String size or length?

Hi!
I am trying to translate from processing to openframeworks and almost there ,
except I can not get the size of the string after I input the string into a function
work in draw ()
string sen(“Merry”);
ofDrawBitmapString(sen.size(),100,300);
will return 5 on screen

but then I call a function:
algo( sen,"",1);
which is declared
string ofApp::algo(string sen, string res, int iteration){
float l = sen.size();

}
then when I debug , to have a look at sen.size is equal to 0.000000000000000000000000000000000000130693749 !!!

must be a real beginner bugs, but I have been spending all morning on this !!!
many thanks for helping me .
PS
appears that sen.length is working
but I have still a question about testing it :
if (sen.length()==0) {…}
does not work when there is nothing more in the string?
How can I test there is no more character in the string?
Many thanks

check this
http://www.cplusplus.com/reference/string/string/
as for testing the string use the empty() method like
if (sen.empty()) {.......}
best!

Many thanks for the link! very useful
best