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It seems that a vector will check if the move constructor is labeled as noexcept before deciding on whether to move or copy elements when reallocating. Is the default move constructor defined as noexcept? I saw the following documentation but it didn't specify this.http://en.cppreference.com/w/cpp/language/move_constructor
Implicitly-declared move constructor
If no user-defined move constructors are provided for a class type (struct, class, or union), and all of the following is true: there are no user-declared copy constructors there are no user-declared copy assignment operators there are no user-declared move assignment operators there are no user-declared destructors the implicitly-declared move constructor is not defined as deleted due to conditions detailed in the next section then the compiler will declare a move constructor as an inline public member of its class with the signature T::T(T&&) A class can have multiple move constructors, e.g. both T::T(const T&&) and T::T(T&&). If some user-defined move constructors are present, the user may still force the generation of the implicitly declared move constructor with the keyword default.
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Particularly, since they don't have the ability to tell if your move constructor is actually noexcept or not, they basically just have to guess. From memory they simply assume that it is noexcept because throwing move constructors are not common and not being able to move would be a critical problem.
For non-union class types (class and struct), the move constructor performs full member-wise move of the object's bases and non-static members, in their initialization order, using direct initialization with an xvalue argument.
A move constructor enables the resources owned by an rvalue object to be moved into an lvalue without copying. For more information about move semantics, see Rvalue Reference Declarator: &&. This topic builds upon the following C++ class, MemoryBlock , which manages a memory buffer.
I think the answer is 15.4/14 (Exception specifications):
An inheriting constructor (12.9) and an implicitly declared special member function (Clause 12) have an exception-specification. If
fis an inheriting constructor or an implicitly declared default constructor, copy constructor, move constructor, destructor, copy assignment operator, or move assignment operator, its implicit exception-specification specifies the type-idTif and only ifTis allowed by the exception-specification of a function directly invoked byf’s implicit definition;fallows all exceptions if any function it directly invokes allows all exceptions, andfhas the exception-specificationnoexcept(true)if every function it directly invokes allows no exceptions.
Basically, it Does What You Think, and the implicitly-declared move constructor is noexcept whenever it can be.
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