Please use getDefault().

The most-used way to do it is (after dp_max_rel_ is initialised to the default value):
```
dp_max_rel_ = param.getDefault("dp_max_rel", dp_max_rel_)
```
The template parameter is unneeded as long as the default argument has the correct type (but beware of default string literals, they must be wrapped in a temporary std::string for defaulting of string parameters).

For the parameter name, it is always unnecessary to wrap the string literal.