C ++中的结构构造函数？

Struct Constructor in C++?

提问by sth

Can a structhave a constructor in C++?

struct在 C++ 中可以有一个构造函数吗？

I have been trying to solve this problem but I am not getting the syntax.

我一直在尝试解决这个问题，但我没有得到语法。

回答by sth

In C++ the only difference between a classand a structis that members and base classes are private by default in classes, whereas they are public by default in structs.

在 C++ 中，aclass和 a之间的唯一区别struct是成员和基类在类中默认是私有的，而在结构中默认是公共的。

So structs can have constructors, and the syntax is the same as for classes.

所以结构体可以有构造函数，语法与类相同。

回答by nos

struct TestStruct {
        int id;
        TestStruct() : id(42)
        {
        }
};

回答by gitarooLegend

All the above answers technically answer the asker's question, but just thought I'd point out a case where you might encounter problems.

以上所有答案在技术上都回答了提问者的问题，但只是想我会指出一个您可能会遇到问题的情况。

If you declare your struct like this:

如果你像这样声明你的结构：

typedef struct{
int x;
foo(){};
} foo;

You will have problems trying to declare a constructor. This is of course because you haven't actually declared a struct named "foo", you've created an anonymous struct and assigned it the alias "foo". This also means you will not be able to use "foo" with a scoping operator in a cpp file:

尝试声明构造函数时会遇到问题。这当然是因为您实际上还没有声明一个名为“foo”的结构，您已经创建了一个匿名结构并为其分配了别名“foo”。这也意味着您将无法在 cpp 文件中使用带有作用域运算符的“foo”：

foo.h:

foo.h：

typedef struct{
int x;
void myFunc(int y);
} foo;

foo.cpp:

foo.cpp：

//<-- This will not work because the struct "foo" was never declared.
void foo::myFunc(int y)
{
  //do something...
}

To fix this, you must either do this:

要解决此问题，您必须执行以下操作：

struct foo{
int x;
foo(){};
};

or this:

或这个：

typedef struct foo{
int x;
foo(){};
} foo;

Where the latter creates a struct called "foo" and gives it the alias "foo" so you don't have to use the structkeyword when referencing it.

后者创建一个名为“foo”的结构并为其提供别名“foo”，因此您struct在引用它时不必使用关键字。

回答by Chap

Yes, but if you have your structure in a union then you cannot. It is the same as a class.

是的，但是如果你的结构在一个联合中，那么你就不能。它与类相同。

struct Example
{
   unsigned int mTest;
   Example()
   {
   }
};

Unions will not allow constructors in the structs. You can make a constructor on the union though. This question relates to non-trivial constructors in unions.

联合将不允许在结构中使用构造函数。不过，您可以在联合上创建一个构造函数。这个问题与联合中的非平凡构造函数有关。

回答by Luqmaan

As the other answers mention, a struct is basically treated as a class in C++. This allows you to have a constructor which can be used to initialise the struct with default values. Below, the constructor takes szand bas arguments, and initializes the other variables to some default values.

正如其他答案所提到的，结构基本上被视为 C++ 中的一个类。这允许您拥有一个构造函数，该构造函数可用于使用默认值初始化结构。下面，构造函数将sz和b作为参数，并将其他变量初始化为一些默认值。

struct blocknode
{
    unsigned int bsize;
    bool free;
    unsigned char *bptr;
    blocknode *next;
    blocknode *prev;

    blocknode(unsigned int sz, unsigned char *b, bool f = true,
              blocknode *p = 0, blocknode *n = 0) :
              bsize(sz), free(f), bptr(b), prev(p), next(n) {}
};

Usage:

用法：

unsigned char *bptr = new unsigned char[1024];
blocknode *fblock = new blocknode(1024, btpr);

回答by Saurabh Raoot

Class, Structure and Union is described in below table in short.

类、结构和联合在下表中简要描述。

回答by GManNickG

Yes. A structure is just like a class, but defaults to public:, in the class definition and when inheriting:

是的。结构就像一个类，但public:在类定义和继承时默认为:

struct Foo
{
    int bar;

    Foo(void) :
    bar(0)
    {
    }
}

Considering your other question, I would suggest you read through some tutorials. They will answer your questions faster and more complete than we will.

考虑到您的其他问题，我建议您阅读一些教程。他们会比我们更快、更完整地回答您的问题。

回答by SwDevMan81

struct HaveSome
{
   int fun;
   HaveSome()
   {
      fun = 69;
   }
};

I'd rather initialize inside the constructor so I don't need to keep the order.

我宁愿在构造函数内部初始化，所以我不需要保持顺序。

回答by heavyd

Yes structures and classes in C++ are the same except that structures members are public by default whereas classes members are private by default. Anything you can do in a class you should be able to do in a structure.

是的，C++ 中的结构和类是相同的，只是结构成员默认是公共的，而类成员默认是私有的。你在课堂上可以做的任何事情，你都应该可以在结构中做。

struct Foo
{
  Foo()
  {
    // Initialize Foo
  }
};

回答by Steve L

Note that there is one interesting difference (at least with the MS C++ compiler):

请注意，有一个有趣的区别（至少对于 MS C++ 编译器而言）：

---

---

If you have a plain vanilla struct like this

如果你有一个像这样的普通香草结构

struct MyStruct {
   int id;
   double x;
   double y;
} MYSTRUCT;

then somewhere else you might initialize an array of such objects like this:

然后在其他地方，您可能会像这样初始化一个此类对象的数组：

MYSTRUCT _pointList[] = { 
   { 1, 1.0, 1.0 }, 
   { 2, 1.0, 2.0 }, 
   { 3, 2.0, 1.0 }
};

however, as soon as you add a user-defined constructor to MyStruct such as the ones discussed above, you'd get an error like this:

但是，一旦您将用户定义的构造函数添加到 MyStruct（例如上面讨论的那些）中，您就会收到如下错误：

    'MyStruct' : Types with user defined constructors are not aggregate
     <file and line> : error C2552: '_pointList' : non-aggregates cannot 
     be initialized with initializer list.

    'MyStruct' : Types with user defined constructors are not aggregate
     <file and line> : error C2552: '_pointList' : non-aggregates cannot 
     be initialized with initializer list.

So that's at least one other difference between a struct and a class. This kind of initialization may not be good OO practice, but it appears all over the place in the legacy WinSDK c++ code that I support. Just so you know...

所以这至少是结构和类之间的另一个区别。这种初始化可能不是好的 OO 实践，但它在我支持的旧版 WinSDK c++ 代码中无处不在。只是让你知道...