在 C++ 中创建默认构造函数
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Create a default constructor in C++
提问by user1824239
This might be a stupid question but I can't find a lot of information on the web about creating your own default constructors in C++. It seems to just be a constructor with no parameters. However, I tried to create my default constructor like this:
这可能是一个愚蠢的问题，但我在网上找不到很多关于在 C++ 中创建自己的默认构造函数的信息。它似乎只是一个没有参数的构造函数。但是，我尝试像这样创建我的默认构造函数：
Tree::Tree() {root = NULL;}
I also tried just:
我也试过：
Tree::Tree() {}
When I try either of these I am getting the error:
当我尝试其中任何一个时，我收到错误：
No instance of overloaded function "Tree::Tree" matches the specified type.
没有重载函数“Tree::Tree”的实例与指定的类型匹配。
I can't seem to figure out what this means.
我似乎无法弄清楚这意味着什么。
I am creating this constructor in my .cppfile. Should I be doing something in my header (.h) file as well?
我正在我的.cpp文件中创建这个构造函数。我也应该在我的头.h文件( ) 中做一些事情吗？
回答by Pete Becker
Member functions (and that includes constructors and destructors) have to be declared in the class definition:
成员函数（包括构造函数和析构函数）必须在类定义中声明：
class Tree {
public:
Tree(); // default constructor
private:
Node *root;
};
Then you can define it in your .cpp file:
然后你可以在你的 .cpp 文件中定义它：
Tree::Tree() : root(nullptr) {
}
I threw in the nullptrfor C++11. If you don't have C++11, use root(0).
我投入了nullptrC++11。如果您没有 C++11，请使用root(0).
回答by Kevin Katzke
C++11allows you to define your own default constructor like this:
C++11允许你像这样定义你自己的默认构造函数：
class A {
public:
A(int); // Own constructor
A() = default; // C++11 default constructor creation
};
A::A(int){}
int main(){
A a1(1); // Okay since you implemented a specific constructor
A a2(); // Also okay as a default constructor has been created
}
回答by Dietmar Kühl
It isn't sufficient to create a definition for any member function. You also need to declarethe member function. This also applies to constructors:
为任何成员函数创建定义是不够的。您还需要声明成员函数。这也适用于构造函数：
class Tree {
public:
Tree(); // declaration
...
};
Tree::Tree() // definition
: root(0) {
}
As a side note, you should use the member initializer list and you should notuse NULL. In C++ 2011 you want to use nullptrfor the latter, in C++ 2003 use 0.
作为旁注，您应该使用成员初始值设定项列表，而不应该使用NULL. 在 C++ 2011 中你想使用nullptr后者，在 C++ 2003 中使用0.
回答by Josh Heitzman
Yes, you need to declare it in your header. For example place the following inside the declaration of the tree class.
是的，您需要在标题中声明它。例如，将以下内容放在树类的声明中。
class Tree {
// other stuff...
Tree();
// other stuff...
};
