使用rdbuf将文件内容复制到缓冲区失败

Question

I'm trying to copy file content to a buffer.

std::ifstream fp(myFile, std::ios::binary)
fp.seekg(0, fp.end); // set cursor at the end
int length = fp.tellg(); // get data size
fp.seekg(0, fp.beg); // go back to buffer begin

char data[1000];
if(length<1000) {
    memcpy(data, fp.rdbuf(), length); // This crash

    std::stringstream contents;
    contents << fp.rdbuf();
    memcpy(data, contents->str().c_str(), length); // works fine
}

Copy using rdbuf directly crash, but copy on stringstream then in buffer works fine.

Does anybody has an explanation ?

Answer 1

It's because the rdbuf function doesn't actually return the data, it returns a stream buffer instance. The input operator << is overloaded to handle this object instance, but memcpy don't know what do do with it.

Answer 2

rdbuf()不返回可以使用memcpy的缓冲区，它返回一个streambuf对象。