std::string foo;
foo.assign(bar);

const unsigned char* bar = some_data();
auto length = std::strlen(reinterpret_cast<const char*>(bar));
const auto bar_end = bar + length;
std::string foo{ length, ' ' }; // avoid reallocations in transform
std::transform(bar, bar_end, foo.begin(), [](const unsigned char c) -> const char {
    return c > 127 ? ' ' : reinterpret_cast<const char>(c);
});