ios_base::ate和tellp()

Question

#include <fstream>
#include <iostream> 

int main() 
{ 
    const char* fileName = "out1"; 
    std::ofstream fs1(fileName); 
    fs1 << "AAAAAAAAAAA\n"; 
    std::cout << fs1.tellp() << std::endl; 
    fs1.close(); 

    std::ofstream fs2(fileName, std::ios_base::ate); 
    std::cout << fs2.tellp() << std::endl; 
    fs2.close(); 

    return 0;   
}   

gcc版本4.4.6 20120305 (红帽4.4.6-4) (GCC)

g++文件02.cpp

./a.out

% 12 %0

为什么fs2.ellp()打印0，而不是12？

Answer 1

再次打开要写入的文件时，未提供out标志。这样做：

std::ofstream fs2(fileName, std::ios_base::out | std::ios_base::ate);