Where is ToCString's header, or how to convert arguments to cstrings?

I'm trying to use ToCString in a NodeJS addon.

String::Utf8Value foo_utf8(args[0]->ToString());
const char *foo = ToCString(foo_utf8);

I can't find it's header, and if I manually prototype it as:

const char* ToCString(const v8::String::Utf8Value& value);

then I get a runtime error of:

node: symbol lookup error: /home/chris/Dropbox/cbackend/build/Release/cbackend.node: 
undefined symbol: _Z9ToCStringRKN2v86String9Utf8ValueE

How can I convert a V8 String argument to a null-terminated 8 bit C string?

I found a workaround, as the ToCString function (wherever it lives) is trivial. Just add:

const char* ToCString(const v8::String::Utf8Value& value) {
  return *value ? *value : "<string conversion failed>";
}

Going by the docs, v8 has operators for cstr conversion:

Local<Value> value = get_value();     // nan† thing, you can ignore this

v8::String::Utf8Value string(value);
char* str = *string;                  // defined as:        char * operator* () 
const char* const_str = *string;      // defined as:  const char * operator* () const

This applies to both njs v0.10 and v0.11.

† https://github.com/rvagg/nan