Difference between `vector<int> v;` and `vector<int> v = vector<int>();`

Question

What is the difference between

std::vector<int> v; 

and

std::vector<int> v = std::vector<int>(); 

Intuitively, I would never use the second version but I am not sure if there is any difference. It feels to me that the second line is simply a default constructor building a temporary object that is then moved by a move assignment operator.

I am wondering whether the second line is not somehow equivalent to

std::vector<int> v = *(new std::vector<int>());  

hence causing the vector itself to be on the heap (dynamically allocated). If it is the case, then the first line would probably be preferred for most cases.

How do these lines of code differ?

Answer 1

Starting from C++17 there's no difference whatsoever.

There's one niche use case where the std::vector = std::vector initialization syntax is quite useful (albeit not for default construction): when one wants to supply a "count, value" initializer for std::vector<int> member of a class directly in the class's definition:

struct S {   std::vector<int> v; // Want to supply `(5, 42)` initializer here. How? }; 

In-class initializers support only = or {} syntax, meaning that we cannot just say

struct S {   std::vector<int> v(5, 42); // Error }; 

If we use

struct S {   std::vector<int> v{ 5, 42 }; // or = { 5, 42 } }; 

the compiler will interpret it as a list of values instead of "count, value" pair, which is not what we want.

So, one proper way to do it is

struct S {   std::vector<int> v = std::vector(5, 42); };