程序内容：
#include "stdafx.h"   
#include<iostream>
#include<cmath>
using namespace std;
class Triangle{
private:
    float A,B,C;
public:
   Triangle(float a,float b,float c)
   {
        A=a;B=b;C=c;
   }
   void perimeter(float a,float b,float c);
   void area(float a,float b,float c);
};
void Triangle::perimeter(float a,float b,float c)
{
    cout<<endl<<"Perimeter is:"<<A+B+C <<endl;
}
void Triangle::area(float a,float b,float c)
{
    cout<<endl<<"area is :"<<sqrt((A+B+C)*(A-B+C)*(-A+B+C)*(A+B-C))/4.0;
}
int _tmain(int argc, _TCHAR* argv[])
{

    float a,b,c;
    cout<<"Please input lengths of side:"<<endl;
    cin>>a>>b>>c;
    Triangle t;
    t.area(a,b,c);
    t.perimeter(a,b,c);
    return 0;
}
调试结果：
1>g:\c++\c++2010使用\三角形类\三角形类\三角形类.cpp(38): error C2512: “Triangle”: 没有合适的默认构造函数可用
1>
1>生成失败。
1>
1>已用时间 00:00:01.84
========== 生成: 成功 0 个，失败 1 个，最新 0 个，跳过 0 个 ==========

我改成这样了：

#include "stdafx.h"  
#include<iostream>
#include<cmath>
using namespace std;

class Triangle{
private:
    float A,B,C;
public:
    void perimeter(float a,float b,float c);
    void area(float a,float b,float c);
};

void Triangle::perimeter(float a,float b,float c)
{
   
    cout<<endl<<"Perimeter is:"<<A+B+C <<endl;
}

void Triangle::area(float a,float b,float c)
{
   
    cout<<endl<<"area is :"<<sqrt((A+B+C)*(A-B+C)*(-A+B+C)*(A+B-C))/4.0;
}

Triangle t;
int _tmain(int argc, _TCHAR* argv[])
{

    float a,b,c;
    cout<<"Please input lengths of side:"<<endl;
    cin>>a>>b>>c;
   
    t.area(a,b,c);
    t.perimeter(a,b,c);
    return 0;
}

结果调试成了 再次请教啊