what's difference between const auto and const type*

Question

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?

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; }

Hint: data_0's type is Data* const data_0;.

1201ProgramAlarm
– 1201ProgramAlarm

2020-10-28 04:28:41 +00:00
Commented Oct 28, 2020 at 4:28 — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Oct 28, 2020 at 4:28

Remy Lebeau · Accepted Answer · 2020-10-28 04:50:11Z

const applies to the thing on its left, unless there is nothing then it applies to the thing on its right.

In const auto data_0 = GetData(0);, the const applies to whatever type auto deduces to, which in this case is Data*. So, the final declaration is:

Data * const data_0 = GetData(0);

The const applies to the pointer itself, not the thing it is pointing at. Thus, data_0 is a const pointer to a non-const Data object. SetVal() is a non-const method, so it can be called on the object.

In const Data* data_1 = GetData(0);, the const applies to Data. So, the final declaration is:

Data const * data_1 = GetData(0);

The const applies to the thing being pointed at, not to the pointer itself. Thus, data_1 is a non-const pointer to a const Data object. SetVal() is not a const method, so it cannot be called on the object.

eerorika · Accepted Answer · 2020-10-28 04:43:36Z

what's difference between const auto and const type*

const auto will be deduced from the initialiser and will be a const qualified type. 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 i.e. Data* const.

const type* is a non-const qualified pointer to a const qualified object of type type. The difference is that one is non-const pointer to const and the other is const pointer to non-const.

why error happend

Because you call a non-const qualified member function through a pointer to const.

I think const auto data_0 should be the same with const Data* data_1

It isn't, and it shouldn't be.

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