[英]what's difference between const auto and const type*
why error happend, I think const auto data_0 should be the same with const Data* data_1, what's the difference bwtween data_0 and data_1?为什么会发生错误,我认为 const auto data_0 应该与 const Data* data_1 相同,那么 data_0 和 data_1 之间有什么区别?
class Data {
public:
Data(int val_) : val(val_) {
}
~Data() {
}
void SetVal(int val_) {
val = val_;
}
private:
int val;
};
Data* GetData(int val) {
return new Data(val);
}
int main () {
const auto data_0 = GetData(0);
const Data* data_1 = GetData(0);
data_0->SetVal(1); // OK
data_1->SetVal(1); // error: passing ‘const Data’ as ‘this’ argument discards qualifiers
return 0;
}
const
applies to the thing on its left, unless there is nothing then it applies to the thing on its right. const
适用于它左边的事物,除非什么都没有,那么它适用于它右边的事物。
In const auto data_0 = GetData(0);
在const auto data_0 = GetData(0);
, the const
applies to whatever type auto
deduces to, which in this case is Data*
. , const
适用于auto
推导出的任何类型,在这种情况下是Data*
。 So, the final declaration is:所以,最终的声明是:
Data * const data_0 = GetData(0);
The const
applies to the pointer itself, not the thing it is pointing at. const
适用于指针本身,而不是它所指向的东西。 Thus, data_0
is a const pointer to a non-const Data
object.因此, data_0
是一个指向非常量Data
对象的常量指针。 SetVal()
is a non-const method, so it can be called on the object. SetVal()
是一个非常量方法,因此可以在对象上调用它。
In const Data* data_1 = GetData(0);
在const Data* data_1 = GetData(0);
, the const
applies to Data
. , const
适用于Data
。 So, the final declaration is:所以,最终的声明是:
Data const * data_1 = GetData(0);
The const
applies to the thing being pointed at, not to the pointer itself. const
适用于所指向的事物,而不适用于指针本身。 Thus, data_1
is a non-const pointer to a const Data
object.因此, data_1
是一个指向 const Data
对象的非常量指针。 SetVal()
is not a const method, so it cannot be called on the object. SetVal()
不是 const 方法,因此不能在对象上调用它。
what's difference between const auto and const type* const auto 和 const type 之间有什么区别*
const auto
will be deduced from the initialiser and will be a const qualified type. const auto
将从初始化器中推导出来,并且是一个 const 限定类型。 In case of data_0
, the type of the initialiser is a pointer to non-const Data
and therefore the deduced type becomes a const qualified pointer to non-const Data
ie Data* const
.在data_0
情况下,初始化器的类型是指向非常量Data
的指针,因此推导出的类型成为指向非常量Data
的常量限定指针,即Data* const
。
const type*
is a non-const qualified pointer to a const qualified object of type type
. const type*
是指向 type type
的 const 限定对象的非常量限定指针。 The difference is that one is non-const pointer to const and the other is const pointer to non-const.区别在于一个是指向const的非常量指针,另一个是指向非常量的const指针。
why error happend为什么会发生错误
Because you call a non-const qualified member function through a pointer to const.因为你通过一个指向 const 的指针调用了一个非常量限定的成员函数。
I think const auto data_0 should be the same with const Data* data_1我认为 const auto data_0 应该与 const Data* data_1 相同
It isn't, and it shouldn't be.不是,也不应该是。
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