Then all the optional parameters readonly vs. These methods e. The only other thing to do is make the constructor for class File to take an OpenFile object:. This constructor gets the actual parameters from the OpenFile object, then actually opens the file:. Since each member function in the chain returns a reference, there is no copying of objects and the chain is highly efficient.
Furthermore, if the various member functions are inline , the generated object code will probably be on par with C-style code that sets various members of a struct. Suppose there is a class called Bar that has a default ctor. As before, this might be defined by someone other than you. Now you want to create a Foo object using a temporary Bar. In other words, you want to create an object via Bar , and pass that to the Foo ctor to create a local Foo object called x :.
Note: The above solution requires yourCode to be able to access the Foo copy constructor. You are guaranteed to find out whether it works or fails at compile-time, so if it compiles cleanly, it will work at runtime.
That was mostly tongue in cheek. The real problem is that people tend to worship consistency, and they tend to extrapolate from the obscure to the common. The explicit keyword is an optional decoration for constructors and conversion operators to tell the compiler that a certain constructor or conversion operator may not be used to implicitly cast an expression to its class type. But sometimes you want to prevent this sort of implicit promotion or implicit type conversion.
You can mix explicit and non- explicit constructors and conversion operators in the same class. For example, this class has an explicit constructor taking a bool but a non- explicit constructor taking a double , and can be implicitly converted to double, but only explicitly converted to bool:.
Variable a is initialized using the Foo double constructor because Foo bool cannot be used in an implicit cast, but true can be interpreted as a double true , that is, as 1.
This may or may not be what you intended, but this is what happens. By default a class is given a copy constructor and a copy assignment that copy all elements, and a move constructor and a move assignment that move all elements. Here we get p2. Here, the default copy gives us h2. This leads to disaster: when we exit f the destructors for h1 and h2 are invoked and the object pointed to by h1. If we need to copy or move, we can of course define the proper initializers and assignments to provide the desired semantics.
Now return to Point. For Point the default copy semantics is fine, the problem is the constructor:. People provide default arguments to get the convenience used for orig and p1. Then, some are surprised by the conversion of 2 to Point 2,0 in the call of f.
This constructor defines a conversion. To require such a conversion to be explicit, declare the constructor explicit :. Constructors build objects from dust. Is there any difference between List x; and List x ;? A big difference! Is the default constructor for Fred always Fred::Fred? Immediate base classes left to right , then member objects top to bottom. Not all compilers issue diagnostic messages for these cases. Is it moral for one member object to be initialized using another member object in the initializer expression?
Yes, but use care and do that only when it adds value. What if one member object has to be initialized using another member object? Should you use the this pointer in the constructor? Does return-by-value mean extra copies and extra overhead? Not necessarily. What about returning a local variable by value? Does the local exist as a separate object, or does it get optimized away?
These are some! Why are classes with static data members getting linker errors? Yes, though with some important caveats. A subtle way to crash your program. What is a technique to guarantee both static initialization and static deinitialization?
However if the object has a non-trivial destructor with important side effects, such as writing to a file or some other non-volatile action, then you need more. If you need to control the order of both static initialization and static deinitialization, meaning if you wish to access a statically allocated object from both constructors and destructors of other static objects, then keep reading.
Otherwise run away. How can I handle a constructor that fails? Why am I getting an error after declaring a Foo object via Foo x Bar? This is really going to hurt; you might want to sit down.
What is the purpose of the explicit keyword? This is a question that comes in many forms. How do I turn off copying? How do I stop implicit conversions? Most commonly, a TypeInitializationException exception is thrown when a static constructor is unable to instantiate a type or for an unhandled exception occurring within a static constructor. For static constructors that aren't explicitly defined in source code, troubleshooting may require inspection of the intermediate language IL code.
The presence of a static constructor prevents the addition of the BeforeFieldInit type attribute. This limits runtime optimization. A field declared as static readonly may only be assigned as part of its declaration or in a static constructor. When an explicit static constructor isn't required, initialize static fields at declaration rather than through a static constructor for better runtime optimization.
The runtime calls a static constructor no more than once in a single application domain. That call is made in a locked region based on the specific type of the class.
No additional locking mechanisms are needed in the body of a static constructor. To avoid the risk of deadlocks, don't block the current thread in static constructors and initializers. For example, don't wait on tasks, threads, wait handles or events, don't acquire locks, and don't execute blocking parallel operations such as parallel loops, Parallel.
Though not directly accessible, the presence of an explicit static constructor should be documented to assist with troubleshooting initialization exceptions. A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method. Static constructors are also a convenient place to enforce run-time checks on the type parameter that cannot be checked at compile time via type-parameter constraints.
The value of x is 1 because the variable initializer is executed before the base class instance constructor is invoked. However, the value of y is 0 the default value of an int because the assignment to y is not executed until after the base class constructor returns. It is useful to think of instance variable initializers and constructor initializers as statements that are automatically inserted before the constructor-body.
The example. Eric Lippert had an interesting post on the related issue of object initialization, which explains the reason for the ordering of constructors and field initializers:.
Part Two. The base constructor will be called first, otherwise, in cases where your "other stuff" must make use of member variables initialized by your base constructor, you'll get compile time errors because your class members will not have been initialized yet. This is true, even if you leave off the :base in which case, the 0-parameter base constructor is called. But if I don't put a call to super in explicitly, super is called.
Constructor calls are called fired from the bottom up, and executed from the top down. Thus, if you had Class C which inherits from Class B which inherits from Class A, when you create an instance of class C the constructor for C is called, which in turn calls the instructor for B, which again in turn calls the constructor for A. Now the constructor for A is executed, then the constructor for B is executed, then the constructor for C is executed.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Base constructor in C - Which gets called first?
Asked 13 years, 1 month ago. Active 4 years, 11 months ago. Viewed 70k times. Mike Comstock Mike Comstock 6, 10 10 gold badges 35 35 silver badges 40 40 bronze badges. Just think logically , child is based on parent so first parent constructor should get executed and then child.
But But when its comes to initializers they execute opposite first child and then parent. So constructor first parent to child and for initializers child to parent. Watch this video it has actual demonstration of he same youtube. Add a comment. Active Oldest Votes. Check out these 2 msdn articles: Why do initializers run in the opposite order as constructors? Part One Why do initializers run in the opposite order as constructors?
Sam Meldrum Sam Meldrum
0コメント