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我在检查 Qt 源代码时遇到了以下代码:

template <typename T>
Q_INLINE_TEMPLATE void QList<T>::node_construct(Node *n, const T &t)
{
    if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic) n->v = new T(t);
    else if (QTypeInfo<T>::isComplex) new (n) T(t);
#if (defined(__GNUC__) || defined(__INTEL_COMPILER) || defined(__IBMCPP__)) && !defined(__OPTIMIZE__)
    // This violates pointer aliasing rules, but it is known to be safe (and silent)
    // in unoptimized GCC builds (-fno-strict-aliasing). The other compilers which
    // set the same define are assumed to be safe.
    else *reinterpret_cast<T*>(n) = t;
#else
    // This is always safe, but penaltizes unoptimized builds a lot.
    else ::memcpy(n, static_cast<const void *>(&t), sizeof(T));
#endif
}

它有一个奇怪的new指令:

new (n) T(t);

据我了解,它似乎不是类型转换。这个结构是什么意思?

4

1 回答 1

7

这是placement new. 它只是调用带有地址的构造函数。因此,T将在 location 构造一个类型的对象n。它也看起来是一个调用复制构造函数的放置新。

于 2013-05-15T18:30:26.353 回答