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Looks like a stupid question. But comment to my answer to one of the SO question made me to think again.
[ comment says, capacity need not be zero for empty vector]
By default my answer would be 0 as there are no elements inside vector. It makes sense to keep the capacity as 0 and on the first allocation it can be increased without any performance hits.
But standard does not say anything one this. ( I checked in Josuttis book as well).
Is it purely implementation specific? Does any STL vendor use some arbitrary number as capcity for the empty vector?
Any thoughts...
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By default my answer would be 0 as there are no elements inside vector. It makes sense to keep the capacity as 0 and on the first allocation it can be increased without any performance hits.
The capacity of a vector represents the maximum number of elements the vector can hold.
std::vector::empty Returns whether the vector is empty (i.e. whether its size is 0). This function does not modify the container in any way. To clear the content of a vector, see vector::clear.
If the vector container is empty what will size() and empty() returns? Size will return 0 and empty will return 1 because if there is no element in the vector the size of that vector will be 0 and empty will be true so it will return 1.
C++ Standard 23.2.4.2 only says that vector::capacity is
The total number of elements that the vector can hold without requiring reallocation.
That means that the actual value is fully implementation specific.
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The capacity can be whatever the implementors feel is correct or necessary.
It should also be noted it's never "safe" to assume you know the current capacity() without a call to that function. If you reserve 10 elements, the implementor is of free to allocate one hundred if it so wants to. Or 11, 42 (preferred) or just 10.
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