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I instantiate an std::vector foo(1000).
foo.size() is now 1000 and foo.capacity() is also 1000.
If I clear the vector with foo.clear(), the size() is now 0, but what is the capacity()? Does the standard say anything about that?
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Starting with C++11, we can call the vector::shrink_to_fit function after clear() , which reduces the vector's capacity to fir the size. It works by “requesting” a reallocation on the vector.
No. That's implied by the fact that iterators, pointers and references prior to the point of erase remain valid. Reducing the capacity would require a reallocation.
clear() removes all the elements from a vector container, thus making its size 0. All the elements of the vector are removed using clear() function.
C++ std::vector Reducing the Capacity of a Vector In C++11 we can use the shrink_to_fit() member function for a similar effect: v. shrink_to_fit();
No, it doesn't. The capacity of a vector never decreases. That isn't mandated by the standard but it's so both in standard library implementations of VC++ and g++. In order to set the capacity just enough to fit the size, use the famous swap trick
vector<T>().swap(foo); In C++11 standard, you can do it more explicitly:
foo.shrink_to_fit(); To clear a vector and consume as little capacity as possible, use the swap trick:
std::vector<T>().swap(foo); This creates an empty vector, swaps its internals with foo, and then destroys the temporary vector, getting rid of the elements that once belonged to foo and leaving foo as if it was freshly created.
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