奇怪的'新'构造函数

Question

我在检查Qt源时遇到了以下代码:

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指示:

new (n) T(t);

据我所知,它似乎不是一个类型.这种结构意味着什么？

Answer 1

这是placement new.它只是调用一个带地址的构造函数.因此,T将在该位置构建类型的对象n.它也看起来是一个新的调用复制构造函数的位置.