[英]Default constructor c++
I am trying to understand how default constructor (provided by the compiler if you do not write one) versus your own default constructor works. 我试图了解默认构造函数(如果不编写,则由编译器提供)与您自己的默认构造函数的工作方式。
So for example I wrote this simple class: 例如,我写了这个简单的类:
class A
{
private:
int x;
public:
A() { std::cout << "Default constructor called for A\n"; }
A(int x)
{
std::cout << "Argument constructor called for A\n";
this->x = x;
}
};
int main (int argc, char const *argv[])
{
A m;
A p(0);
A n();
return 0;
}
The output is : 输出为:
Default constructor called for A 要求A的默认构造函数
Argument constructor called for A 参数构造函数要求A
So for the last one there is another constructor called and my question is which one and which type does n have in this case? 因此,对于最后一个,有另一个调用的构造函数,我的问题是在这种情况下n具有哪种类型和哪种类型?
A n();
declares a function, named n
, that takes no arguments and returns an A
. 声明一个名为
n
的函数,该函数不带任何参数并返回A
Since it is a declaration, no code is invoked/executed (especially no constructor). 由于它是一个声明,因此不会调用/执行任何代码(尤其是没有构造函数)。
After that declaration, you might write something like 声明之后,您可能会写类似
A myA = n();
This would compile. 这样可以编译。 But it would not link!
但这不会链接! Because there is no definition of the function
n
. 因为没有定义函数
n
。
A n();
could be parsed as an object definition with an empty initializer or a function declaration. 可以使用空的初始化程序或函数声明将其解析为对象定义。
The language standard specifies that the ambiguity is always resolved in favour of the function declaration (§8.5.8). 语言标准规定始终要解决模糊性,而要使用函数声明(第8.5.8节)。
So n
is a function without arguments returning an A
. 因此
n
是一个不带参数返回A
的函数。
For the last one NO constructor gets called. 对于最后一个,没有构造函数被调用。
For that matter no code even gets generated. 因此,甚至不会生成任何代码。 All you're doing is telling (declaring) the compiler that there's a function
n
which returns A
and takes no argument. 您要做的只是告诉(声明)编译器,这里有一个函数
n
返回A
并且不带参数。
No there is not a different constructor. 不,没有其他构造函数。
A n();
is treated as a declaration of function taking no arguments and returning A
object. 被视为不带参数并返回
A
对象的函数声明。 You can see this with this code: 您可以使用以下代码查看此内容:
class A
{
public:
int x;
public:
A(){ std::cout << "Default constructor called for A\n";}
A(int x){
std::cout << "Argument constructor called for A\n";
this->x = x;
}
};
int main(int argc, char const *argv[])
{
A m;
A p(0);
A n();
n.x =3;
return 0;
}
The error is: 错误是:
main.cpp:129: error: request for member 'x' in 'n', which is of non-class type 'A()' main.cpp:129:错误:请求'n'中的成员'x',这是非类类型'A()'
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