefloata0bia
avirtualfloatareaco
stretur
bia
bia

classcirclepublicbaseprotected
floatradiuspublic
circlefloatr0radiusrvirtualfloatareaco
stretur
314radiusradius
i
tmai
tria
glet34squares5circlec2
basepptcout