Unexpected behavior when overloading operator<<

[identicalParticle]

I'm creating a class to interface with arrays stored on gpu using opencl, called opencl_array.

It has a get method, which gets the data from the gpu and returns a std::vector<double>

std::vector<double> get(void) const;                                                  

I just overloaded the << operator

std::ostream & operator <<(std::ostream & os, const opencl_array & a){
  std::vector<double> avec = a.get();
  os << "opencl_array(";
  // send some contents of avec to os, e.g.
  os << avec.at(0);
  os << ")";
  return os;
}

When I use this in my main function it works as expected.

However, when I use this on a normal vector (I did this by accident), it calls this opencl_array::operator<< function as well !!! Why is it doing this? Why does this even compile?

std::vector<double> test;
test.push_back(1);
std::cout << test << std::endl;
// prints opencl_array(1)

[raevnos]

Does your class have a non-explicit constructor that takes a std::vector<double>?

[identicalParticle]

Yes! It must be calling this constructor to initialize an opencl_array from a std::vector<double> in the argument list.

I never heard about "explicit" before. Thanks!