"void value not ignored as it ought to be" on non-void function

Question

Here's my code

  void execute() {
    for (u32 f = 0; f < numForces; f++)
    {
      u32 length = end - start;
      PhysicalObject* arr = objects + start;
      std::vector<bool> tmp(length);
      bool* tmpArr = tmp.data();
      forces[f].affected(arr, length, tmpArr);
      for(u32 index = 0; index < length; index++)
        if (tmp[index])
          forces[f].apply(arr[index]);
    }
  }

the line

      bool* tmpArr = tmp.data();

throws me the error: "void value not ignored as it ought to be" I've looked into the documentation and I'm certain vector's data method is non void and should return a pointer: > http://www.cplusplus.com/reference/vector/vector/data/

Any ideas of what could be going wrong?

Important things:

I'm including vector
My compiling flags on gcc: -Wall -Wextra -Wcast-qual -pedantic -pedantic-errors -Wfatal-errors -Wno-missing-braces -Werror
the function is a class but I haven't added the rest as it just removes focus from the problem and it's a class made to run in parallel which uses a complex architecture.

That's a special problem with the specialization of `std::vector`, you can't get the address of a single bit. — πάντα ῥεῖ, Jul 15 '18 at 20:37
This question's answer might be useful, https://stackoverflow.com/questions/6485496/how-to-get-stdvector-pointer-to-the-raw-data — yadhu, Jul 15 '18 at 20:39
To fix this you can probably make a simple struct that wraps a bool value and store that in the vector — MivVG, Jul 15 '18 at 20:39

score 5 · Answer 1 · answered Jul 15 '18 at 20:40

5

Unfortunately vector<bool> is special.

https://en.cppreference.com/w/cpp/container/vector_bool

You'd probably be better off switching to vector<uint8_t>

answered Jul 15 '18 at 20:40

john

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`std::vector>` might also be suitable alternative. – πάντα ῥεῖ Jul 15 '18 at 20:42

score 3 · Answer 2 · answered Jul 15 '18 at 22:54

To briefly quote Scott Meyers from his Effective STL book, item 18: "Avoid using vector<bool>". The semantics of that specialization means that it is not completely conforming to the container requirements in the standard, because it doesn't actually store bools; it stores a packed representation using bits instead.

Whenever you need access to individual "bools" or you need a pointer to the data sequence (as in your example), you're in trouble, because there is no way to get a pointer to a single bit.

As mentioned by @john above, vector<uint8_t> may serve you well. Or, if you rewrite the function to use iterators instead of pointers, you could try with deque<bool>.

"void value not ignored as it ought to be" on non-void function

2 Answers2