c++ - 为什么允许此举？

Question

vector 在调整大小时将尝试使用 move 语义将对象从旧数组 move 到新数组。但是如果 vector 中的模板化对象不支持不抛出 noexcept move 构造函数然后它将恢复使用复制构造，以便 strong exception guarantee被保留下来。

但是当我尝试这样做时:

#include <vector>

class X
{
    public:
        // Needs default constructor
        X() {}

        // Copy operations disabled.
        X(X const&)             = delete;
        X& operator=(X const&)  = delete;

        X(X&&)              // throwable move constructor
        {}
        X& operator=(X&&)   // throwable move assignment.
        {return *this;}
};

int main()
{
    // Vector of Size zero
    std::vector<X>  data;

    // Vector of Size ten.
    // Since the move constructor can potentially throw
    // We have to copy elements when we do a resize
    //
    // But X has a disabled copy semantics
    // Thus I would expect a compile time error here.
    data.resize(10);
}

此编译没有错误或警告:

> g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix
> g++ -std=c++11 test.cpp
>

Answer 1

没有为具有抛出 move 构造函数的不可复制元素提供强异常安全保证。

[vector.capacity]/p12-14(强调我的):

void resize(size_type sz); 

12 Effects: If sz <= size(), equivalent to calling pop_back() size() - sz times. If size() < sz, appends sz - size() default-inserted elements to the sequence.

13 Requires: T shall be MoveInsertable and DefaultInsertable into *this.

14 Remarks: If an exception is thrown other than by the move constructor of a non-CopyInsertable T there are no effects.

请注意，这不需要 T成为CopyInsertable .

在内部，实现可能使用 std::move_if_noexcept ，尽管名称如此，但实际上是“如果没有异常(exception)或如果不可复制则 move ”。