std::string... A string can hold both letters and numbers. Though if you need to any calculations with the numbers in the string you will need to extract them with a stringstream.
For example lets say you have a line in a text document that looks like "123123 Hello". You could read that line in and whatever other lines there are like so.
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int main()
{
std::vector<std::string> linesOfFile;
std::string line;
std::ifstream readFile("testing.txt");
// Read each line in the file and store them in a vector
while (std::getline(readFile, line))
linesOfFile.push_back(line);
// Print all the lines
for (const std::string& i : linesOfFile)
std::cout << i << std::endl;
}