Problem with String& argument of a function

Question

I am trying to send SMSs using a GSM module with Arduino Mega. I have followed a tutorial and downloaded the SIM908IDE100 library. I am using the function

SendSMS(String&, const char [27]);.

It works fine when I type:

sms.SendSMS("1234567890","Text message");

where "1234567890" is the telephone number I want to send the SMS to and "Text message" is the text of the SMS.

If I instead try to pass to the function a String containing the telephone number

String number = "1234567890"; sms.SendSMS(number,"Text message");

I get the following error:

exit status 1 no matching function for call to 'SMSGSM::SendSMS(String&, const char [27]);'

I don't understand what String followed by an ampersand means. What am I supposed to pass the function as its first argument?

• Juraj: are you sure it is not the sms.SendSMS("1234567890","Text message"); giving you "no matching function"?
• arderino: @Juraj Yes I am sure. The sms.SendSMS("1234567890","Text message"); works fine and the SMS is sent without any problem.
• Mikael Patel: Hint: Try sms.SendSMS(String("1234567890"),"Text message");

Answer 1

The function prototype in sms.h is

char SendSMS(char *number_str, char *message_str);

if number is String, you should have

sms.SendSMS((char*) number.c_str(),"Text message");

c_str() is a function of the Strinng class. It returns a pointer to internal char array of the String object.

The cast to char* is necessary because the author of the library didn't make the parameter constant with const char*. (It is a good practice, to declare pointers and references as constants.)

The error message no matching function for call to 'SMSGSM::SendSMS(String&, const char [27]); shows the arguments you try to use. String& is reference to some instance of the String type

• arderino: if I use sms.SendSMS(number.c_str(),"Text message"); I get the error: no matching function for call to 'SendSMS(const char*, const char [13])'
• Juraj: sms.SendSMS((char*) number.c_str(),"Text message");
• arderino: thank you very much, it works now. I still don't fully understand how (char*) works, would you mind to briefly explain it? thanks again
• Juraj: tutorialspoint.com/cprogramming/c_strings.htm