How do I print vector values of type glm::vec3 that have been passed by reference?

glm has an extension for this. Add #include "glm/ext.hpp" or "glm/gtx/string_cast.hpp"

Then to print a vector for example:

glm::vec4 test;
std::cout<<glm::to_string(test)<<std::endl;

I think the most elegant solution might be a combination of the two answers already posted, with the addition of templating so you don't have to reimplement the operator for all vector/matrix types (this restricts the function definition to header files, though).

#include <glm/gtx/string_cast.hpp>

template<typename genType>
std::ostream& operator<<(std::ostream& out, const genType& g)
{
    return out << glm::to_string(g);
}

glm::vec3 doesn't overload operator<< so you can't print the vector itself. What you can do, though, is print the members of the vector:

std::cout << "{" 
          << vertices[i].x << " " << vertices[i].y << " " << vertices[i].z 
          << "}";

Even better, if you use that a lot, you can overload operator<< yourself:

std::ostream &operator<< (std::ostream &out, const glm::vec3 &vec) {
    out << "{" 
        << vec.x << " " << vec.y << " "<< vec.z 
        << "}";

    return out;
}

Then to print, just use:

std::cout << vertices[i];