std::cout output 的时机很奇怪

Question

#include <iostream>
#include <thread>
#include <chrono>
using namespace std;

int main()
{
    std::cout << "1";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    std::cout << "2";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    std::cout << "3";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    return 1;
}
  

I want std::cout to be printed every second.

But when I run this code, the result value 123 is printed at the same time.

Why?

And please tell me how to fix it.

Answer 1

std::cout is buffered:

C++11 27.4.2 [narrow.stream.objects]/3: The object cout controls output to a stream buffer associated with the object stdout

so you need to explicitly flush this buffer to get what you want. Here are some ways to do this:

1. Use operator<< with std::flush

std::cout << "1" << std::flush;

2. Use the free function std::flush(std::basic_ostream)

std::cout << "1";
std::flush(std::cout);

3. Use the member function std::cout::flush()

std::cout << "1";
std::cout.flush();

4. Use operator<< with std::endl

std::cout << "1" << std::endl;

This will of course also insert a line-break and is probably not what you want.

Answer 2

You may need to flush the standard stream after each cout otherwise they may wait till the end of the program to flush all at once.

Answer 3

Its due to buffering. After each cout , you are required to flush the buffer, by using any of the statements like std::flush(std::cout); .

This may help you.

#include <iostream>
#include <thread>
#include <chrono>
using namespace std;

int main(){
    std::cout << "1";
    //std::this_thread::sleep_for( std::chrono::milliseconds(1000) );
    std::flush(std::cout);

    std::cout << "2";
    //std::this_thread::sleep_for( std::chrono::milliseconds(1000) );
    std::flush(std::cout);

    std::cout << "3";
    //std::this_thread::sleep_for( std::chrono::milliseconds(1000) );
    std::flush(std::cout);

    return 0;
}