How to get the size of data stored in “any” in c++17?

问题

suppose i have a function like this

int writetofile(wstring name, any sdata){
  ...
   return error;
}

This function have no idea about what data would be stored but would need to know the size of the data stored in sdata. Although it is easy to determine the type of data stored in the sdata but i don't think there is some easy way to know about the size of data in sdata.

i have a data structure which has members of type wstring. Now We cant write that data structure directly to the file as it contains that wstring. As far as i have researched on internet, the best way to write wstring or string, is to write size first of the string and then the string. Then when i would read the string first reading the size then after this read that much of size.

for this i have made a function.

int filemanager::write(any data, fileid uid, DWORD *byteswritten) const
{
    // files is a map<fileid, fileinfo> where fileinfo is a struct which has 
    // members including file's name and handle 
    // fileid is a typedef of int
    if (!files.count(uid)) return -1;
    if (!data.has_value()) return -2;
    if (data.type() == typeid(wstring)) {
        DWORD sz1, sz2;
        wstring str = any_cast<wstring>(data);
        size_t sz3 = str.length()*sizeof(wchar_t);
        if (sz3 == 0) return -2;
        if (FALSE == WriteFile(files[uid].handle, &sz3, sizeof(size_t), &sz1, NULL)){
            return GetLastError();
        }
        if (FALSE == WriteFile(files[uid].handle, str.c_str(), sz3, &sz2, NULL) && sz2 != sz3) {
            return GetLastError();
        }
        if (byteswritten != nullptr) *byteswritten = sz1 + sz2;
    }
    else {
        // now if the type is not a wstring then just write it to a file
        // here i would need the size of the data stored in the data
    }
    return 0;
}

回答1:

std::any is not the correct tool for what you're trying to do. It is a tool primarily intended for communicating between points A and B, where both points know what the type is, but the communication needs to happen through some intermediate code C that doesn't need to know the type.

If B needs to try a bunch of different casts to see which value was provided, any is not the right tool for that job. The cast is there for type safety reasons: to have a well-defined failure path in the event that the wrong any was provided. That is, to make sure that A and B are communicating correctly. It's not there so that you can try a bunch of different things.

You can't ask any what the size of the stored object is because you should already know the answer. And even if you didn't know the answer, you could never validly use that answer.

Take your use case, for example. any is not TriviallyCopyable, so it is not legal C++ to copy its bytes directly to a file and then copy them back. And even if it were conceptually OK to do so, any may only be storing a pointer to the object it contains. So you'd just be writing a pointer to the file.

Serialization in C++ is not going to be as simple as you're trying to make it.

来源：https://stackoverflow.com/questions/51882028/how-to-get-the-size-of-data-stored-in-any-in-c17

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