How do I read exactly 128 bytes from an fstream into a string object?
I wrote some code to read the first 128 bytes of a file and print it and then the last 128 bytes of the file and print that. The last part works, since you can easily iterate to EOF, but how do I get exactly 128 bytes from the front? The code below doesn't work since you can't add 128 to an ifstream iterator, it's not indexable, only incrementable (it seems).
Sure I could make an iterator and *++ it 128 times, but there must be a single line straightforward way to do it, right?
#include <iostream>
#include <fstream>
#include <string>
int main(int argc, char **argv)
{
std::ifstream ifs ("input.txt",std::ifstream::in | std::ifstream::binary);
if (ifs.good())
{
// read first 128 bytes into a string
ifs.seekg(0,std::ifstream::beg);
std::string first128((std::istreambuf_iterator<char>(ifs)),
(std::istreambuf_iterator<char>(ifs))+128);
std::cout << first128 << std::endl;
// read last 128 bytes into a string
ifs.seekg(-128,std::ifstream::end);
std::string last128((std::istreambuf_iterator<char>(ifs)),
std::istreambuf_iterator<char>());
std::cout << last128 << std::endl;
return 0;
}
return 1;
}
char buffer[129];
ifs.read (buffer,128);
buffer[128] = '\0';
first128 = buffer;
How about this then:
template <typename Itr, typename Out>
void copy_n(Itr it, size_t count, Out out)
{
for(size_t i=0;i<count;++i)
out = *it++;
}
...
std::string first128;
std::istreambuf_iterator<char> it(ifs);
copy_n( it, 128,
std::back_inserter<std::string>(first128) );