Question

这是一个PNG类，在类文档中列出了两个构造函数。

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.

我使用以下命令来调用构造函数：

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);
}

但是下面有错误：

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);

有人可以提一下我的构造函数调用有什么问题吗？ THX

Answer 1

你应该这样写：

PNG*

使用new应该会得到$scope.allBrands = function(){ var brands = []; return BrandService.get().then(function(data) { angular.forEach(data, function(value, key) { return brands.push(value.name); }); return brands; }); }; console.log($scope.allBrands());，即指向对象的指针。

Answer 2

你应该这样写：

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动态分配对象，但是当您不这样做时，不要使用它。除非您确实需要，否则不应使用new。

Answer 3

类型为X的operator new将为X分配内存并返回X*类型的地址。
所以你应该在指针变量中收集它：

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

C ++ Candidate构造函数不可行：没有已知的转换

3 个答案: