Question

Say, I have a string and a vector of bools. Based on the characters in the string, I want to set the corresponding vector indices to true.

std::vector<bool> is_present(256, false);
for (int i = 0; i < str.size(); ++i)
{
    is_present[str[i]] = true;
}

From what I understand, the standard does not define the signed-ness of a char. Depending on the platform, it may be signed or unsigned. On most platforms, signed char will be an 8-bit two’s complement number (-128 to 127), and unsigned char will be an 8-bit unsigned integer (0 to 255).

If that is the case, is there a possibility that str[i] will return a negative number and cause a memory fault in is_present[str[i]]? or is the char getting typecast to vector<bool>::size_type which is unsigned and hence no problems can occur?

Also, is it better to use vector<bool> is_present(pow(2, CHAR_BIT)), false) instead of hardcoding it to 256?

Answer 1

Always cast a char to an unsigned char if you want to be definite about the values.

You can say 1u << CHAR_BIT to get the desired size.

std::vector<bool> is_present(1u << CHAR_BIT, false);
for (int i = 0; i < str.size(); ++i)
{
    is_present[static_cast<unsigned char>(str[i])] = true;
}