c++ - 我怎么知道变量是否可以直接写入二进制文件？

Question

我写了这个函数:

template <typename T>
ofstream bfwrite(ofstream &os, const T &data) {
    os.write(reinterpret_cast<char*>(data), sizeof(data));
    return os;
}

我知道这对某些类型不起作用，因为例如，它们指向堆上的数据。所以我想做一个编译时检查，但我在引用中找不到合适的函数。

有3种选择:is_trivially_copyable、is_pod或者直接使用别人的序列化库。学习一门语言时，你能做的一切其他人都已经完成了，所以我会坚持前两个选项之一。is_trivially_copyable 对我来说足够安全。有关详细信息，请参阅已接受的答案。

Answer 1

您需要的是 http://en.cppreference.com/w/cpp/types/is_trivially_copyable特质。

Objects of trivially-copyable types are the only C++ objects that may be safely copied with std::memcpy or serialized to/from binary files with std::ofstream::write()/std::ifstream::read().

具体来说，我强调的是:

In general, a trivially copyable type is any type for which the underlying bytes can be copied to an array of char or unsigned char and into a new object of the same type, and the resulting object would have the same value as the original.

template <typename T>
ofstream bfwrite(ofstream &os, const T &data)
{
    static_assert(std::is_trivially_copyable<T>::value, "T must be trivially copyable");
    os.write(reinterpret_cast<char*>(data), sizeof(data));

    return os;
}

或者，如果您需要 sfine ，像这样:

template <typename T>
typename std::enable_if<std::is_trivially_copyable<T>::value,
  ofstream>::type bfwrite(ofstream &os, const T &data)
{
    os.write(reinterpret_cast<char*>(data), sizeof(data));

    return os;
}