Custom STL Allocator with a custom constructor

Question

I'm using the STL allocator mentioned here.
The only change I'm making is that I'm inheriting from a base class called Object, and I use base class' new and delete functions for allocation.

 class MyAlloc :public Object{ ...... }

I want to use the parameterized constructor of the base class which will be based on parameter sent to the STLAllocator, which would be something like this.

 MyAlloc(A *a) : Object(a) { ... }

And then use this constructor like :

 A *a = new A(); std::vector<int,MyAlloc<int> (a) > v;

I'm not able to achieve this. It is resulting in compilation error :
'a'cannot appear in a constant-expression
template argument 2 is invalid
Thanks in advance..:)

Mike Seymour · Accepted Answer · 2013-09-02 15:01:52Z

9

You specify the type of the allocator as a template argument and, if you don't want a default-constructed one, a value as a constructor argument:

std::vector<int,MyAlloc<int>> v((MyAlloc<int>(a)));

Note that I added an extra pair of parentheses to avoid the "most vexing parse". In this case, we can't avoid that using brace-initialisation, since that will try to use the initialiser list to populate the vector.

edited Sep 2, 2013 at 15:01

answered Sep 2, 2013 at 14:48

Mike Seymour

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4 Comments

Mike Seymour Over a year ago

@gx_: Good point; I only parsed it with my brain, which is a bit buggy today.

302Found Over a year ago

Thanks for the help.. solutions works fine with std::vector but is not working with std::map. I have used it in the same way : std::map<std::string, std::string, std::less<std::string>, MyAlloc<std::pair<const std::string,std::string> > > x ((MyAlloc<std::pair<const std::string, std::string>(a))); Results here : ideone.com/C8hY19

Mike Seymour Over a year ago

@codinonwheels: Your allocator needs to meet the allocator requirements described in C++11 17.6.3.5 (or C++98 20.1.5, if you're stuck in the past). pointer is one of several members and nested types that it needs to provide.

302Found Over a year ago

Actually, looks like my allocator satisfies the requirements. I know this because if I don't have the value as construct argument, it works fine. The error crops in once I have a value as construct argument. Also, I wanted to know the value passed is right or not..! Please bear me.. I'm learning allocators.

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