Sprintf() error

There is a Binary2String() in my project.

It works well for a long time. But it threw exception when vec[0] = 255 on sprintf(temp, "%02d ", vec[i]);.

Original Code as this,

void Binary2String(const vector<unsigned char>& vec, string& result)
{

 if(vec.size() == 0)
  return;

 for(size_t i = 0; i < vec.size(); i++)
 {
  char temp[4] = {0};
  sprintf(temp, "%02d ", vec[i]);
  result.push_back(temp[0]);
  result.push_back(temp[1]);
  result.push_back(temp[2])开发者_JS百科;

 }
}

Then I fixed the bug as this, I can't understand the vector and sprintf() completed.

Does it make sense or not? (I am new to C++. Thank you)

        char temp[6] = {0};
  sprintf(temp, "%02d ", vec[i]);
  result.push_back(temp[0]);
  result.push_back(temp[1]);
  result.push_back(temp[2]);
        result.push_back(temp[3]);
  result.push_back(temp[4]);

First of all, %02d is not the correct format since values can go up to 3 digits (%03d would be more appropriate). And as Chris states in the comment below, %03hhu would be the technically correct specifier (to understand %03hhu, take a look at this cheat sheet).

Second, You need 5 bytes to store the chars "2", "5", "5", " ", 0 (you have an extra space at the end of %02d). Either drop the space, or extend the buffer to at least 5 chars (as you have done).

Finally, you can convert to string much more reliably this way:

template<typename T> std::string ToString(const T& in) {
    std::basic_ostringstream<char> o;
    if (!(o << in)) {
        // error, throw an exception
    }
    return o.str();
}