Error when using the std::allocator in the template class

Question

Trying to figure out how to use std :: allocator.

#include <iostream> template <typename T, typename A> struct vector_base { A allocator; T* data; size_t size; size_t space; vector_base(const A &alloc, size_t n) :allocator(alloc), data(alloc.allocate(n)), size(n), space(n) {} ~vector_base() {allocator.deallocate(data, space);} }; int main() { std::allocator<int> my_allocator; vector_base<int, std::allocator<int>> vector(my_allocator, 10); return 0; }

Error:

error: passing ‘const std::allocator’ as ‘this’ argument discards qualifiers [-fpermissive] :allocator(alloc), data(alloc.allocate(n)), size(n), space(n)

alloc.allocate(n) you try to call a non-const member function on a const reference — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 26, 2019 at 12:27

Hatted Rooster · Accepted Answer · 2019-05-26 13:09:25Z

4

alloc is a const&, you're trying to call allocate on it which is a non-const method, that's not allowed.

Call allocate on your initialized member instead:

 vector_base(const A &alloc, size_t n) :allocator(alloc), data(allocator.allocate(n)), size(n), space(n) {}

edited May 26, 2019 at 13:09

answered May 26, 2019 at 12:28

Hatted Rooster

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4 Comments

L. F. Over a year ago

I would suggest not depending on the order in which the data members are declared. I would put data = allocator.allocate(n) in the function body. This way, the code doesn't break if someone moves the declaration of allocator after data.

Hatted Rooster Over a year ago

@L.F. Can't be sure OP runs 11+, but yeah usually you would do that

L. F. Over a year ago

I think you misunderstood. By “body” I mean the body of the constructor, not in-class member initializer ;-)

Swift - Friday Pie Over a year ago

wouldn't compiler still move initialization around, because order of initialization is predefined? Though, what may happen, is that someone changes order of fields in class' declaration. THEN it is a problem.

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