Return Pointer To Non-Modifiable Member Array C++ gcc warning

Question

I would like to return a pointer to an array owned by a class. However, I do not want to allow users to modify that data or pointer. According to how I understand things you need return a constant pointer to constant data using the following syntax.

const Type *const Class::Method() {
    return this->data_; 
}

However gcc gives the following warning when compiling.

warning: type qualifiers ignored on function return type

Why is this warning provided by gcc? What does it mean? If this is not the right syntax for what I want, what is?

I am working with a C interface that expects a const float*, and for performance reasons I would like to shy away from instantiating a std::vector. — Matthew Hoggan, Mar 03 '13 at 04:35

score 2 · Answer 1 · answered Mar 03 '13 at 04:48

The warning you get is because the final const is overlooked. Take it off and you're set.

You don't need to return a const pointer to const data, just a pointer to const data (which is const Type*). Just as it doesn't make sense to return a const int, it doesn't make sense to return a T* const, because as soon as the value is assigned to a new variable, that const is discarded.

Jesse Good · Accepted Answer · 2013-03-03T05:41:33.033

1

The top level const is ignored for built-in types. As there is a rule in C++[3.14p4]: class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types.. In your case const Type* const, the top level const making the pointer const is ignored.

You could add const to the end: const Type * Class::Method() const {...}. This would prevent the pointer from being modified inside the member function. However, since the member function returns prvalue which is non-modifiable, it is not necessary to do this to prevent modification of the pointer member outside of the class (and this is also the reason why this rule exists in C++). It may be useful when you want to call the function with a constant reference to a Class object, etc., but for what you are doing, this doesn't seem necessary.

edited Mar 03 '13 at 05:41

answered Mar 03 '13 at 04:32

Jesse Good

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I don't think `const Type * Class::Method() const` should be part of this answer: that might be the right thing to do but it doesn't have anything to do with what he's asking for. – Stephen Lin Mar 03 '13 at 05:10
(especially since his code seems to compile without error right now) – Stephen Lin Mar 03 '13 at 05:12
@StephenLin: I've reworded the answer to clarify (Thanks). – Jesse Good Mar 03 '13 at 05:18

score 0 · Answer 3 · answered Mar 03 '13 at 04:32

0

First const is right, the second const does not make any sense. Just look at this example:

const int foo();
int a = foo();

The fact if foo returns const int or just int does not change anything in this case. Same for Type *.

answered Mar 03 '13 at 04:32

Slava

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Your example does not relate to the problem. – Matthew Hoggan Mar 03 '13 at 04:43
Try to compile const int foo() { return 0; } with the same warning level and see how it is unrelated... – Slava Mar 03 '13 at 05:15

Return Pointer To Non-Modifiable Member Array C++ gcc warning

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