test.cpp

#include <iostream>
#include <vector>

#undef CUSTOM_BUFFER

#if defined CUSTOM_BUFFER
static const int BUFFER_SIZE = 1024 * 16;
#endif

int main()
{
#if defined CUSTOM_BUFFER
    char buffer[BUFFER_SIZE];
    std::cin.rdbuf()->pubsetbuf(buffer, BUFFER_SIZE);
#endif
    std::ios_base::sync_with_stdio(false);
    std::vector<int> data;
    int input;

    while (std::cin >> input)
    {
        data.push_back(input);
    }
    std::cout << "Data loaded." << std::endl;

    return 0;
}

test.py

import sys
data = [int(x) for x in sys.stdin.read().split()]

time.txt

$ c++ -O2 -Wall -Wextra test.cpp            
$ time ./a.out < million-integers.txt
Data loaded.
./a.out < million-integers.txt  4.06s user 0.01s system 99% cpu 4.072 total
$ time python test.py < million-integers.txt
python3 test.py < million-integers.txt  0.38s user 0.06s system 99% cpu 0.438 total

$ c++ --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.2.0
Thread model: posix