16.6 Buffering Problems

Buffered I/O does not write immediately to the file. Instead, the data is kept in a buffer until there is enough for a big write, or until the buffer is flushed. The following program is designed to print a progress message as each section is finished.

std::cout << "Starting"; 
do_step_1(  ); 
std::cout << "Step 1 complete"; 
do_step_2(  ); 
std::cout << "Step 2 complete"; 
do_step_3(  ); 
std::cout << "Step 3 complete\n"; 

Instead of writing the messages as each step completes, std::cout puts them in a buffer. Only after the program is finished does the buffer get flushed, and all the messages come spilling out at once.

The I/O manipulator std::flush forces the flushing of the buffers. Properly written, the above example should be:

std::cout << "Starting" << std::flush; 
do_step_1(  ); 
std::cout << "Step 1 complete" << std::flush; 
do_step_2(  ); 
std::cout << "Step 2 complete" << std::flush; 
do_step_3(  ); 
std::cout << "Step 3 complete\n" << std::flush; 

Because each output statement ends with a std::flush, the output is displayed immediately. This means that our progress messages come out on time.

The C++ I/O classes buffer all output. Output to std::cout and std::cerr is line buffered. In other words, each newline forces a buffer flush. Also, C++ is smart enough to know that std::cout and std::cerr are related to std::cin and will automatically flush these two output streams just before reading std::cin. This makes it possible to write prompts without having to worry about buffering: std::cout << "Enter a value: "; // Note: No flush std::cin >> value;