Initialize a vector with a custom allocator

Question

I'm trying to initialize a vector with an explicit allocator, but the code below seems not working:

vector<int> a1(allocator<int>()); cout << a1.size(); //Error C2228: left of '.size' must have class/struct/union

There is also a working version:

vector<int> a2(*(new allocator<int>())); cout << a2.size(); // OK

And another:

vector<int> a3(a2, allocator<int>()); cout << a3,size(); // OK

I want to know why only the first version is not working. I'm using MSVC with C++17, and it seems the compiler won't regard a1 as an object. Thanks.

a1 is a function declaration. You fell victim to the most vexing parse. — HolyBlackCat
– HolyBlackCat, Commented Apr 18, 2021 at 21:21
@HolyBlackCat Thanks a lot! I'm new to this terminology actually. I will check to figure out how it works. — Jim Reid
– Jim Reid, Commented Apr 18, 2021 at 21:27

Jim Reid · Accepted Answer · 2021-04-20 21:46:47Z

Thanks to everyone who gives me suggestions, this is known as "most vexing parse" and can be fixed by uniform initialization as:

vector<int> a1(allocator<int>{});

So that a1 is now a vector. Otherwise, a1 will be interpreted as a function, that receives a parameter of type std::allocator<int> (*)() and returns vector<int>.

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