new-operator

问题 I see the following constructs: new X will free the memory if X constructor throws. operator new() can be overloaded. The canonical definition of an operator new overload is void *operator new(size_t c, heap h) and the corresponding operator delete . The most common operator new overload is placement new, which is void *operator new(void *p) { return p; } You almost always cannot call delete on the pointer given to placement new . This leads to a single question: How is memory cleaned up when

问题 In JavaScript, what's the difference between var x = Object(); and var x = new Object(); ? 回答1: This is pulled directly from the ECMAScript specification: 15.2.1 The Object Constructor Called as a Function When Object is called as a function rather than as a constructor, it performs a type conversion. 15.2.1.1 Object ( [ value ] ) When the Object function is called with no arguments or with one argument value, the following steps are taken: If value is null, undefined or not supplied, create

问题 In JavaScript, what's the difference between var x = Object(); and var x = new Object(); ? 回答1: This is pulled directly from the ECMAScript specification: 15.2.1 The Object Constructor Called as a Function When Object is called as a function rather than as a constructor, it performs a type conversion. 15.2.1.1 Object ( [ value ] ) When the Object function is called with no arguments or with one argument value, the following steps are taken: If value is null, undefined or not supplied, create

问题 Is there a way to use the new keyword to allocate on the stack (ala alloca ) instead of heap ( malloc ) ? I know I could hack up my own but I'd rather not. 回答1: To allocate on the stack, either declare your object as a local variable by value , or you can actually use alloca to obtain a pointer and then use the in-place new operator: void *p = alloca(sizeof(Whatever)); new (p) Whatever(constructorArguments); However, while using alloca and in-place new ensures that the memory is freed on

问题 Is there a way to use the new keyword to allocate on the stack (ala alloca ) instead of heap ( malloc ) ? I know I could hack up my own but I'd rather not. 回答1: To allocate on the stack, either declare your object as a local variable by value , or you can actually use alloca to obtain a pointer and then use the in-place new operator: void *p = alloca(sizeof(Whatever)); new (p) Whatever(constructorArguments); However, while using alloca and in-place new ensures that the memory is freed on

问题 According to C++ Primer 4th edition, page 755, there is a note saying: Modern C++ programs ordinarily ought to use the allocator class to allocate memory. It is safer and more flexible. I don't quite understand this statement. So far all the materials I read teach using new to allocate memory in C++. An example of how vector class utilize allocator is shown in the book. However, I cannot think of other scenarios. Can anyone help to clarify this statement? and give me more examples? When should

问题 Say I have a class called Money which has parameters Dollars and Cents I could initialize it in the followings 2 ways: Money a(3,15); Money *b=new Money(3,15); My question is when should I use (1) and when should I use (2) 回答1: The first one creates a Money object on the stack, its lifespan is within the scope of when it was created. Meaning when you hit a } it goes out of scope and the memory is returned. Use this when you want to create an object within one function. The second one creates

问题 I need to take hql that is currently : select distinct a from Asset as a where ... and change it to select new com.org.AssetDTO(a.id, a.address, a.status) from Asset as a where ... My problem is with the distinct keyword. Where does it belong in an hql query where you're using the new Object query type. One thought was to use a sub-select and have my distinct there. I've tried adding distinct a.id but that doesn't work. 回答1: Ok for anyone interested the proper syntax is select distinct new

问题 I need to take hql that is currently : select distinct a from Asset as a where ... and change it to select new com.org.AssetDTO(a.id, a.address, a.status) from Asset as a where ... My problem is with the distinct keyword. Where does it belong in an hql query where you're using the new Object query type. One thought was to use a sub-select and have my distinct there. I've tried adding distinct a.id but that doesn't work. 回答1: Ok for anyone interested the proper syntax is select distinct new

问题 I read that the new modifer hides the base class method. using System; class A { public void Y() { Console.WriteLine("A.Y"); } } class B : A { public new void Y() { // This method HIDES A.Y. // It is only called through the B type reference. Console.WriteLine("B.Y"); } } class Program { static void Main() { A ref1 = new A(); // Different new A ref2 = new B(); // Polymorpishm B ref3 = new B(); ref1.Y(); ref2.Y(); //Produces A.Y line #xx ref3.Y(); } } Why does ref2.Y(); produce A.Y as output?