在c ++中的空构造函数 [英] Empty constructor in c++

问题描述

我理解的部分，我会得到一些随机值，但是在代码片段中的 Foo（）构造函数像默认public构造函数，编译器当我们没有定义构造函数时提供？

Well I understand the part that I will be getting some random value, but is theFoo() constructor in the snippet acting just like the default public constructor which the compiler supplies when we have no constructor defined?

#include<iostream>
using namespace std ;

class Foo{
        int i  ;
    public:
        Foo(){
        }
        void disp(){
            cout<<"i = "<<i  ;
        }
};

int main(){
    Foo bar1,bar2 ;
    bar1.disp();
    cout<<"\n";
    bar2.disp();
}

我看到一些人写这样的空构造函数，

I have seen some people writing an empty constructor like this, but I could't understand exactly why/when is it to be used?

推荐答案

没有参数的用户定义的ctor，没有ctor-init-列表和空主体的行为与默认的ctor几乎相同。

有一个区别，它不算作一个简单的ctor，

A user-defined ctor without arguments, without ctor-init-list and with an empty body behaves nearly the same as the default-ctor.
There is one difference though, it does not count as a trivial ctor, ever!

这样的默认默认值会避免这种差异和伴随的潜在性能下降：

Explicitly defaulting like this instead would avoid that difference and the concomittant potential performance-degradation:

Foo() = default; // Needs C++11

什么是default一个类函数声明之后是什么意思？

参见< type_traits> 以检测差异： http://en.cppreference.com/w/cpp/types/ is_constructible

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