This code:

std::cout << "std::cout for 2112 =  " << 2112 << 'n';
YAML::Emitter out;
out << 2112;
std::cout << "Emitter for 2112 = " << out.c_str() << 'n';

produces this output:

std::cout for 2112 =  2112
Emitter for 2112 = 2,112

I think this shows that the comma is not added to the yaml string because of some locale / std stream settings, and seems to be coming from yaml-cpp itself.
I could convert the number to a string first and send this to the Emitter, but is there a simpler way to prevent the thousands separator from being included in the yaml string?