调用C ++默认构造函数 [英] C++ Default Constructor Called

问题描述

在下面的代码中，类B的成员类型为类A(varA1).我想创建一个B类对象，其中的成员varA1打算使用A类中的非默认构造函数A(int v1).

In the code below, class B has a member that is of type class A (varA1). I want to create a class B object where the member varA1 is intended to use the non-default constructor A(int v1) in class A.

#include <iostream>
using std::cout; using std::endl; using std::string;

class A {
public:
    A() { cout << "A default constructor" << endl;}
    A(int v1);
private:
    int var1;
};
A::A(int v1) {
    cout << "A int constructor" << endl;
    var1 = v1;
}

class B {
public:
    B() { cout << "B default constructor" << endl;}
    B(int v1);
private:
    int var1;
    A varA1;
};
B::B(int v1) {
    cout << "B int constructor" << endl;
    var1 = v1;
    A varA1(int v1);
}

int main()
{
    A TestA(1);
    B TestB(1);

    return 0;
}

但是，当我运行上面的代码时，我得到以下输出:

However when I run the code above I get the following output:

A int constructor
A default constructor
B int constructor

我在这里一定做错了.我需要更改什么，以便B类在A类中使用非默认构造函数A(int v1)?

I must be doing some wrong here. What do I need to change so that the B class uses the non-default constructor A(int v1) in class A?

我正在使用ubuntu 14.04LTS.GNU G ++ 4.9和5.1给出了相同的结果.

I am using ubuntu 14.04LTS. Both GNU G++ 4.9 and 5.1 gave the same results.

在此先感谢您的阅读和回答.

Thanks in advance for reading and answering.

推荐答案

使用成员初始化列表:

B::B(int v1) : var1(v1), varA1(v1) {
    cout << "B int constructor" << endl;
}

请注意，成员的初始化(构造)顺序与在类中声明的顺序相同，因此，在成员初始化列表中切换顺序不会改变构造发生的顺序(希望编译器会警告您这个的).如果您尝试从 var1 构造 varA1 ，并且在类中的 varA1 之后声明了 var1 ，则此小细节就变得很重要.定义.

Note that members are initialized (constructed) in the same order that they're declared in the class, so switching orders in the member initialization list won't change the order in which construction happens (and hopefully your compiler will warn you of this). This little detail becomes important if you try to construct varA1 from var1 and var1 is declared after varA1 in the class definition.

顺便说一句，所有这行代码都在(在 B :: B(int v1)构造函数内部)中完成:

And by the way, all this line does (inside the B::B(int v1) constructor):

A varA1(int v1);

正向声明一个名为 varA1 的函数，该函数接受一个 int 参数并返回一个 A 对象.这与最令人烦恼的解析半类似，尽管实际上并非如此最烦人的解析.

is forward declare a function named varA1 that takes an int parameter and returns an A object. This is semi-similar to the most vexing parse, though this isn't really a case of the most vexing parse.

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