C++ 从 'std::__tuple... {aka 'unsigned int*'} 转换为 'uint32_t' {aka 'unsigned int'} 失去精度

Question

You may have to forgive me as I'm new to C++ and may have made some fundamental errors with the code I have worked up so far.

static tuple<read_result, uint8_t*> m_scan_record(bool skip, uint32_t& size, FILE* file)
{
    read_result result;
    
    tuple<read_result, uint32_t*> rd_rec_size_result = m_read_generic_t<uint32_t>(file);

    result = (read_result)get<0>(rd_rec_size_result);
    
    if (result != read_success )
    {
        return tuple<read_result, uint8_t*>(result, nullptr);
    }

    size = (uint32_t) get<1>(rd_rec_size_result);

    if ( skip )
    {
        fseek(file, size, SEEK_CUR);
    }
// ...
}

template<typename T>
static tuple<read_result, T*> m_read_generic_t(FILE* file)
{
    T ret = 0;
    
    read_result result = m_read_from_file_to_buffer(&ret, sizeof(T), file);

    if (result == read_success)
    {
        return tuple<read_result, T*>(result, &ret);
    }

    return tuple<read_result, T*>(result, nullptr);
}

When I compile this code I am getting this error:

cast from ‘std::__tuple_element_t<1, std::tuple<read_result, unsigned int*> >’ {aka ‘unsigned int*’} to ‘uint32_t’ {aka ‘unsigned int’} loses precision [-fpermissive]

My intentions and what I am expected to do/happen:

1. In the declaration of m_scan_record , the size argument is declared with a & which is intended to allow me to pass the value by reference, analogous to using the REF c# keyword

2. I make a call to generic (template) function m_read_generic_t which is called with the specified type <unit32_t> and therefore (according to its definition) will return a type of tuple<read_result, uint32_t*>

3. Once I have the tuple returned by m_read_generic_t , I want to take the unit32_t value pointed to by the second value of the tuple, and put that value into the size variable mentioned at point 1, above, which presumably will then be accessible to the calling function one step further up the stack.

4. From the above points you can hopefully see that my intention (and I appreciate that I may be far away in reality:) is that at this line:

   size = (uint32_t) get<1>(rd_rec_size_result);

all I am doing is simply grabbing a 'pointed to' value and putting it into a variable of a matching type, much like the oft-cited textbook example:

uint32_t v = 123;
uint32_t* ptr_to_v = &v;
uint32_t x = ptr_to_v;   // x == 123

Clearly this is not what is really going on with my code, though, because if it were, I presume that the cast would be un-needed. But if I remove it, like this:

size = get<1>(rd_rec_size_result);

then I get a compile-time error:

a value of type "std::__tuple_element_t<1UL, std::tuple<read_result, uint32_t *>>" cannot be assigned to an entity of type "uint32_t"

I believe therefore that I am doing something badly wrong - but I can't work out what. Is this to do with the way I am taking the pointer out of the tuple; or is there something else going on when it comes to the getting a uint32_t value from a uint32_t*?

This is all in a C++ environment on Ubuntu 20.04, FWIW

Many thanks in advance for any/all suggestions; please go easy on me!

Answer 1

tuple<read_result, uint32_t*> rd_rec_size_result = ...

The 2nd member of this tuple, as explicitly declared here, is a pointer to a uint32_t . That's what uint32_t * means, in C++.

size = (uint32_t) get<1>(rd_rec_size_result);

This retrieves the uint32_t * and attempts to convert it to a uint32_t . C++ does not work this way. Although this conversion can be forced your compiler has every right to believe that whatever this code is trying to do it must be wrong.

Perhaps I was wondering initially, your intention was to dereference the pointer. This is the reason for your compilation error, in any case. If your intention was to, truly, dereference this pointer, then this would've been a simple matter of changing this to

size = *get<1>(rd_rec_size_result);

However, that's not going to be the end of your troubles. Even after this compilation error is fixed, this way, the shown code will still be badly, badly broken.

This is because m_read_generic_t returns a pointer to a local object, which will get destroyed when the function returns, and attempting to dereference this pointer, here, will make demons fly out of your nose .

The real fix here is to change m_read_generic_t to not return a pointer as the 2nd value in the tuple in the first place, thus eliminating the compilation error in the first place.