Manipulating byte vector through float pointer

Question

Is it possible to manipulate an std::vector<unsigned char> through its data pointer as if it were a container of float?

Here is an example that compiles and (seemingly?) runs as desired (GCC 4.8, C++11):

#include <iostream>
#include <vector>

int main()
{
    std::vector<unsigned char> bytes(2 * sizeof(float));
    auto ptr = reinterpret_cast<float *>(bytes.data());
    ptr[0] = 1.1;
    ptr[1] = 1.2;
    std::cout << ptr[0] << ", " << ptr[1] << std::endl;
    return 0;
}

This snippet successfully writes/reads data from the byte buffer as if it were an array of float. From reading about reinterpret_cast I'm afraid that this might be undefined behavior. My confidence in understanding the type aliasing details is too little for me to be sure.

Is the code snippet undefined behavior as outlined above? If so, is there another way to achieve this sort of byte manipulation?

Answer 1

Legal answer

No, this is not permitted.

C++ isn't just "a load of bytes" — the compiler (and, more abstractly, the language) have been told that you have a container of unsigned chars, not a container of floats. No floats exist, and you can't pretend that they do.

The rule you're looking for, which is known as strict aliasing, may be found under [basic.lval]/8.

The opposite would work, because it is permitted (via a special rule in that same paragraph) to examine the bytes of any type via an unsigned char*. But in your case, the quickest safe and correct way to "get" a float from something that starts life as unsigned char is to std::memcpy or std::copy those bytes into an actual float that exists:

std::vector<unsigned char> bytes(2 * sizeof(float));
float f1, f2;

// Extracting values
std::memcpy(
   reinterpret_cast<unsigned char*>(&f1),
   bytes.data(),
   sizeof(float)
);

std::memcpy(
   reinterpret_cast<unsigned char*>(&f2),
   bytes.data() + sizeof(float),
   sizeof(float)
);

// Putting them back
f1 = 1.1;
f2 = 1.2;

std::memcpy(
   bytes.data(),
   reinterpret_cast<unsigned char*>(&f1),
   sizeof(float)
);

std::memcpy(
   bytes.data() + sizeof(float),
   reinterpret_cast<unsigned char*>(&f2),
   sizeof(float)
);

This is fine as long as those bytes form a valid representation of float on your system. Granted it looks a little unwieldy, but a quick wrapper function will make short work of it.

A common alternative, assuming you only care about floats and don't need a resizable buffer, is to produce some std::aligned_storage then do a bunch of placement new into the resulting buffer. Since C++17, you could alternatively play around with std::launder, though resizing the vector (read: reallocating its buffer) would also be inadvisable in that scenario.

Also, these approaches are quite involved and result in complex code that not all your readers will be able to follow. If you can launder your data such that it "is" a sequence of floats, you may as well just make yourself a nice std::vector<float> in the first place. Per the above, it is permitted to get and use an unsigned char* to that buffer if you wish.

It ought to be noted that there is much code out there in the wild that uses your original approach (particularly in older projects with a barebones C heritage). On many implementations, it may appear to work. But it is a common misconception that it is valid and/or safe, and you're prone to instruction "re-ordering" (or other optimisations) if you rely on it.

---

Hedge-betting answer

For what it's worth, if you disable strict aliasing (GCC permits this as an extension, and LLVM doesn't even implement it), then you can probably get away with your original code. Just be careful.

Answer 2

Is it possible to manipulate an std::vector through its data pointer as if it were a container of float?

Not quite. Your example has UB indeed.

However, you can reuse the storage of those bytes to create the floats there. Example:

float* ptr = std::launder(reinterpret_cast<float*>(bytes.data()));
std::uninitialized_fill_n(ptr, 2, 0.0f);

After this, the lifetime of the unsigned char objects has ended, end there are floats there instead. Using ptr is well defined.

Whether this would be useful for you is another matter. Start with a simpler design first: Why not simply use std::vector<float>?