如何防止`std::cin` 或`\\n` 刷新`std::cout` 的缓冲区？

Question

Let's imagine my program needs input from the user at different times. I want this input to prevent flushing the cout buffer. Can I set cin and cout on different stream buffers?

Example in question: a program that reads two numbers in a line, n1 n2 , and depending on the first number being either 0 , 1 , or 2 :

• n1 = 0 : writes the second number n2 to a vector v
• n1 = 1 : outputs v[n2] in cout
• n1 = 2 : pop_back() on v

The MWE is:

#include <iostream>
#include <vector>

using namespace std;

int main() {
    int size, n1, n2;
    vector<int> v;
    cin >> size;

    while(size--){
        cin >> n1;

        if (n1 == 0)
        {
            cin >> n2;
            v.push_back(n2);
        }
        else if (n1 == 1)
        {
            cin >> n2;
            cout << v[n2] << '\n';
        }   
        else if (n1 == 2)
            v.pop_back();
    }

return 0;
}

Say I have this test input

8
0 1
0 2
0 3
2
0 4
1 0
1 1
1 2

correct output should be

1
2
4

The program above yields outputs interspersed with the input lines instead.

But I would like them all printed together at end program, without using different means eg storing them in some container etc.

So I think I should operate on the buffer, but how?

Answer 1

You could write to your own std::stringstream buffer, and then output that to std::cout when you're ready.

MWE:

#include <iostream>
#include <sstream>
#include <stdexcept>
#include <vector>

using std::cin;
using std::cout;
using std::istream;
using std::runtime_error;
using std::stringstream;
using std::vector;

static auto get_int(istream& in) -> int {
    int n;
    if (!(in >> n)) {
        throw runtime_error("bad input");
    }
    return n;
}

int main() {
    auto ss = stringstream();
    auto v = vector<int>();
    auto size = get_int(cin);

    while(size--) {
        auto n1 = get_int(cin);

        if (n1 == 0) {
            auto n2 = get_int(cin);
            v.push_back(n2);
        } else if (n1 == 1) {
            auto n2 = get_int(cin);
            ss << v[n2] << '\n';
        } else if (n1 == 2) {
            v.pop_back();
        }
    }

    cout << ss.str();
}

Answer 2

No need to modify the buffer. Instead of cout << v[n2] you can store v[n2] in a second vector and print it out on the outside of the loop.