使用C ++构造函数的不同方法 [英] Different ways of using C++ constructors

问题描述

A a1（5）; A a2 = A（5）？这两个作品，但我真的想知道他们之间的区别，因为我使用方法2在我的项目之一，我遭受一个错误，是固定的，在我改变方法1之后。提前感谢！

What the difference between A a1(5); and A a2 = A(5) ? Both of the works, but I really want know the difference between them, because I used method 2 in one of my project and I suffered from a bug which is fixed after I change to method 1. Thanks in advance!

class A {
public:
    int val;
    A() : val(0) {}
    A(int newVal) : val(newVal) {}
};

int main()
{
    A a1(5);  // method 1
    A a2 = A(5); // method 2
}

推荐答案

A a1(5);  // method 1
A a2 = A(5); // method 2

第一个称为直接初始化，第二个称为复制初始化。

The first one is called direct initialization, the second one is called copy-initialization.

如果您使复制构造函数无法访问或/和不将其定义为第二个将不会编译：

The second one will NOT compile if you make the copy-constructor inaccessible or/and don't define it as:

class A {
public:
    int val;
    A() : val(0) {}
    A(int newVal) : val(newVal) {}
private:
    A(A const&);  //the second one will not compile
};

现在第二个不能编译。请注意，在这两种情况下都不会编译：

Now the second one will not compile. Note that it will not compile in both cases:

• 如果定义了复制构造函数，但不可访问$ c> private 或 protected ）。


• 如果复制构造函数已声明。

• If the copy-constructor is defined, but is inaccessible (either it is private or protected).
• If the copy-constructor is declared, but not defined.

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