std::vector against std::vector with custom allocator

Question

Well I'm using my own allocator for a vector that has fast run-time requirements to avoid calling new/delete. The problem is now that code like this is not compiling:

std::vector<char> x(1000);
std::vector<char,myalloc<char>> y(1000);
x = y;

This obviously occurs because the implementation of the std::vector's allocator is compile-time and changes the type. But code like the above should be allocator-independent.

For POD I can do memcpy of course, but is there any other method? I'm really close to implementing my own vector that would use a custom allocator dynamically, specified at run time and get rid of std::vector.

Can you use assign? en.cppreference.com/w/cpp/container/vector/assign — jtbandes
– jtbandes, Commented Mar 2, 2020 at 7:14
@jtbandes yes but this would require a big edit in the code. — Michael Chourdakis
– Michael Chourdakis, Commented Mar 2, 2020 at 7:16
x.assign(y.begin(), y.end()) or (C++11) x.assign(std::begin(y), std::end(y)) should do it. A vector's operator=() doesn't have overloads for that to work, regardless of what you think "should" be the case. If you really want a type which works that way, create a simple struct/class type - templated if required - that contains a vector (with whatever element type and allocator you use) and ensure its operator=() works as required. — Peter
– Peter, Commented Mar 2, 2020 at 7:30
If I'm not mistaken you can use std::pmr::vector instead of just std::vector everywhere and everything should "just work". — sklott
– sklott, Commented Mar 2, 2020 at 7:37

L. F. · Accepted Answer · 2020-03-02 07:42:02Z

3

But code like the above should be allocator-independent.

Assigning the elements should indeed be allocator-independent. But the copy assignment operator of std::vector does more than just assign elements — it also takes care of allocators. So it should not be allocator-independent.

std::vector does provide an interface for assigning elements only: the assign member function. It can be used like this:

x.assign(y.begin(), y.end());

Since C++17, polymorphic allocators allow allocator-aware containers to have type-erased allocator support, so you can consistently use std::pmr::vector<char>, but it has some problems you need to consider — overhead of type erasure, different allocator propagation semantics, different interface, etc.

answered Mar 2, 2020 at 7:42

L. F.

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2 Comments

nada Over a year ago

Good concise answer. I just want to add begin() and end() can also be used right in the ctor like std::vector<whatever> y(x.begin(), x.end());

L. F. Over a year ago

@nada Yeah. In fact, they can be used in any context where a range denoted by a pair of iterators is needed

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