printing vector elements : cout doesn't work while printf works

I'm trying to access the elements of a vector using this simple example:

#include <iostream>
#include <vector>

typedef std::vector<uint8_t> uint8_vec_t;

int main(void) {
    uint8_vec_t v = {1, 2, 3, 4};
    uint8_vec_t::iterator itr;
    std::cout << "v.size() = " << v.size() << std::endl;
    int i = 0;
    for ( itr = v.begin(); itr != v.end(); ++itr, i++) {
        std::cout << "std::cout: v[" << i << "] = " << v[i] << std::endl;
        std::cout << "std::cout: *itr =" << *itr << std::endl;
        printf("printf: v[%d] = %d\n", i, v[i]);
        printf("printf: *itr = %d\n", *itr);
    }
    //
    return 0;
}

And I get the following output:

v.size() = 4
std::cout: v[0] = 
std::cout: *itr =
printf: v[0] = 1
printf: *itr = 1
std::cout: v[1] = 
std::cout: *itr =
printf: v[1] = 2
printf: *itr = 2
std::cout: v[2] = 
std::cout: *itr =
printf: v[2] = 3
printf: *itr = 3
std::cout: v[3] = 
std::cout: *itr =
printf: v[3] = 4
printf: *itr = 4

I provide the same value to cout and printf but only printf prints the value to the terminal. Why cout doesn't print the value while I have no issue when using printf ?

Thanks for your help.