C ++候選構造函數不可行：沒有已知的轉換

Question

這是一個PNG類，在類文檔中列出了以下兩個構造函數。

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.

我使用以下代碼來調用構造函數：

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);
}

但是出現以下錯誤：

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);

誰能給我一些暗示，說明我的構造函數調用有什么問題嗎？ 謝謝

Answer 1

您應該這樣寫：

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應該為您提供PNG*即指向對象的指針。

Answer 2

您應該這樣寫：

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

C ++不是Java-您可以使用new來動態分配對象，但如果不這樣做，則不要使用它。 除非確實需要，否則不應該使用new 。

Answer 3

運營商新的類型X將分配內存為X ，並返回它的地址是型X* 。
因此，您應該將其收集到一個指針變量中：

PNG* in_image = new PNG("in.png");        // in_image points to the newly created PNG object
size_t width = in_image.width();
size_t height = in_image.height();
PNG* out_image = new PNG(width,height);   // another object is created and out_image points to it