casting

问题 Why this is not legal: class Base { public: Base(){}; virtual ~Base(){}; }; class Derived : public Base{}; void takeDerived(Derived * c){}; // main void(*ptr)(Base*) = static_cast<void(*)(Base*)>(&takeDerived); // doesn't work // but this work ok, as well as reinterpret_cast // void(*ptr)(Base*) = (void(*)(Base*))(&takeDerived); Derived is a Base . Why can't it be casted in function parameter? For example, I can do this easily even without casting: void takeBase(Base* c){}; takeBase(new

问题 I am writing a library that stores Eigen expression templates as member variables to do the complicated calculations it needs to do. However, it seems like I'm not able to store or return these expression templates unless they are directly converted in MatrixXd or alike. This forces every step to be saved to a temporary, and ruins the efficiency of the whole design. Here's a short example that causes the trouble. Holder just holds an Eigen matrix, and Summer takes two holders and outputs the

问题 Is there a way to cast parameter in the controller's action from one type to another in ASP.NET MVC 5? Example. Action method is the following: public string Test(Guid input) { return ""; } If the method is invoked with parameters "input=hello" then I get the error with the message: "The parameters dictionary contains a null entry for parameter 'input' of non-nullable type 'System.Guid' for method 'System.String Test(System.Guid)' in 'RealtyGuide.WebSite.Controllers.HomeController'. An

问题 I'm working with the NetLink socket library ( https://sourceforge.net/apps/wordpress/netlinksockets/ ), and I want to send some binary data over the network in a format that I specify. The format I have planned is pretty simple and is as follows: Bytes 0 and 1: an opcode of the type uint16_t (i.e., an unsigned integer always 2 bytes long) Bytes 2 onward: any other data necessary, such as a string, an integer, a combination of each, etc.. the other party will interpret this data according to

问题 Let's say I have a book database and I want to check whether the CLRS book has the correct authors. Assuming private static final String CLRS_title = "Introduction to Algorithms"; @Test public void CLRS_is_written_by_CLRS(){ //given SelectConditionStep<Record> query = create .select() .from( ( BOOK.leftOuterJoin(BOOK_AUTHOR).on(BOOK.ID.eq(BOOK_AUTHOR.BOOKID)) ).leftOuterJoin(AUTHOR).on(AUTHOR.ID.eq(BOOK_AUTHOR.AUTHORID)) ) .where(BOOK.TITLE.eq(CLRS_title)) ; //when Result<Record> result =

问题 I have created the following simple table for this test: DROP TABLE DateFieldTest CREATE TABLE DateFieldTest ( DateField Date NOT NULL ) INSERT INTO DateFieldTest VALUES ('2013-07-15') INSERT INTO DateFieldTest VALUES ('2013-07-16') INSERT INTO DateFieldTest VALUES ('2013-07-17') INSERT INTO DateFieldTest VALUES ('2013-07-05') INSERT INTO DateFieldTest VALUES ('2013-07-06') INSERT INTO DateFieldTest VALUES ('2013-07-07') INSERT INTO DateFieldTest VALUES ('2013-06-05') INSERT INTO

问题 i have a pyodbc row with value (9, ) I am trying to read the value into a simple integer variable, and i find that its so hard to do so. (i am new to python so i probably miss something dumb) doing a simple int(a_row[0]) get - TypeError: int() argument must be a string or a number, not 'pyodbc.Row' i try to cast the row into a list first, and its the same result i had to force it into a integer value to work row_id=(2, ) temp_num=0 temp_num = [number for number in row_id[0]] temp_num=int(temp

问题 I have 2 classes: class A { public void A_1(Object b) { ... Type t = b.GetType(); (t.FullName)b.B_1(); //It doesn`t work! Error in cast } .... } class B { public void B_1() { ... } .... } A a = new A(); B b = new B(); a.A1(b); How to cast object correctly? 回答1: If you want to cast an object of any type to an object of another type, you do this: // Will Throw an exception at runtime if it cant be cast. B newObject = (B)oldObject; // Will return null at runtime if the object cannot be cast B

问题 In C++ for Windows, I have some object factory that is supposed to create a series of Info object by passing a pointer to the object to a Create function and returning a created object. void CreateInfoObject(AbstractInfo** info); // The creation function AbstractInfo is a base class of which we have many types of Info objects derive. I thought I could now create an Info object as follows: MyInfoObject* InfoObj = NULL; // derived from AbstractInfo object InfoFactory fc; fc.CreateInfoObject(

问题 The 0x3f800000 represents 1.0 in single precision floating maths. I tried this but could not get the correct result from the program: set x 0x3f800000 set y [expr double($x)] puts $y I just want to "cast" the value of x into a float so it will print on screen as float. How do I do this in tcl? Please note that in the original problem that I am trying to solve, a tcl script reads value from a hardware register in Quartus II System Console debug system. However, I have given a simple example