C++ Candidate constructor not viable: no known conversion

Question

This is a PNG class with two constructors listed as below in class document.

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.

I use the following to invoke constructors:

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);
}

But got errors below:

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);

Could anyone give some hints on what is wrong with my constructor invoke? Thx

Answer 1

You should write it like this :

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);

Using new should get you a PNG* ie a pointer to the object.

Answer 2

You should write it like this:

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

C++ is not Java - you can use new to dynamically allocate an object, but you don't use it when you're not doing that. You shouldn't use new unless you actually need it.

Answer 3

operator new for type X will allocate memory for X and return it's address which is of type X* .
So you should collect it in a pointer variable:

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