How to break up a string into a vector fast? - c++

Question

I am processing CSV and using the following code to process a single line.
play with code
std::vector<std::string> string_to_vector(const std::string& s, const char delimiter, const char escape) {
std::stringstream sstr{s};
std::vector<std::string> result;
while (sstr.good()) {
std::string substr;
getline(sstr, substr, delimiter);
while (substr.back() == escape) {
std::string tmp;
getline(sstr, tmp, delimiter);
substr += "," + tmp;
}
result.emplace_back(substr);
}
return result;
}
What it does: Function breaks up string s based on delimiter. If the delimiter is escaped with escape the delimiter will be ignored.
This code works but is super slow. How can I speed it up?
Do you know any existing csv processing implementation that does exactly this and which I could use?

Answer 1

The fastest way to do something is to not do it at all.
If you can ensure that your source string s will outlive the use of the returned vector, you could replace your std::vector<std::string> with std::vector<char*> which would point to the beginning of each substring. You then replace your identified delimiters with zeroes.
[EDIT] I have not moved up to C++17, so no string_view for me :)
NOTE: typical CSV is different from what you imply; it doesn't use escape for the comma, but surrounds entries with comma in it with double quotes. But I assume you know your data.
Implementation:
#include <iostream>
#include <vector>
#include <string>
std::vector<char*> string_to_vector(std::string& s,
const char delimiter, const char escape)
{
size_t prev(0), pos(0), from(0);
std::vector<char*> v;
while ((pos = s.find(delimiter, from)) != s.npos)
{
if (pos == 0 || s[pos - 1] != escape)
{
s[pos] = 0;
v.push_back(&s[prev]);
prev = pos + 1;
}
from = pos + 1;
}
v.push_back(&s[prev]);
return v;
}
int main() {
std::string test("this,is,a\\,test");
std::vector<char*> v = string_to_vector(test, ',', '\\');
for (auto& s : v)
std::cout << s << " ";
}