问题
Calling clear() on a vector will call the destructors of whatever is stored in the vector, which is a linear time operation. But is this the case when the vector contains primitive types like int or double?
回答1:
I believe the answer is implementation dependent. It takes at most linear time, but some implementations may choose to optimize this.
Per 'Does clearing a vector affect its capacity?', neither MSVC nor G++ decrease the capacity of their vectors, even when .clear is called. Looking at the G++ headers, it is evident that .clear is constant-time with the default allocator, as long as the elements are scalar (primitive arithmetic types or pointers).
回答2:
Think about this from the POV of how a vector is likely implemented. When you invoke:
delete [] internalPtr;
What happens?
- The heap must reclaim a contiguous block of space
- destructors must fire or each object in internalPtr
The former must still happen for primitive types, but destructors don't exist for them. So delete[] will execute based entirely on how fast the heap can delete a block of memory
回答3:
In this link:
http://www.cplusplus.com/reference/vector/vector/clear/
It says complexity of clear() is linear in size (destructions).
回答4:
Well.. it says that clear() is linear, but we also know that it calls the destructor of every item...
http://www.cplusplus.com/reference/vector/vector/clear/
What if the destructor call ist not linear?
However, on primitives the destructor-call is linear (or constant, this is not important unless it is not more than linear)
so yes, on primitives is clear() always a linear operation
来源：https://stackoverflow.com/questions/14970349/is-stdvectorprimitiveclear-a-constant-time-operation