std::vector, default construction, C++11 and breaking changes

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南笙
南笙 2020-11-27 16:41

I ran today against a quite subtle issue I\'d like to have your opinion on.

Consider the following garden-variety shared-body-idiom class:

struct S
{         


        
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  •  情深已故
    2020-11-27 16:58

    Does the C++03 standard mandate that std::vector must have a constructor defined as above, i.e. with a default argument? In particular is there a guarantee that the entries of the vector object get copied instead of default constructed?

    Yes, the specified behavior is that x is copied n times so that the container is initialized to contain with n elements that are all copies of x.


    What does the C++11 Standard say about this same point?

    In C++11 this constructor has been turned into two constructors.

    vector(size_type n, const T& x, const Allocator& = Allocator()); // (1)
    explicit vector(size_type n);                                    // (2)
    

    Except for the fact that it no longer has a default argument for the second parameter, (1) works the same way as it does in C++03: x is copied n times.

    In lieu of the default argument for x, (2) has been added. This constructor value-initializes n elements in the container. No copies are made.

    If you require the old behavior, you can ensure that (1) is called by providing a second argument to the constructor invocation:

    std::vector v(42, S());
    

    I see this as a possibility for a breaking change between C++03 and C++11. I see this as a possibility for a breaking change between C++03 and C++11. Has this issue been investigated? Solved?

    Yes, as your example demonstrates, this is indeed a breaking change.

    As I am not a member of the C++ standardization committee (and I haven't paid particularly close attention to library-related papers in the mailings), I don't know to what degree this breaking change was discussed.

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