What does ~ mean in the YML configuration file in

问题:

What does ~ mean in the YML configuration file in Symfony 2?

For example:

NotBlank: ~

回答1:

In Yaml, ~ means null. (source)

The actual meaning in Symfony 2 depends on what you are configuring. Most likely it will mean "use defaults" or it will be used where it is needed to put a configuration key but there is nothing to actually configure for that key (as in the NotBlank example).