tellp 在空 ostringstream 上的标准行为

#include<iostream>
#include <sstream>

int main() {
    std::ostringstream oss;
    std::cout << "Initial tellp position: " << oss.tellp()<< std::endl;

    oss << "Hello, world!";
    std::cout << "After writing, tellp position: " << oss.tellp()<< std::endl;

    return 0;
}

Initial tellp position: 0
After writing, tellp position: 13