Compilation error using "template <class T>" (prototypes generated incorrectly) [imported]

[cmaglie]

This is Issue 472 moved from a Google Code project.
Added by 2011-02-06T15:31:57.000Z by leomi...@gmail.com.
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Defect, Priority-Medium, Component-IDE, Component-PreProcessor

Original description

I found an error using the last revision of Arduino IDE (0022).
This code doesn't compile correctly:

template <class T> int SRAM_writeAnything(int ee, const T& value)
{
    const byte* p = (const byte*)(const void*)&value;
    int i;
    for (i = 0; i < sizeof(value); i++)
      EEPROM.write(ee++, *p++);
    return i;
}


template <class T> int SRAM_readAnything(int ee, T& value)
{
    byte* p = (byte*)(void*)&value;
    int i;
    for (i = 0; i < sizeof(value); i++)
      *p++ = EEPROM.read(ee++);
    return i;
}

The IDE returns this error:
error: expected ',' or '...' before '&' token
error: ISO C++ forbids declaration of 'T' with no type
error: 'T' has not been declared

Using version 0021 it works fine.

[nickgammon]

Can we promote this? Under version 1.5.8 of the IDE:

// generic minimum
template <typename T> T minimum (T a, T b) 
  {
  if (a < b)
    return a;
  return b;
  }  // end of minimum

void setup () 
  { 
  int foo = minimum (6, 7);
  }
void loop () { }

Gives error message:

sketch_oct08b.ino:3:1: error: 'T' does not name a type
Error compiling.

The code generated by the IDE and sent to the compiler is:

#line 1 "sketch_oct08b.ino"

#include "Arduino.h"
T minimum (T a, T b);
void setup ();
void loop ();
#line 2
template <typename T> T minimum (T a, T b)
  {
  if (a < b)
    return a;
  return b;
  }

void setup ()
  {
  int foo = minimum (6, 7);
  }
void loop () { }

Clearly the function-prototype generation is not recognizing this part:

template <typename T>

... as being part of a function declaration. Surely that is easy to fix?

[matthijskooijman]

Seems this would need to be fixed in these regexes:

https://github.com/arduino/Arduino/blob/master/app/src/processing/app/preproc/PdePreprocessor.java#L320-L321

Not sure if we'll actually make things better here. We should really make this preprocessing stuff deprecated...

[nickgammon]

I don't particularly mind that (except it would take a while to take effect). The preprocessing stuff would be OK, if it worked consistently, but it doesn't. And then you have to have function prototypes sometimes which confuses people as to why you sometimes need them and sometimes not.

Here's another example, which doesn't use templates:

typedef void (*GeneralMessageFunction) ();

//void checkPin (const int pin, GeneralMessageFunction response); // prototype
void checkPin (const int pin, GeneralMessageFunction response)
  {
  }  // end of checkPin

void setup () { }
void loop () { }

With the function prototype commented out you get the error:

sketch_oct09a.ino:2:31: error: 'GeneralMessageFunction' has not been declared

The other side-effect is that often (in these cases) the wrong line gets highlighted by the IDE because the "real" problem is in the hidden prototype line which has been generated, and which the user doesn't see.

It's not that hard to teach people to use prototypes, and indeed if you follow a simple rule they aren't usually necessary:

Declare functions before you call them.

So, if you put setup() and loop() last, and the things they call ahead of them (earlier in the source), everything works without prototypes anyway.

[ffissore]

Please checkout this thread [1] on devs mailing list and try one of the linked IDEs. It should fix almost all the problems reported in this issue. Typedefs are still incorrectly handled so @nickgammon example still won't work
[1] https://groups.google.com/a/arduino.cc/forum/#!topic/developers/4X2T3rCxXWM

[ffissore]

With this latest build https://groups.google.com/a/arduino.cc/d/msg/developers/4X2T3rCxXWM/WbQ2_dGzuhsJ even typedefs will work fine

[ffissore]

Fixed by #3779

[danlsk]

I am using 1.6.8. I got this error: 'T' does not name a type. I don't see it fixed yet. Let me know if other version fixes this.

[Chris--A]

@danlsk
You'll have to provide the code you are using (a small complete example). Gammon's code does not cause errors in 1.6.12/13.

Compiles fine:

// generic minimum
template <typename T> T minimum (T a, T b) 
  {
  if (a < b)
    return a;
  return b;
  }  // end of minimum

void setup () 
  { 
  int foo = minimum (6, 7);
  }
void loop () { }

[danlsk]

template <class T>
T sum (T a, T b){
  T result;
  result = a + b;
  return result;
}


void setup() {
  Serial.begin(9600);
  int i=5, j=6, k;
  double f=2.0, g=0.5, h;
  
  k = sum<int>(i,j);
  h = sum<double>(f,g);  
  
  Serial.println (k);
  Serial.println (h);
  
}

void loop() {
}

[danlsk]

Alright, I just figured out that it cannot break into 2 lines and must be on a single line.
template <class T> T sum (T a, T b)
instead of

template <class T>
  T sum (T a, T b)

But this one below works in 2 lines:

 template <class T>
 class mypair {
    T a,b;
  public:
    mypair (T first, T second)
      {a=first;b=second;}
    T getmax();
};

template <class T>
T mypair<T>::getmax(){
  T retval;
  retval = a>b? a : b;
  return retval;
}

void setup() {
  Serial.begin(9600);
  mypair <int> myobject (100,75);
  Serial.println(myobject.getmax());
}

void loop() {}

[Chris--A]

Yeah, the class function works on multiple lines as the IDE does not generate a prototype for it. Only file scoped functions are generated.

IMO the multi line error should be fixed as templates can become very involved and a single line detracts from readability dramatically. Especially now that C++11 has been enabled, templates are a staple diet of the language and are more prevalent that ever (standard template lib for one).

At least there is a workaround for now.

[facchinm]

This arduino/arduino-builder#182 PR should solve the multiline problem once for all , but the code is horribly convoluted and needs a big cleanup before merging. Anyway, if you could test it and report any bug you may encounter, it would be great!

[Chris--A]

Will do, I can spend a little time trying to break it for you :)

[facchinm]

@Chris--A ,
I rebased and refactored the code, it should be much better now 😄
@ArduinoBot should produce a build really soon so I'd really like you (and anyone interested) to test it!

[Chris--A]

just got home from work, will give it a spin shortly.

[Chris--A]

Unfortunately there was no luck with the new code, test results here: arduino/arduino-builder#182 (comment)