C++抽象类无法实例化

Question

我有这个抽象类

class ImageIO {
protected:
    ImageIO() {}
public:
    virtual bool load(const std::string & filename, const std::string & format) = 0;
    virtual bool save(const std::string & filename, const std::string & format) = 0;
}

以及实现

class Image : public Array2D<Color>, public ImageIO {

private:
    static int case_insensitive(string s) {
        transform(s.begin(), s.end(), s.begin(), tolower);

        if (s.compare("ppm") == 0) {
            return 1;
        }
        return 0;
    }

public:
    bool load(const std::string& filename, const std::string& format) {
        if (case_insensitive(format)) {
            float* data = ReadPPM(filename.c_str(), (int*)width, (int*)height);

            for (int i = 0, j = 0; i < width * height; i++, j += 3) {
                buffer[i] = Color(data[j], data[j + 1], data[j + 2]);
            }

            delete[] data;
            return true;
        }

        return false;
    }

    bool save(const std::string& filename, const std::string& format) {
        if (case_insensitive(format)) {

            float* data = new float[width * height * 3];
            for (int i = 0, j = 0; i < width * height; i++, j += 3) {
                data[j] = buffer[i].x;
                data[j + 1] = buffer[i].y;
                data[j + 2] = buffer[i].z;
            }

            bool write_check = WritePPM(data, width, height, filename.c_str());
            delete[] data;
            return write_check;
        }

        return false;
    }
};

如果我尝试这样做，我会不断收到抽象类无法实例化的错误

图像测试 = Image();

在我的主要内容中。 我还尝试在头文件中声明 t= 这两个方法

class Image : public Array2D<Color>, public ImageIO {

private:
    static int case_insensitive(std::string);
public:
    bool ImageIO::load(const std::string& filename, const std::string& format);
    bool ImageIO::save(const std::string& filename, const std::string& format);
};

但随后我遇到了 Unresolved external symbol 错误。我实现这两个函数的方式或者我声明类的方式是否有问题？

编辑:如果我在头文件中声明这两个方法，则错误为:

error LNK2019: unresolved external symbol "public: virtual bool __thiscall image::Image::[image::ImageIO]::load(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (?load@?QImageIO@image@@Image@2@UAE_NABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0@Z) referenced in function _main
error LNK2019: unresolved external symbol "public: virtual bool __thiscall image::Image::[image::ImageIO]::save(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (?save@?QImageIO@image@@Image@2@UAE_NABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0@Z) referenced in function _main

如果我不这样做，错误是

error C2259: 'image::Image': cannot instantiate abstract class

我还添加了覆盖，但没有任何变化。

Answer 1

要专门回答有关无法解析的外部符号的问题，您不需要在保存/加载函数上使用 ImageIO 父说明符。您声明 Image 本身具有这些函数，它们覆盖父级上的抽象函数。然后，您在 cpp 文件中实现它们时需要指定 Image。

图像.hpp

class Image : public Array2D<Color>, public ImageIO {

private:
    static int case_insensitive(std::string);
public:
    bool load(const std::string& filename, const std::string& format) override;
    bool save(const std::string& filename, const std::string& format) override;
};

图像.cpp

bool Image::load(const std::string& filename, const std::string& format) {
    if (case_insensitive(format)) {

        //etc.
    }

    return false;
}

bool Image::save(const std::string& filename, const std::string& format) {
    if (case_insensitive(format)) {

       //etc.
    }

    return false;
}