C++ 继承（覆盖构造函数）

Question

I am learning OpenGL w/ C++. I am building the asteroids game as an exercise. I'm not quite sure how to override the constructors:

projectile.h

class projectile
{

protected:
    float x;
    float y;

public:

    projectile();
    projectile(float, float);

    float get_x() const;
    float get_y() const;

    void move();
};

projectile.cpp

projectile::projectile()
{
    x = 0.0f;
    y = 0.0f;
}

projectile::projectile(float X, float Y)
{
    x = X;
    y = Y;
}

float projectile::get_x() const
{
    return x;
}

float projectile::get_y() const
{
    return y;
}

void projectile::move()
{
    x += 0.5f;
    y += 0.5f;
}

asteroid.h

#include "projectile.h"

class asteroid : public projectile
{
    float radius;

public:
    asteroid();
    asteroid(float X, float Y);
    float get_radius();
};

main.cpp

#include <iostream>
#include "asteroid.h"

using namespace std;

int main()
{
    asteroid a(1.0f, 2.0f);

    cout << a.get_x() << endl;
    cout << a.get_y() << endl;
}

error I'm getting:

main.cpp:(.text+0x20): undefined reference to `asteroid::asteroid(float, float)'

Answer 1

You need a asteroid.cpp .

Even though inheriting from projectile , for non-default constructors (ie, asteroid(float,float) ), you still need to define the child class constructor.

You'll also need to define get_radius , as it's not defined in your base class.

Here's how that might look (I've taken the liberty of passing values for radius into both ctors):

#include "asteroid.h"

asteroid::asteroid(float r)
    : projectile()
{
    radius = r;
}

asteroid::asteroid(float x, float y, float r)
    : projectile(x, y)
{
    radius = r;
}

float asteroid::get_radius()
{
    return radius;
}

Answer 2

您可以使用:语法来调用父级的构造函数：

asteroid(float X, float Y) : projectile (x ,y);

Answer 3

Ok, just figured it out.

I actually don't have asteroid constructors defined because I thought they would inherit. But I think I have to do the following in asteroid.h:

asteroid(float X, float Y) : projectile(X, Y){];