ostream equivalent of %.2f or %.2lf

You can use std::fixed and std::setprecision from the iomanip header:

#include <iostream>
#include <iomanip>
int main(void) {
    double d = 1.0 / 2;
    std::cout << std::fixed << std::setprecision(2) << d << std::endl;
    return 0;
}

This outputs 0.50 as desired.

Use setprecision in combination with fixed.

According to section 22.4.2.2.2 of the standard, precision specifications on iostreams have exactly the same effect as they do for printf. And fixed gives the exact same behavior as printf's %f.