how can I read a text file byte by byte?

Use the .get() method. Here is an example:

istream fileReader("filename.txt");
char byte;
while( fileReader.get(byte) ){
  cout << byte;
}

Are you sure this method return the byte??

when I tried to use it,I find that it's return the character from the txt file.

a charecture is one byte long. to prove this try this code

#include <iostream>

int main()
{
    char ch;
    std::cout << sizeof(char);
    std::cin.get();
    return 0;
}
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