C++ 候选构造函数不可行 : no known conversion

Question

这是一个 PNG 类，在类文档中列出了两个构造函数，如下所示。

PNG::PNG    (   string const &  file_name   )   
Creates a PNG image by reading a file in from disk.

Parameters
file_name   Name of the file to be read in to the image.

PNG::PNG    (   size_t  width, size_t   height )        
Creates a default PNG image of the desired dimensions (that is, a width x height opaque white image).

Parameters
width   Width of the new image.
height  Height of the new image.

我使用以下调用构造函数:

int main(){

    PNG in_image=new PNG("in.png");
    size_t width=in_image.width();
    size_t height=in_image.height();
    PNG out_image=new PNG(width,height);
}

但出现以下错误:

main.cpp:5:6: error: no viable conversion from 'PNG *' to 'PNG'
    PNG in_image=new PNG("in.png");
        ^        ~~~~~~~~~~~~~~~~~
./png.h:62:9: note: candidate constructor not viable: no known conversion from
  'PNG *' to 'const PNG &' for 1st argument; dereference the argument with *
    PNG(PNG const & other);
    ^
./png.h:55:9: note: candidate constructor not viable: no known conversion from
  'PNG *' to 'const string &' (aka 'const basic_string<char,
  char_traits<char>, allocator<char> > &') for 1st argument
    PNG(string const & file_name);
    ^
main.cpp:8:6: error: no viable conversion from 'PNG *' to 'PNG'
    PNG out_image=new PNG(width,height);
        ^         ~~~~~~~~~~~~~~~~~~~~~
./png.h:62:9: note: candidate constructor not viable: no known conversion from
  'PNG *' to 'const PNG &' for 1st argument; dereference the argument with *
    PNG(PNG const & other);
    ^
./png.h:55:9: note: candidate constructor not viable: no known conversion from
  'PNG *' to 'const string &' (aka 'const basic_string<char,
  char_traits<char>, allocator<char> > &') for 1st argument
    PNG(string const & file_name);

谁能给我一些提示，说明我的构造函数调用有什么问题？谢谢

Answer 1

你应该这样写:

PNG *in_image=new PNG("in.png");

size_t width=in_image->width();
size_t height=in_image->height();

PNG *out_image=new PNG(width,height);

使用 new 应该会得到一个 PNG*，即指向对象的指针。