Vector of Vector specific syntax

Question

I usually declare (and define) my vector of vectors with the following syntax,

vector<vector<int>> graph = *new vector<vector<int>>(n, *new vector<int>(n, 0));

Here n is already defined. And this usually works fine in most compilers. But a few days ago I tried importing the source file to another system, and it was filled with compilation errors something like,
"Expected primary expression after '>>' "
I do not remember the exact errors, but do know that after adding spaces between '>' and '>' on both sides the error was removed. i.e.

vector<vector<int> > graph = *new vector<vector<int> >(n, *new vector<int>(n, 0));

I know that the syntax requires us to add spaces between both '>'s, but I would like to know if there is any difference between the compilers, because both used c++11 and even the same IDEs. I have used this syntax for very long and it would be very frustrating to edit each one. It would be easier to know which compilers it works on.

"I usually declare (and define) my vector of vectors with the following syntax" Oh God, no. — DeiDei, Dec 16 '17 at 14:43
Why are you declaring a non pointer with a pointer that is wasting memory — Jake Freeman, Dec 16 '17 at 14:44
`*new ...` is a memory leak. How are you ever going to `delete` that pointer? — melpomene, Dec 16 '17 at 14:53
Please don't define your vectors like that. Please define them exactly the same way every tutorial ever does it. — Galik, Dec 16 '17 at 14:54
The ability to avoid having to put a space between the `> >` was introduced in `C++11`. Before that you needed a space. — Galik, Dec 16 '17 at 14:56

Tobias Ribizel · Answer 1 · 2017-12-16T16:40:03.723

I'll give you a two-part answer:

vector<...> v = *new vector<...>(...)

Basically, you shouldn't do this.
Vector does its own memory management, so there is no need for raw new. Also, in this line you allocate memory for a vector on the heap (which is only 12 or 24 bytes, depending on your system), then assign this vector to the vector v on the stack (which might involve a copy of the contents of the whole vector). The vector on the heap is never deleted and thus the memory is leaked.

Better:

vector<vector<int>> graph = vector<vector<int>>(n, vector<int>(n, 0));

or just

vector<vector<int>> graph(n, vector<int>(n, 0));

Now the answer to your original question: The C++ standard started allowing >> to close nested templates starting from C++11 (see this related question), so you need to configure your compiler to use at least the C++11 standard (normally using the -std=c++11 flag). However, nearly all recent compilers I know already use this standard by default.

For a more detailed answer you would need to tell us which IDE and compiler you use.

Vector of Vector specific syntax

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