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If there is no typename keyword, the compiler cannot tell whether size_type is a member of std::vector<T> or something else. For example, if std::vector<T> has a static member of size_type.
size_type
std::vector<T>
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If there is no typename keyword, the compiler cannot tell whether
size_typeis a member ofstd::vector<T>or something else. For example, ifstd::vector<T>has a static member of size_type.