#不知道是什么问题，这怎么改

#include <iostream>
using namespace std;

class Shape 
{
public:
    virtual float getArea() = 0;
};

class Circle : public Shape
{
public:
    Circle(float r) { radius = r; }
    void setRadius() 
    { 
        cout << "请输入圆的半径=" << endl;
        cin >> radius;
    }
    float getArea()
    {
        return 3.14 * radius * radius;
    }

private:
    float radius;
};

class Rectangle : public Shape
{
public:
    Rectangle(float l,float w) { length = l, width = w; }
    void setLength() 
    {
        cout << "请输入长方形的长=" << endl;
        cin >> length;
            cout << "请输入长方形的宽=" << endl;
        cin >> width;
    }
    float getArea()
    {
        return length*width;
    }
private:
    float length;
    float width;
};

class Tringle : public Shape
{
    Tringle(float a, float b,float c); 
public:
    void setA()
    {
        cout << "请输入三角形的边长a=" << endl;
        cin >> a;
        cout << "请输入三角形的边长b=" << endl;
        cin >> b;
        cout << "请输入三角形的边长c=" << endl;
        cin >> c;
    }
    float getArea()
    {
        return sqrt(0.5 * (a + b + c) * (0.5 * (a + b + c) - a) * (0.5 * (a + b + c) - b) * (0.5 * (a + b + c) - c));
    }
private:
    float a;
    float b;
    float c;
};

int main()
{
    Shape* s[3]; // 定义一个指针数组

    s[0] = new Circle();
    s[1] = new Rectangle();
    s[2] = new Tringle();

    int i;
    for (i = 0; i < 3; i++) 
    {
        s[i]->getArea();
    }

    for (i = 0; i < 3; i++) // 用完后要释放堆上创建的对象
    {
        delete s[i];
    }
    return 0;
}

帮你修改好了

#define _USE_MATH_DEFINES

#include <iostream>
#include <cmath>
#include <stdexcept>

class Shape
{
public:
    virtual ~Shape() {}
    virtual float getArea() const = 0;
};

class Circle : public Shape
{
public:
    Circle(float radius)
    {
        setRadius(radius);
    }

    float getRadius() const { return _radius; }

    void setRadius(float radius)
    {
        if (radius <= 0.0f)
            throw std::invalid_argument("invalid circle");
        _radius = radius;
    }

    float getArea() const
    {
        return M_PI * _radius * _radius;
    }

private:
    float _radius;
};

class Rectangle : public Shape
{
public:
    Rectangle(float length, float width)
    {
        setLength(length);
        setWidth(width);
    }

    float getLength() const { return _length; }

    void setLength(float length)
    {
        if (length <= 0.0f)
            throw std::invalid_argument("invalid rectangle");
        _length = length;
    }

    float getWidth() const { return _width; }

    void setWidth(float width)
    {
        if (width <= 0.0f)
            throw std::invalid_argument("invalid rectangle");
        _width = width;
    }

    float getArea() const
    {
        return _length * _width;
    }

private:
    float _length;
    float _width;
};

class Tringle : public Shape
{
public:
    Tringle(float a, float b, float c)
    {
        set(a, b, c);
    }

    float getA() const { return _a; }
    float getB() const { return _b; }
    float getC() const { return _c; }

    void set(float a, float b, float c)
    {
        if (a <= 0.0f || b <= 0.0f || c <= 0.0f || (a + b) <= c || (a + c) <= b || (b + c) <= a)
            throw std::invalid_argument("invalid triangle");
        _a = a;
        _b = b;
        _c = c;
    }

    float getArea() const
    {
        float s = (_a + _b + _c) / 2.0f;
        return sqrt(s * (s - _a) * (s - _b) * (s - _c));
    }

private:
    float _a;
    float _b;
    float _c;
};

int main()
{
    try
    {
        Shape *s[3];

        float r;
        std::cout << "请输入圆的半径=";
        std::cin >> r;
        s[0] = new Circle(r);

        float l, w;
        std::cout << "请输入长方形的长=";
        std::cin >> l;
        std::cout << "请输入长方形的宽=";
        std::cin >> w;
        s[1] = new Rectangle(l, w);

        float a, b, c;
        std::cout << "请输入三角形的边长a=";
        std::cin >> a;
        std::cout << "请输入三角形的边长b=";
        std::cin >> b;
        std::cout << "请输入三角形的边长c=";
        std::cin >> c;
        s[2] = new Tringle(a, b, c);

        for (int i = 0; i < 3; i++)
            std::cout << "图形" << i + 1 << "的面积=" << s[i]->getArea() << '\n';

        for (int i = 0; i < 3; i++)
            delete s[i];
    }
    catch (const std::exception &e)
    {
        std::cerr << e.what();
    }
    return 0;
}