读、写和 as_bytes 函数

Question

In Programming and principles chapter 11, the author gives the following code to demonstrate binary i/o:

#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<cmath>


#include<sstream>
#include <fstream>
#include <iomanip>
using namespace std;
    template<class T>
    char* as_bytes(T& i) // treat a T as a sequence of bytes
{
    void* addr = &i; // get the address of the first byte
    // of memory used to store the object
    return static_cast<char*>(addr); // treat that memory as bytes
}
int main()
{
    // open an istream for binary input from a file:
    cout << "Please enter input file name\n";
    string iname;
    cin >> iname;
    ifstream ifs {iname,ios_base::binary}; // note: stream mode
    // binary tells the stream not to try anything clever with the bytes
    // open an ostream for binary output to a file:
    cout << "Please enter output file name\n";
    string oname;
    cin >> oname;
    ofstream ofs {oname,ios_base::binary}; // note: stream mode
    // binary tells the stream not to try anything clever with the bytes

    vector<int> v;

    // read from binary file:
    for(int x; ifs.read(as_bytes(x),sizeof(int)); ) // note: reading bytes
        v.push_back(x);
    // . . . do something with v . . .
    // write to binary file:
    for(int x : v)
        ofs.write(as_bytes(x),sizeof(int)); // note: writing bytes
    return 0;
}

I have some questions:

1. Why is he reading a address to a uninitialized variable?
2. Why does the program cut off some chars at the end of the file?
3. Why and how is he pushing an uninitialized variable to a vector?

Answer 1

Problem 1 and 3

In

for(int x; ifs.read(as_bytes(x),sizeof(int)); )

x is passed into the function uninitialized, but x 's undefined value is not going to be used.

The read function is going to use the space allocated to x as a container. It will read one int 's worth of data from ifs and store it in x , giving x a known value that can then be safely used. Because the body of the loop will not enter unless an int was read from the file

v.push_back(x);

is guaranteed to have a valid value for x . That's assuming the input file contains valid int s.

Problem 2

ifs is being read in blocks of int size. If the size of the file is not evenly divisible by the size of an int the final read will fail. The body of the loop is only entered if the read is successful, so no data is added to the vector and no data will be read from the vector and written to the output file.