std::fstream stream("filename",std::ios::in | std::ios::out | std::ios::binary);
char byte;
stream.read(&byte,1);
// stream.seekp(1);
int bytesCount = 4096;
auto bytesVec = std::vector<char>(bytesCount,'c');
char* bytes = bytesVec.data();
std::cout << stream.bad() << std::endl;
stream.write(bytes,bytesCount);
std::cout << stream.bad() << std::endl;
如果我执行这个代码,那么第一个bad()返回false,但是第二个返回true,实际上没有写入.
如果我将bytesCount减小到小于4096的值(大概是一些内部缓冲区的大小),那么第二个bad()返回false,但仍然没有写入.
如果我取消注释seekp()行,写入开始工作：bad()返回false,并且字节实际上被写入.
为什么在这里需要seekp()为什么没有它呢？ seekp()是正确的方法吗？
我在Windows 7上使用Visual Studio 2012.
解决方法
在MS的fstream的更新模式下打开文件的写操作
库继承自其C< stdio.h>实现.
C标准(我引用C99,但在这一点上与C89没有区别)
在7.19.5.3/6状态：
When a file is opened with update mode (‘+’ as the second or third character in the
above list of mode argument values),both input and output may be performed on the
associated stream. However,output shall not be directly followed by input without an
intervening call to the fflush function or to a file positioning function (fseek,
fsetpos,or rewind),and input shall not be directly followed by output without an
intervening call to a file positioning function,unless the input operation encounters end-
of-file.
(我的重点).
所以你的stream.seekp(1)解决方案,下载到C fseek是正确的.
GNU C库没有此标准限制,因此您发布的代码有效
如同GCC建成的那样.
MS< fstream>图书馆符合C标准的继承
这个限制. fstreams使用basic_filebuf< charT,traits>实现.在(C11)标准的这个模板的第27.9.1.1/2段中,它简单地说：
The restrictions on reading and writing a sequence controlled by an object of class basic_filebuf are the same as for reading and writing with the Standard C library FILEs.
