不能将参数2从'mxArray **'转换为'mwArray&' [英] cannot convert parameter 2 from 'mxArray **' to 'mwArray &'

问题描述

我想从Matlab使用deloytool创建一个C ++共享库，并在MVS中使用它。我编译一个函数名'foo.m'，结果我得到文件列表（.h，.cpp，.lib，...），我发现'fooCpplib.h'中的函数如下：

  extern LIB_fooCpplib_CPP_API void MW_CALL_CONV foo（int nargout，mwArray& y，const mwArray& x）;然后我创建了一个MVS项目（2010），窗体窗体应用程序，有2个文本框和2个点击按钮，一个名为inBox的文本框，另一个名为outBox的文本框。 button_click中的代码如下：
  private：System :: Void button1_Click（System :: Object ^ sender，System :: EventArgs ^ e）{
 double input = System :: Double :: Parse（inBox-> Text）; 
 mxArray * x_ptr; 
 mxArray * y_ptr = NULL; 
 double * y; 
 
 //创建一个mxArray以输入到mlfFoo 
 x_ptr = mxCreateDoubleScalar（input）; 
 
 //调用实现函数
 //注意第二个输入参数应该是& y_ptr而不是y_ptr。 
 foo（1，& y_ptr，x_ptr）; 
 
 // mlfFoo的返回值是一个mxArray。 
 //使用mxGetpr获取它包含的数据的指针。 
 y =（double *）mxGetPr（y_ptr）; 
 
 //以
 outBox-> Text =+ * y的形式显示结果; 
 
 //清理内存
 mxDestroyArray（x_ptr）; 
 mxDestroyArray（y_ptr）; 
} 
  
当我构建项目时，发生错误如下：
 错误C2664：'foo'：无法将参数2从'mxArray **'转换为'mwArray&'
是不相关的;转换需要reinterpret_cast，C风格的转换或函数式
转换。 
  
注意：我已经在.cpp源文件中包含了'fooCpplib.h'。
 
 
 
任何人都可以帮助我这个！ 
谢谢！
解决方案
当参数声明为
  TypeName& argName 
  
 c $ c> argName 是一个引用。在C ++中你可以通过引用传递参数，这允许函数修改你传递给它的变量（除了别的以外）。
 
 
 
如果你有一个指针，函数需要引用，您需要使用星号解除引用指针，而不是使用带&号的指针的地址：
  foo（1，* y_ptr，* x_ptr）; 
 // ^ ^ 
 // | | 
 //这里和这里
  
你可以想到变量，指针，指针指针，等等，根据间接级别。变量具有零间接水平;指针具有一个间接水平; 
 
 
 
添加一个&符号增加了间接的水平;添加星号会减少它。像变量一样，引用的间接级别为零。如果你有一个指针，并且你需要一个变量，你必须通过添加一个星号来减少间接的水平。
 
 
 
你的代码中的另一个问题是 foo 期望引用 mwArray ，带有一个w，但是你引用 mxArray ，带有x。类型需要匹配，否则编译器不会占用你的程序。
 
I want to create a C++ Shared library from Matlab using deloytool and use it in MVS. I compile a function name 'foo.m', as the result I got list of files (.h,.cpp,.lib,...) and I found the function in 'fooCpplib.h' is as follow:
extern LIB_fooCpplib_CPP_API void MW_CALL_CONV foo(int nargout, mwArray& y, const mwArray& x);
Then I create a MVS project (2010), window form application, with 2 textbox and 2 click button, one textbox named inBox, and another named outBox. The code inside button_click is as follow:
private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
double input = System::Double::Parse(inBox->Text);
mxArray *x_ptr;
mxArray *y_ptr=NULL;
double *y;

// Create an mxArray to input into mlfFoo
x_ptr = mxCreateDoubleScalar(input);

// Call the implementation function 
// Note the second input argument should be &y_ptr instead of y_ptr. 
foo(1,&y_ptr,x_ptr);

// The return value from mlfFoo is an mxArray. 
// Use mxGetpr to get a pointer to data it contains.
y = (double*)mxGetPr(y_ptr);

// display the result in the form
outBox->Text = ""+*y;

//clean up memory
mxDestroyArray(x_ptr);
mxDestroyArray(y_ptr);
}
When I build the project, the error was occurred as below:
error C2664: 'foo' : cannot convert parameter 2 from 'mxArray **' to 'mwArray &'
Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style
cast.
Note: I already included 'fooCpplib.h' in the .cpp source file.

Could anyone help me about this!
Thank you!
解决方案
When a parameter is declared as
TypeName &argName
it means that argName is a reference. In C++ you can pass parameters by reference, which allows the function modify the variable that you pass to it (among other things).

If you have a pointer, but a function expects a reference, you need to dereference the pointer in the call with an asterisk, rather than taking the address of a pointer with an ampersand:
foo(1,*y_ptr,*x_ptr);
//    ^      ^
//    |      |
//  Here and here
You can think of variables, pointers, pointers to pointers, etc. in terms of level of indirection. Variables have a level of indirection of zero; pointers have level of indirection of one; pointers to pointers have level of indirection of two, and so on.

Adding an ampersand increases the level of indirection; adding an asterisk decreases it. Like variables, references have level of indirection of zero. If you have a pointer and you need a variable, you must decrease the level of indirection by prepending an asterisk.

Another problem in your code is that foo expects references to mwArray, with a "w", but you are passing references to mxArray, with an "x". The types need to match, otherwise the compiler is not going to take your program.

                        
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